Utah invasive weed long black seed with hook

Garlic Mustard

Garlic mustard (Alliaria petiolata) is an invasive herb that has spread throughout much of the United States over the past 150 years, becoming one of the worst invaders of forests in the American Northeast and Midwest. While it is usually found in the undergrowth of disturbed woodlots and forest edges, recent findings have shown that garlic mustard has the ability to establish and spread even in pristine areas. This spread has allowed it to become the dominant plant in the undergrowth of some forests, greatly reducing the diversity of all species. Garlic mustard is one of very few non-native plants to be able to successfully invade forest understories.

Origin and Expansion

Garlic mustard is a non-native species originating from Europe and parts of Asia. It is believed that garlic mustard was introduced into North America for medicinal purposes and food. The earliest known report of it growing in the United States dates back to 1868 on Long Island, NY. It has since spread throughout the eastern United States and Canada as far west as Washington, Utah, and British Columbia.

First year garlic mustard basal flower rosette – Jil M. Swearingen, USDI National Park Service, Bugwood.org Second year flowers – David Cappaert, Michigan State University, Bugwood.org

Biology

Garlic mustard has a biennial life cycle, that is, it takes two years to fully mature and produce seeds. Seeds germinate in February to early March of the first year and grow into a short rosette by the middle of the summer. In the plant’s second year, a stalk develops, flowers form, and the plant dies by June. Siliques, four-sided seedpods, develop in May, containing small black seeds lined up in a row. On average, a garlic mustard plant will produce 22 siliques, each of which can contain as many as 28 seeds. A particularly vigorous plant may produce as many as 7,900 seeds (Nuzzo, 1993) although the average is more likely to be in the 600 seed range. The seeds generally germinate within one to two years, but may remain viable for up to five years in the seed bank. Seed dispersal is mainly by humans or wildlife carrying the seeds.

Characteristics and Identification

Identification of first year plants can be difficult; the task is made easier by smelling the garlic odor produced when the leaves of the plant are crushed. The basal leaves of an immature plant are dark-green and kidney shaped with round teeth (scalloped) along the edges; average size of the leaves is 6 to 10 cm in diameter. The petiole, or leaf stalk, of first year plants are 1 to 5 cm long. In its second year, the alternating stem leaves become more triangular shaped, 1 to 5 cm long, and have sharper teeth, with leaves becoming gradually smaller towards the top of the stalk. Leaf stalks of mature plants are hairy. As with the younger plants, second year plants have a garlic odor when crushed but the odor is less obvious with increasing age.

Garlic mustard flowers arrive in early April and die by June. Flowers develop on an unbranched (occasionally weakly branched) stalk and have 4 small white petals arranged symmetrically. Flowers are approximately 6 to 7 mm in diameter with 3 to 6 mm petals. Individual flowers contains six stamens, two shorter and four longer. Mature flowering plants reach 3.5 feet tall, although shorter flowering specimens may be found.

Impacts

Garlic mustard has the potential to form dense stands that choke out native plants in the understory by controlling light, water, and nutrient resources. Plants most affected by these dense stands are herbaceous species that occur in similar moist soil forest habitats and grow during the spring and early summer season. Although unsupported by the lack of long-term research into garlic mustard impacts, the plant has been circumstantially tied to decreased native herbaceous species richness in invaded forests. Researchers have found that garlic mustard is allelopathic (it releases chemicals that hinder the growth of other plant species) and has inhibited growth of both grasses and herbs in laboratory settings (Michigan State University, 2008). Some researchers also believe that these compounds may hinder the beneficial relationships some plant species have with soil fungi (Roberts and Anderson, 2001). Experimental trials have shown that removal of garlic mustard leads to increased diversity of other species, including annuals and tree seedlings (MSU, 2008).

Garlic mustard is one of the few invasive plants able to dominate the understory of forests in the Northeast and Midwest – Victoria Nuzzo, Natural Area Consultants, Bugwood.org

Other aspects of the forest ecosystem may be altered due to the change in the vegetative community tied to garlic mustard invasion. While the impacts to wildlife are not completely understood, altering the plant diversity can cause a change in leaf litter availability, potentially impacting salamanders and mollusks (MSU, 2008). Insects, including some butterflies, may be affected through the lost diversity in plants and loss of suitable egg-laying substrate (MSU, 2008). Garlic mustard may also affect the tree composition by creating a selective barrier that some seedlings, such as the chestnut oak (Quercus prinus), may not be able to overcome (MSU, 2008). These changes in tree composition could have significant long-term effects.

Prevention, Control and Management

There are few effective natural enemies of garlic mustard in North America. Herbivores, or animals that eat plant material, such as deer (Odocoileus virginianus) and woodchucks (Marmota monax) only remove up to 2% of the leaf area in a stand of garlic mustard (Evans et al. 2005). This level of herbivory is ineffective in controlling reproduction or survival of garlic mustard. Although 69 herbivorous insects have been found to be associated with garlic mustard in Europe, less than a dozen have been found on North American infestations of the species (Hinz and Gerber, 1998).

Manual removal of plant has been shown to prevent the spread of garlic mustard. Pulling by hand must remove at least the upper half of the root to prevent a new stalk from forming; this is most easily accomplished in the spring when the soil is soft. Hand-pulling should be performed before seeds are formed and needs to be continued for up to five years in order to deplete any established seed bank. This method works best in smaller pockets of invasion or in areas recently invaded to help prevent the development of a seed bank.

Chemical applications can also be effective for controlling garlic mustard, particularly in areas too large for removal by hand. In dense stands where other plant species are not present, a glyphosate-based herbicide such as Roundup® can be an effective method for removal. Glyphosate herbicides are non-selective, so caution must be used when non-target species are in the area. Chemical applications are most affective during the spring (March-April) when garlic mustard is one of the few plants actively growing. Fall applications may be used; however other plant species still in their growing season may be harmed. Readers are advised to check with local regulatory agencies to determine the regulations involved with chemical treatments.

The best method for controlling garlic mustard, or any other invasive plant, is to prevent its establishment. Disturbances in the forest understory that would allow for rapid invasion should be minimized. This would include limiting foot traffic, grazing, and erosion-causing activities. Monitoring the forest understory and removing any garlic mustard plants as soon as they are introduced will help to prevent the establishment and spread of this invader.

New York Distribution Map

This map shows confirmed observations (green points) submitted to the NYS Invasive Species Database. Absence of data does not necessarily mean absence of the species at that site, but that it has not been reported there. For more information, please visit iMapInvasives.

Utah invasive weed long black seed with hook

AUTHORSHIP AND CITATION:
Zouhar, Kris. 2009. Isatis tinctoria. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: https://www.fs.fed.us/database/feis/plants/forb/isatin/all.html [].

TAXONOMY:
The scientific name of Dyer’s woad is Isatis tinctoria L. (Brassicaceae) [27,31,32,33,39,45,55,74,81,82,85,86].

OTHER STATUS:
Information on state-level noxious weed status of plants in the United States is available at Plants Database.

DISTRIBUTION AND OCCURRENCE

GENERAL DISTRIBUTION:
Dyer’s woad is not native to North America but was introduced by some of the first immigrants from Europe to Plymouth Colony in the early 1600s. Despite its early introduction, it is not widely established outside of cultivation in New England (review by [51]). As of this writing (2009), it occurs outside of cultivation throughout much of western North America, from British Columbia south to California and New Mexico (excluding Arizona) and east to Montana, Wyoming, and Colorado. In eastern North America, it occurs in Newfoundland [39], Quebec, Ontario, New York, New Jersey, West Virginia, Virginia, and Illinois [39,78]. Plants Database provides a distribution map of Dyer’s woad in the United States and Canada.

Dyer’s woad is considered native to southeastern Russia and is thought to have spread throughout the eastern hemisphere in prehistoric times. It has been cultivated as a dye crop and valued as a medicinal herb in Europe since the 13th century (review by [80]). A review by Callihan and others [12] suggests that Dyer’s woad was grown for its blue dye in West Virginia and surrounding states, and introductions to western North America occurred as contaminants in alfalfa (Medicago sativa) seed imported to California and Utah in the early 1900s. Dyer’s woad was routinely offered for sale in seed trade catalogues in Pennsylvania and several other New England states prior to 1850 [50]. Dyer’s woad was still available for sale from plant nurseries in the United States in the 1990s [52].

At the time of this writing (2009), Dyer’s woad seems most common on crop- and rangelands in southeastern Idaho, northern and central Utah, and western Wyoming [36], but it also occurs in southeastern Oregon, northwestern California, and Montana and is sporadic across northern Nevada ([36], reviews by [6,19]). In northern Utah, it often invades mountain sides in such numbers that continuous masses of yellow color extend over many acres, and fields appear black in fall as seed pods turn dark [35]. It is less common in eastern North America, where it has rarely escaped from cultivation. It occurs in 2 counties in Illinois [55] and is occasionally found as a weed in the northeastern United States and adjacent Canada [27]. In 1938 it was reportedly very abundant along roadsides and in vacant lots in parts of Virginia, and thought to be “spreading rapidly” [25]; according to Plants Database [78], Dyer’s woad occurs primarily in the northern part of the state.

HABITAT TYPES AND PLANT COMMUNITIES:
Plant community associations of nonnative species are often difficult to describe accurately because detailed survey information is lacking, there are gaps in understanding of nonnative species’ ecological relationships, and nonnative species may still be expanding their North American range. Dyer’s woad likely occurs in plant communities other than those discussed here and listed in the Fire Regime Table.

Throughout the Intermountain West, Dyer’s woad is locally dense and often spreads into big sagebrush communities (Artemisia tridentata) in Idaho, Utah, Montana, Wyoming, California, and Nevada [36,63]. According to a review of nonnative invasive plants in sagebrush ecosystems [63], Dyer’s woad is “highly invasive and capable of dominating a site” once introduced. Invasive populations of Dyer’s woad are most commonly described in northern Utah and southern Idaho, where it usually occurs in plant communities dominated by big sagebrush and/or bluebunch wheatgrass (Pseudoroegneria spicata) [21,82]. On study sites where Dyer’s woad occurred at 2 foothill locations on the western slope of the Wellsville Mountains, Utah, potential natural vegetation is dominated by big sagebrush and bluebunch wheatgrass, although a long history of early spring through fall grazing had caused retrogression to early-seral or “poor” conditions for livestock use; the major plant species there were big sagebrush, broom snakeweed (Gutierrezia sarothrae), curlycup gumweed (Grindelia squarrosa), and the nonnatives cheatgrass (Bromus tectorum), bulbous bluegrass (Poa bulbosa), and Dyer’s woad [84]. Associated species on these sites may also include Sandberg bluegrass (Poa secunda) [59], and nonnatives such as medusahead (Taeniatherum caput-medusae), jointed goatgrass (Aegilops cylindrica), Mediterranean sage (Salvia aethiopis), and several knapweed species (Centaurea spp.) [83].

A study using remote sensing to predict potential distribution of Dyer’s woad in Utah found that it occurred in 55 of the 60 cover types or plant communities identified by spectral classification and that Dyer’s woad infestations were most frequently associated with 10 of these. Details regarding the plant communities were not given, but it is suggested that satellite remote sensing methodology may be a useful tool for estimating potential weed distribution over large, vegetatively diverse land areas [14].

Specimens of Dyer’s woad and associated species were collected at 40 xeric to mesic sites on rangeland, agricultural land (nonirrigated pastures and crops and irrigated alfalfa fields), and disturbed areas (roadsides, railroad embankments, gravel pits, and levees) in Idaho [12]. Where Dyer’s woad occurred in rangeland habitats, all communities were dominated by big sagebrush. Other species associated with Dyer’s woad on rangeland sites included Rocky Mountain juniper (Juniperus scopulorum), bigtooth maple (Acer grandidentatum), quaking aspen (Populus tremuloides), curlleaf mountain-mahogany (Cercocarpus ledifolius), antelope bitterbrush (Purshia tridentata), mountain snowberry (Symphoricarpos oreophilus), threetip sagebrush (Artemisia tripartita), rubber rabbitbrush (Chrysothamnus nauseosus), several native forbs, and several native and nonnative grasses including bluebunch wheatgrass and cheatgrass. Callihan and others [12] provide lists of associated species on rangeland, agricultural land, and disturbed areas. The latter 2 site types were occupied mostly by nonnative and/or invasive plants including Dyer’s woad.

In 2003, Dyer’s woad was reported on 1 site out of 542 surveyed on the Bridger-Teton National Forest in western Wyoming. The cover type was not given, but associates included common St Johnswort (Hypericum perforatum) and Scotch cottonthistle (Onopordum acanthium) [58].

As of 1991, a population of Dyer’s woad had persisted for “many years” in a bluebunch wheatgrass-Idaho fescue (Festuca idahoensis) mountain grassland community on the lower southwestern slope of Mt Sentinel in west-central Montana [45].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

Botanical description: Dyer’s woad is typically a biennial [21,27,28,31,32,33,36,40] or a short-lived, usually monocarpic, perennial [21,28,31,32,33,36,40]. A review by Callihan and others [12] and a laboratory study by Asghari [4] suggest that buds on Dyer’s woad root crowns sometimes survive after the plant has flowered, allowing the plants to persist and possibly produce additional flower and seed crops (see Vegetative regeneration). Dyer’s woad is sometimes described as a winter annual [36,54,69]. A field study on northern Utah rangeland found that most Dyer’s woad individuals were biennial or monocarpic perennials, but none displayed winter annual life histories. All Dyer’s woad plants that set seed died [20,21]. A second study in the same area found that 1% of Dyer’s woad individuals studied flowered during the first growing season [21].

Aboveground description: Dyer’s woad begins as a rosette with several long-petioled basal leaves [23,27,31,32,33,40] about 1.6 to 4 inches (4-10 cm) long on average [23,40] but reaching up to 7 inches (18 cm) long [31,32,33,82] and 0.3 to 1.6 inches (0.8-4 cm) wide [82]. Basal leaves are usually covered with simple hairs [31,82]. According to Varga and Evans [80], approximately 20 stalks begin to develop from each rosette, but usually 7 or fewer mature. Other sources indicate that Dyer’s woad usually has 1 main stem [27,31,32,33] that is simple below and branched above [31,32,33]. Stems are erect and may range from about 14 inches (35 cm) [82] to 47 inches (120 cm) tall [27,31,32,33], with several authors describing a typical range of 20 to 35 inches (50-90 cm) tall [23,40,74]. Plants are typically glabrous throughout [23,27,33,40,82] or hirsute with long, simple hairs at the base [82]. Stem leaves are narrower than basal leaves, mostly about 1 to 4 inches (2-10 cm) long [27], and are gradually reduced upwards [82].

Dyer’s woad flowers are about 6 mm wide [27] with petals about 3.5 mm long [31,32,33]. Flowers are borne in numerous, compound racemes forming a large, terminal panicle [27,31,32,33,36]. Dyer’s woad fruits are samaroid, indehiscent silicles ranging from 8 to 18 mm long and 2.5 to 7 mm wide with a single, median seed [27,28,31,32,33,36,40,74]. Silicles have strongly flattened valves [27,28,32,33] and are sometimes described as winged ([28], review by [54]). Fruits are dark to black at maturity [31,36,74] and droop from a short, slender pedicel [27] that is ascending to reflexed [31] or recurved [36]. According to Weber [81], Dyer’s woad is the only crucifer that produces hanging, indehiscent fruit resembling samaras of Fraxinus.

A 1983 field survey of Dyer’s woad in Idaho revealed some morphological variation: Some Dyer’s woad plants in Bear Lake County had very long basal leaves and were more pubescent than others described elsewhere. One specimen of Dyer’s woad along North Canyon in Caribou County was almost 5 feet (150 cm) tall. At Border Summit on dry and gravelly soils at 6,300 feet (1,920 m), Dyer’s woad was generally shorter (16 to 24 inches (40-60 cm) tall) and denser than those observed in other areas. A rust fungus was observed on some Dyer’s woad plants in Caribou and Bear Lake counties: Infected plants appeared severely stunted, though the disease was not of epidemic proportions on these sites [12]. The fungus was later identified as Puccinia thlaspeos, and has been recorded on other weedy members of the Brassicaceae in North America ([12] and references therein). See Biological control for more information on this rust fungus.

Belowground description: The root system of Dyer’s woad is dominated by a taproot [20,21] that is variously described as “robust” [28], “thick” [36], “fleshy” [80], or “woody” [12]. Dyer’s woad taproots can reach or exceed 5 feet (1.5 m) in depth ([36], review by [80]). Smaller lateral roots are concentrated in the upper 8 to 12 inches (20-30 cm) of the soil profile ([20], review by [37]) and spread laterally about 16 inches (40 cm) [20]. The root system of Dyer’s woad in a foothill rangeland pasture in northern Utah that had been continuously grazed by domestic sheep for over a decade had a mean taproot length of about 35 inches (90 cm) for rosettes and about 39 inches (100 cm) for mature plants. Mean total root length was about 85 inches (217 cm) for rosettes and 102 inches (258 cm) for mature plants, although the measurement method used (trench profile method) underestimates total root length because most of the fine roots are lost. Mature Dyer’s woad plants had 43% of total mapped root length in the upper 8 inches (20 cm) of the soil profile, while rosettes had 31% of total mapped root length at this depth, suggesting that lateral branching of Dyer’s woad roots occurs predominantly in the second year of growth. The authors note that this 2-layered rooting pattern is similar to that of sagebrush (Artemisia spp.), which may confer an advantage in semidesert steppe in the Intermountain West [20,21].

Dyer’s woad plants collected from disturbed sites in Utah were nonmycorrhizal; this was expected because members of the Brassicaceae family are predominantly nonmycorrhizal [61].

Dyer’s woad flowers, unripe fruits, and ripe fruits (top to bottom).
©Steve Dewey, Utah State University, Bugwood.org

Dyer’s woad is characterized by rapid vegetative growth during spring that typically enables it to produce seed by late spring or early summer on midelevation sites. The period of rapid growth by Dyer’s woad may overlap with the period of peak water extraction by bluebunch wheatgrass on some sites in some years, suggesting there may be belowground interference between these co-occurring species [21] (see Successional Status). Dyer’s woad plants were studied on northern Utah foothill sites at 4,850 to 5,000 feet (1,480-1,525 m) elevation during 2 studies: one from May 1982 to November 1983, and the other during the 1984 growing season. See Seedling establishment and plant growth for similar information from an experimentally established Dyer’s woad population in the same area. Young Dyer’s woad plants were marked and phenologically categorized between May 1982 and November 1983. Phenological stages were as follows: dormant, leaf growth, stem growth, floral buds developing, flowering, seed development, seed ripening, seed dissemination, and dead. Leaf growth occurred in both fall and spring, and flowering occurred in late spring. Time between stem growth and seed development was about 8 weeks. Mean stem growth was about 4 inches (10 cm) per week from mid-April until the end of May. Plants were dormant in both summer and winter, corresponding with hot, dry conditions or cold temperatures, respectively. Sixty-five percent of marked plants died and 1% flowered during the 1st growing season. Of the 35% that survived to the 2nd year, about half flowered and produced fruit. All plants that set seeds died; about 12% remained vegetative and may have produced fruit in the 3rd year [21].

Dyer’s woad plants observed on Utah foothill sites during the 1984 growing season started vegetative growth by 16 April 1984, less than 1 week after snowmelt. Basal diameter increased between 16 April and 7 May and thereafter remained fairly constant. Likewise, rosette diameter increased during the same period, leveled off by 23 May, and then declined as basal leaves withered and flowering stems developed. Stem growth began during the last week of April, and flowering began the second week of May, reaching its peak about 23 May. Height of flowering stalks increased rapidly between 7 May and 11 June. Seed developed between 9 June and 15 June. By the end of June, most of the seeds had ripened [20].

Root crown buds on Dyer’s woad plants that have flowered sometimes survive, allowing plants to persist and flower again. The growth of the flowering shoot reduces carbohydrates stored in the taproot during the previous season (review by [12]).

Typical flowering dates by geographic area are given in the following table:

Dyer’s woad flowering dates by geographic area
Area Flowering dates
California April to June [57]
Illinois May to June [55]
Nevada April to July [40]
Utah midspring [59]
Utah (Uinta Basin) May to July [28]
Virginia May to June [85]
Intermountain West May to June [36]
Northeast and adjacent Canada May to July [27]
Pacific Northwest April to August [32]

Dyer’s woad fruits ripen between June and October throughout its range [23]. Dyer’s woad seeds become viable relatively early during seed production [36].

A survey in Idaho in 1983 found that timing of flowering and seed dispersal were related to elevation. Flowering and dispersal dates observed in that survey were as follows [12]:

Phenology of Dyer’s woad in several counties at different elevations in Idaho [12]
County Elevation (m) Phenological stage Dates
Northern Bannock 1,829 Rosette and bolting 3 June
Northern Bannock up to 1,402 Flowering 23 May
Jefferson and Bonneville 1,341-1,463 Flowering 26 May
Caribou 2,073 Flowering 23 June
Caribou 2,079 Flowering 5 July
Caribou 2,316 Flowering 12 July
Bear Lake 2,256 Flowering as late as 14 July
Caribou below 1,981 Full bloom 17 June
Bear Lake 2,256 Full bloom 29 June
Central and southern Bannock and Franklin Full bloom 7 June
Franklin county 1,585 Full to late bloom 10 June
Eastern Oneida 1,067 Late bloom to seed set 15 June
Clark 1,707-2,012 Late bloom to ripe fruit 22 July
Adams 899 Dispersing ripe fruit 26-28 July
Blaine 1,295 Dispersing ripe fruit 26-28 July
Southwestern Oneida 1,492-1,463 Dispersing ripe fruit 20 July

Pollination and breeding system: Results from laboratory studies in Italy showed an outcrossing breeding system in Dyer’s woad. The effects of selfing and crossing on seed production, germinability, and progeny growth were assessed. Self-pollinated plants produced fewer siliques (7.1 g/plant) with lower weight (6.0 mg) and lower seed germinability (8.2%) than outcrossed plants (44.1 g, 8.0 mg, and 46% for each character, respectively). Self-pollinated progenies generally showed lower height growth than outcrossed progenies [72].

Flower and seed production: Dyer’s woad requires a cold vernalization period to induce flowering. A greenhouse study in Utah found that both 1-year old Dyer’s woad plants that had previously flowered (crown rosettes) and 4-month old seedling rosettes required exposure to cold temperatures (39 °F (4 °C) or less) for a minimum of 23 to 47 days to induce flowering [3,4]. The 2 types of rosette responded differently to cold treatments, which ranged from 0 to 93 days at 39 °F (4 °C), suggesting that cold tolerance is dependent not only on length of cold exposure but also on plant age. No seedling rosettes died during any length of cold exposure, while 50% of crown rosettes died after 93 days of cold exposure, and 30% died after 47 days of cold exposure. There was no difference in survival of crown rosettes after 23 days of cold exposure and that of controls [4]. Continual disturbance, such as defoliation, delays flowering of Dyer’s woad [20] (see Physical or mechanical control).

Reviews describe “prolific” or “abundant” seed production in Dyer’s woad [12,19,54]. A review by McConnell and others [54] suggests that some plants produced more than 10,000 seeds in 1 year, although the source of this information is not given. Dyer’s woad plants studied on Utah rangelands produced about 350 to 500 seeds each [20,21].

Seed production may vary among plants established in different seasons and on different microsites. A field study in Utah found that Dyer’s woad plants that established in fall had slightly larger rosettes, taller flowering stalks, and produced more fruit (563 fruits/plant) than those that established in spring (345 fruits/plant). Mean fruit production of plants established in spring was similar among plants growing near sagebrush (293 fruits/plant) and those growing in interspace microsites (317 fruits/plant). Fruit weights were similar among all groups (3.9 mg/fruit) [20,21]. In a related study in the same area, average fruit production was 383 fruits/plant [21].

Seed dispersal: Dyer’s woad fruits do not release the seed at maturity, but fall to the ground intact [19]. The majority of Dyer’s woad fruits disperse within a few meters of parent plants. Long-distance dispersal may occur with the aid of humans, livestock, wildlife, and water [20].

Most Dyer’s woad fruits shed soon after reaching maturity, although some remain on the plants until winter. Fruits are firmly attached to plants, and some abrasive force such as wind or rain is needed to detach them. A field study in Utah recorded daily Dyer’s woad fruit dispersal from 25 June 1985 until 27 August 1985. Most of the fruits were shed in the first 10 days of the study; thereafter, the dispersal rate declined substantially, leveling off after 4.5 weeks. Ninety-five percent of all trapped fruits fell within 21 inches (54 cm) of parent plants, and mean dispersal distance was positively correlated with the height at which seeds were released (r²=0.85). The greatest distance that fruits traveled via wind was about 8 feet (2.4 m). The relationship between windspeed and number of fruits dispersed was “poor”; however, most fruits scattered in the direction of prevailing winds. Dyer’s woad fruits remaining on plants until winter may disperse much greater distances when blown over the surface of crusted snow [20,21]. Fruits may be further transported by ants, as was observed during studies on Utah rangelands [20].

Long-distance spread of Dyer’s woad fruits and seeds must be aided by vectors such as humans, livestock, wildlife, and water. Humans may disperse fruits in their clothing, vehicles, tools or machinery [20,21,80]. Roadsides and railways are effective avenues of seed dispersal [19]. Long-distance dispersal is likely when Dyer’s woad seed is a contaminant in alfalfa or other crop seed (review by [12]); or when mature, seed-bearing Dyer’s woad plants are cut and baled with alfalfa in infested fields, and this baled hay is shipped to where it is used as livestock feed [19,20,21]. Contaminated hay is one of the major causes of Dyer’s woad spread [36].

Livestock and wildlife may carry fruits in mud on their hooves or in their fur [36]. The curved pedicel of Dyer’s woad fruits may act as a hook to aid in dispersal by animals. Dyer’s woad fruits remaining on plants past the first snowfall may be dispersed by herds of deer and elk in the winter months, when herd use of foothill sites is highest [20]. Farah [20] speculates that a high incidence of Dyer’s woad infestations on south-facing slopes on Utah rangeland may be related to deer and elk use of these sites in winter. Birds and rodents may also contribute to long-range dispersal of Dyer’s woad [20].

Downhill and downstream dispersal of Dyer’s woad fruits may be aided by water; flattened wings facilitate this mode of dispersal. Dyer’s woad populations along the banks of drainage systems in Utah may have established after this type of dispersal [20,21].

Seed banking: Information on seed banking in Dyer’s woad was lacking, and it had not been determined how long seeds are viable in the soil, as of 2009. Anecdotal accounts from Europe suggest that Dyer’s woad sometimes appears after grasslands are tilled; authors contend that these are sites of former woad crops where the seeds have remained dormant in the soil, presumably for many years (King 1966 as cited by [87]).

While Dyer’s woad seeds may have no dormancy, they are contained in fruits that have water soluble germination inhibitors such that few seeds germinate immediately in the field, presumably until the inhibitors are leached from the fruit [87]. The inhibitors in the fruit may allow Dyer’s woad seed to persist in the soil seed bank [19]. Because the inhibitors are removed by leaching, they do not seem likely to contribute to long-term persistence of seed in the soil, because they would be leached by precipitation, allowing germination under favorable conditions [87].

Evidence from field studies indicates that some Dyer’s woad seeds remain viable in the soil for at least 10 to 12 months. Dyer’s woad fruits (1,200 total) were harvested from a Utah study site on 8 July 1982 and buried under about 0.4 inch (1 cm) of soil. Each month, 120 fruits were removed from the field, and seeds were removed from fruits and tested for germination and viability. Germination tests were conducted at 77 °F (25 °C) with 12 hours each of alternating light and darkness, and germinated and viable seeds were counted after 14 days. Germination rates of Dyer’s woad seed stored in the field ranged from 99% in September 1982 (after 1 month of burial) to 44% in May 1983 (after 9 months of burial). Seed viability remained high, fluctuating between 73% and 100%, and did not decrease over time. Whether Dyer’s woad seed can germinate after being stored in the soil longer than 10 months is not known. In a related study on the same site,

Germination: Dyer’s woad seeds separated from the fruits do not exhibit dormancy and readily germinate under a variety of conditions, though they do not readily germinate when they remain intact within the fruit. Dyer’s woad seeds do not usually dehisce from the fruits under field conditions; thus, the intact fruit imposes dormancy [87]. The majority of Dyer’s woad seeds collected in August 1969 and separated from the fruits germinated after incubation for 168 hours at temperatures from 37 to 77 °F (3-25 °C). Conversely, when intact fruits were incubated, germination was low and erratic. Seedlings elongated much more slowly from fruits than from seeds [87].

Reduced germination and seedling elongation from intact fruits were likely due to chemistry rather than due to a physical obstruction. In a laboratory study, not only were germination and seedling emergence reduced from intact Dyer’s woad fruits, but the presence of intact fruits or fruit leachate also reduced germination and seedling emergence in both threshed Dyer’s woad seed and in seeds of several other species (see Successional Status for details). Washing Dyer’s woad fruits in tap water for 48 hours increased germination, and washing fruits for 96 hours almost eliminated germination inhibition. In the field, some Dyer’s woad seedlings established from fruits that overwintered [87].

Germination inhibitors present in freshly sown seed are likely leached over winter, thereby allowing greater germination of overwintered seeds. In a field study in Utah, germination of Dyer’s woad seeds sown in October 1984 was 10 times higher in spring 1985 than fall 1984. The author speculates that seeds that germinated shortly after being sown may have been in damaged fruits [20].

Dyer’s woad seed germination is likely inhibited by shade. High percentages (>85%) of Dyer’s woad seed germinated under red, yellow, and white light within 4 days. Significantly lower percentages germinated under far red and blue light (15% and 37%, respectively) (P<0.05), and germination time was longer. Far red and blue light simulate light conditions under a dense canopy [75].

Seedling establishment and plant growth: Seedling establishment, survivorship, growth, and eventual reproductive output (see Seed production) may vary among Dyer’s woad seedlings established in the fall versus those established in spring, and among microsites. Dyer’s woad population demographics were studied over a 2-year period on a Utah rangeland where 100,000 Dyer’s woad fruits were collected during the summer of 1984 and sown on 8 September 1984 in a “well-vegetated” area lacking Dyer’s woad. During the study period precipitation was 18% above the estimated long-term average, and mean monthly temperatures were slightly below the long-term average [20,21]. The following information comes primarily from this single study and is therefore limited in scope; Dyer’s woad may display different population dynamics on other sites. See Seasonal development for more precise phenological information from Dyer’s woad populations in the same area.

Seedling establishment: For freshly shed seeds, establishment rates were lower 1 month after sowing in the fall (0.3%) than during the following spring (2.7%) [20,21], which is consistent with findings of Young and Evans [87] that Dyer’s woad fruits contain water-soluble germination-inhibiting substances that would have leached over winter. Germination in fall 1985 was twice that in fall 1984; these differences were not associated with differences in either precipitation or mean monthly temperatures. Germination from the original seed input ceased after fall 1985 [20,21] (see Seed banking).

Microsites near sagebrush plants seem to provide a more favorable microenvironment for Dyer’s woad seedling establishment than interspace microsites. Seedling densities were 170 and 26 Dyer’s woad plants/m² on sagebrush and interspace microsites, respectively [20,21].

Survival: Survivorship patterns were similar in fall- and spring-established Dyer’s woad populations, with peak mortality in summer. Cohorts of Dyer’s woad that established in October 1984 (n=285) experienced little mortality during the following winter, slight mortality in early spring 1985, and peak mortality during the summer. Thirty-six of these plants survived the summer drought, overwintered again, flowered, and set seed in spring of 1986. None of the Dyer’s woad seedlings that established during the spring of 1985 (n=2,664) flowered in the same year. Of the spring-established cohort, 371 individuals survived the summer drought and overwintered. Eighty-seven percent of these plants flowered and produced seeds in spring of 1986, and the other 13% remained vegetative. Peak mortality in both Dyer’s woad populations occurred during a period with high temperatures and negligible precipitation, suggesting that the main source of mortality was water stress; there was no evidence of predation or pathogens. The authors note that the developing roots of young rosettes of Dyer’s woad are unlikely to access soil moisture from deep soil layers, where moisture occurs during hot and dry conditions above ground; but they caution that a causal relationship between seedling mortality and soil moisture deficit was not established because soil water content was not measured [20,21].

A life table analysis for Dyer’s woad showed constriction of population growth at 2 transitions: seed to seedling (establishment) and young rosettes to mature rosettes. The establishment rate was 3%; and only 23% of young rosettes survived to mature rosettes. Once plants became mature rosettes, the probability of surviving to reproduce was 81%. All flowering individuals set seed, with an average fruit production of 496 fruits/plant [20,21].

Neither microsite characteristics nor seedling density appeared to impact mortality rates in Dyer’s woad populations. Mortality of Dyer’s woad plants growing near sagebrush and those in the interspaces were similar (73% and 74% respectively), despite a 7-fold difference in seedling density [20,21].

Growth and reproductive output: Fall germination of Dyer’s woad favors both vegetative growth and reproductive output (see Seed production); however, spring germination was more important than fall germination in terms of overall population growth: Higher germination rates in spring resulted in more individual plants and higher total fruit production from spring-germinated cohorts than fall-germinated cohorts. Fall-germinated individuals had nominally greater rosette sizes than spring-germinated individuals during most of the study period, and differences were most pronounced at the start of the spring 1986 growing season. Stem growth was initiated in both cohorts during the last week of March 1986, and rapid stem growth occurred up to 18 May 1986. By 20 April, the fall cohort was taller. The fall cohort had significantly greater fruit production/plant (P<0.1), but fruit weights were similar and the spring population had more plants [20,21].

Neither microsite characteristics nor plant density in Dyer’s woad cohorts appeared to translate into better vegetative and reproductive performance: rosette size, height of flowering stalks, and seed production were similar between these 2 groups [20,21].

Vegetative regeneration: Several sources indicate that Dyer’s woad plants may sprout when the top growth is removed at ground level [19,20,21,36,67]. Sprouting seems to originate from buds on Dyer’s woad root crowns ([4], review by [12], personal communication [15]). Numerous vague references to vegetative or asexual regeneration in Dyer’s woad were found in the literature: “Clonal growth has been observed but is not common” [37]; “Asexual reproduction may occur from this underground root system” [80]; “. the weed can spread from underground portions of the root system. ” [6]; “It has a large fleshy taproot from which it may reproduce asexually” [19]; and “Damaged plants often resprout from buds located on the root crown and, less frequently, from the roots” [67]. However, vegetative regeneration in Dyer’s woad seems to be restricted to sprouting from the root crown following aboveground damage.

Dyer’s woad is likely to survive and sprout following aboveground damage and defoliation [19,20,21,36,67], depending on timing, frequency, and severity of damage. A review by Evans [19] states that while undisturbed Dyer’s woad plants typically behave as biennials or winter annuals, perennial behavior can be elicited by mowing, hand-pulling, or breaking the bolting stalk above ground. This is supported by evidence from a field study where plants were clipped at varying intensities, frequencies, and dates: Significant mortality and reduction in reproductive performance occurred when at least 60% of the aboveground phytomass had been removed on or after 23 May (P<0.05) [20] (see Physical or mechanical control for details and methodology). Fuller (1985 as cited by [20]) demonstrated that to substantially reduce flowering capacity and cause adequate mortality before 23 May, Dyer's woad had to be clipped 2 inches (5 cm) below ground. "This suggests that regeneration of Dyer's woad, following clipping damage, results from activation of crown buds and those located on the roots just beneath ground level". Young rosettes are less likely than older plants to survive defoliation due to the lack of development of the root system in young rosettes [20].

A review by Callihan and others [12], a laboratory study by Asghari [4], and observations by Dewey (personal communication [15]) suggest that buds on Dyer’s woad root crowns sometimes survive after the plant has flowered, allowing the plants to persist and possibly produce additional seed crops. Callihan and others [12] note, “Frequently, crown buds on plants that have flowered will survive, allowing plants to persist for three or more seasons.” Asghari [4] used 1-year-old Dyer’s woad rosettes that had previously bolted and flowered in a vernalization study: several of these rosettes bolted and produced seed in the greenhouse. Dewey (personal communication [15]) notes repeated observations of established (flowered) Dyer’s woad plants damaged by tillage, mowing, or fire that have re-emerged and flowered again later in the same summer or in the following season. He suggests that this resprouting is from buds atop the plant’s main taproot, not from creeping roots or rhizomes: He has never seen 2 Dyer’s woad plants connected to each other under ground.

SITE CHARACTERISTICS:
In the western United States, Dyer’s woad most commonly establishes and persists on rangelands and disturbed sites such as roadsides, rights-of-way, fence rows, uncultivated croplands (e.g., alfalfa and small grain fields, orchards), pastures, old fields, and “waste places” ([17,28,31,32,36,82], reviews by [19,54]). Characteristics of sites supporting Dyer’s woad in eastern North America were not described in available literature (2009). A Virginia flora describes Dyer’s woad as infrequent and occurring on disturbed sites [85].

Climate: Dyer’s woad is native to parts of Russia, where the climate may be similar to that of the Intermountain West (review by [2]). Few studies of Dyer’s woad report climate data. On study sites where Dyer’s woad occurred on coarse, well-drained soils at 2 foothill locations on the western slope of the Wellsville Mountains in northern Utah, mean annual precipitation is 16 inches (400 mm), and mean annual air temperature is °F (9 °C) [21,84]. A review by Parker [59] suggests that Dyer’s woad has a moisture requirement of 14 to 18 inches (356-457 mm) per year. Specimens of Dyer’s woad were collected at 40 xeric to mesic sites in Idaho [12].

Elevation : Elevations ranges for Dyer’s woad were given for the following areas:

Landforms and soils: Western rangelands invaded by Dyer’s woad typically occur on uplands, foothills, hillsides, and mountain valleys (review by [59]). A survey of Dyer’s woad in southeastern Idaho found that it occurred primarily on the east side of valleys, extending up canyons, and generally on south-facing, steep to flat slopes in full sun [12]. Infestations are frequently observed on steep hillsides in rugged, inaccessible mountain terrain (review by [19]). Dry foothill sites typically support native bunchgrass, sagebrush, and mountain brush communities [36,45,82] (see Habitat Types and Plant Communities). Dyer’s woad is thought to be well suited to the dry, coarse, rocky soils on these sites (reviews by [2,19,59,80]) and is “a weed of dry places” in much of the Pacific Northwest [33]. Dyer’s woad occurs on mesic (adequate moisture throughout most of season) and mesic-xeric (abundant moisture early in season, becoming drier later on) valleys in Montana [7,44]. In England, Dyer’s woad often occurs in old lime pits and chalk quarries (review by [80]) and is said to prefer alkaline soils on western rangelands (reviews by [59,80]).

Although many sources suggest that Dyer’s woad is well suited to coarse, rocky soils with low water-holding capacity (reviews by [2,19,59,80]), Dyer’s woad grew larger and had greater nitrate aquisition on a relatively moist site with fine soil textures than on a drier, coarse textured soil in a Utah field study (see table below) [48]. Differences in these variables were not related to proximity, life form, or diversity of neighboring plants (see Successional Status).

Establishment in early succession: In a small-plot (1.5 × 1.5 m) experiment Dyer’s woad seedling establishment was consistently higher in disturbed than undisturbed plots regardless of growth form composition of plots. Plots were composed of 24 plants of either crested wheatgrass (Agropyron cristatum × A. desertorum), western yarrow (Achillea millefolium) or Wyoming big sagebrush (Artemisia tridentata var. wyomingensis), and were either left intact or disturbed by removing 4 plants from the center and lightly scarifying with a rake. Four hundred Dyer’s woad seeds were sown in each plot. Dyer’s woad seedling density was 52% to 66% higher in disturbed plots than intact plots (P<0.01) [48]. According to Monaco and others [56], the ability of Dyer's woad to establish on disturbed sites in early succession may be determined by its "colonizing ability", not its competitive ability for soil nitrogen (see below).

Persistence: Dyer’s woad can establish and persist on many types of anthropogenically disturbed sites (see Site Characteristics), and commonly occurs on semi-arid rangelands with a long history of livestock grazing (e.g., [84]). A review by DiTomaso [16] lists Dyer’s woad among nonnative plants that tend to be avoided by livestock, which can favor a rapid shift in dominant species in grazed rangeland plant communities where these unpalatable plants occur. Another review by Parker [59] classified Dyer’s woad as an “invader” in terms of its response to grazing. Field studies in northern Utah [20,84] indicate that Dyer’s woad is readily grazed by domestic sheep prior to flowering; however, little damage is done to the plants (see Biological control).

Competition experiments on old fields in Utah suggest traits in Dyer’s woad that facilitate its persistence in disturbed, semiarid shrub-steppe ecosystems. In a greenhouse experiment, Dyer’s woad exhibited low plasticity in response to nitrogen availability, suggesting a low nitrogen requirement, low nitrogen productivity, or both. The authors note that these qualities are associated with the ability of a species to survive and persist under stressed, nutrient-poor conditions [56]. In a similar experiment, nitrate acquisition of Dyer’s woad was less than that of crested wheatgrass, greater than that of big sagebrush (P<0.01), and similar to that of western yarrow (P<0.01). Dyer's woad was less competitive for nitrate than cheatgrass, and similar to forage kochia (Kochia prostrata). These results suggest that superior competition for soil nitrogen is not the primary mechanism responsible for the dominance and proliferation of Dyer’s woad [48].

Young and Evans [87] suggest that perennial grasses seem to coexist moderately well with Dyer’s woad, perhaps due to differences in root systems, although the height, leaf size, and leaf arrangement of Dyer’s woad may give it an advantage in shading range grasses. Other researchers note that the periods of rapid growth by Dyer’s woad and peak water extraction by bluebunch wheatgrass may overlap on some sites in some years, suggesting there may be belowground interference between these co-occurring species [21].

Establishment and persistence in late succession: Evidence of Dyer’s woad’s ability to invade established vegetation comes from field studies in Utah [20,21] and California [87]. In a “well-vegetated” area on a Utah rangeland that had not been grazed by livestock for several decades, Dyer’s woad established from seed sown by researchers [20,21]. In a study in northern California [87], Dyer’s woad established in annual grass communities considered “ecologically closed” [66]. These annual grasslands, dominated by medusahead or cheatgrass, were thought to represent a culmination of plant succession, and invasion and dominance by Dyer’s woad prompted an investigation into the mechanism allowing its establishment (see Allelopathy).

Results from small-plot experiments in Utah suggest that sites supporting a diversity of species or life forms may be more resistant to Dyer’s woad establishment than those dominated by single species or life form. Species used were a combination of native sagebrush-steppe species and nonnative species widely used for revegetation within sagebrush-steppe communities. Dyer’s woad seedling establishment was consistently higher in single-species (western yarrow) forb plots than in 4-species forb plots, mixed life form plots (consisting of grasses, forbs and shrubs), or single-species shrub (Wyoming big sagebrush) plots. Dyer’s woad establishment was consistently higher in 4-species shrub plots than 4-species forb plots. Dyer’s woad establishment in single-species grass plots (crested wheatgrass) and 4-species grass plots was inconsistent between years [48].

Shade tolerance: While Dyer’s woad tends to occur on open, sunny sites (see Habitat Types and Plant Communities and Site Characteristics), it exhibits some degree of shade tolerance. Callihan and others [12] note Dyer’s woad occurrence in many types of plant communities in Idaho, including those dominated by trees and large shrubs. In the greenhouse, Dyer’s woad responded to increased shade through morphological modifications (increased leaf area, specific leaf area, and shoot:root ratio) to improve its light-harvesting ability. These responses may favor the ability to establish and persist on harsh, nutrient-poor sites as well as shaded, undisturbed sites. Dyer’s woad also demonstrated morphological plasticity in response to variable water conditions, especially under shaded conditions. The authors suggest that high plasticity in heterogeneous environments may allow Dyer’s woad to establish and spread into new sites without the lag time required for local adaptation [56]. However, germination of Dyer’s woad seeds may be inhibited by shade [75].

Allelopathy: Laboratory studies suggest that Dyer’s woad fruits probably contain allelopathic substances [87], although the allelopathic chemicals have not been identified. In the laboratory, the presence of Dyer’s woad fruits inhibited germination of Dyer’s woad, tumble mustard (Sisymbrium altissimum), and alfalfa seeds; reduced root length in seedlings of Dyer’s woad, tumble mustard, medusahead, cheatgrass, and alfalfa; and reduced shoot length in seedlings of Dyer’s woad and tumble mustard. Germination and root length were also reduced for several species incubated on substrates treated with Dyer’s woad fruit leachate, as shown in the table below. Medusahead responded similarly, although data were not provided [87].

Because Dyer’s woad produces a large number of fruits, and these fruits seem to suppress germination of associated species, successional trajectories may be altered in communities dominated by Dyer’s woad, with Dyer’s woad maintaining dominance by reducing establishment of other species. As a biennial or short-lived perennial, Dyer’s woad does not have to establish seedlings every year to maintain dominance in annual communities. The researchers noted, however, that some annual grasses established in Dyer’s woad stands in the field [87].

FIRE EFFECTS AND MANAGEMENT

Dyer’s woad infestation (yellow) in the foothills east of Brigham City, Utah, 9 months after late summer wildfire in 1995.
©Steve Dewey, Utah State University, Bugwood.org

FIRE EFFECTS:
Immediate fire effect on plant: As of this writing (2009) no information was available in the literature regarding the immediate effects of fire on Dyer’s woad plants or seeds. Dyer’s woad is likely top-killed by fire; buds on the root crown are likely to survive fire and sprout new leaves and flowering stalks (see Vegetative regeneration and Fire adaptations and plant response to fire).

Fire adaptations and plant response to fire:
Fire adaptations: As of this writing (2009) no information was available in the literature regarding Dyer’s woad fire adaptations. Dyer’s woad’s ability to sprout following other types of aboveground damage (see Vegetative regeneration and Physical or mechanical control) suggests that it is likely to persist and sprout following fire as well.

Plant response to fire: As of this writing (2009) no information was available in the literature regarding Dyer’s woad response to fire. The photos taken by Steven Dewey (above) were taken in late May or early June of 1996 in the foothills east of Brigham City, Utah. Prior to the late summer 1995 wildfire at that site, the plant community was a mixture of sagebrush, rabbitbrush (Chrysothamnus spp.), native forbs, cheatgrass and Dyer’s woad. Relative canopy cover of Dyer’s woad before the fire was estimated at less than 10%. Dyer’s woad increased dramatically following the wildfire. These and other observations in northern Utah indicate that Dyer’s woad often persists and spreads following wildfire in that area (Dewey 2009, personal communication [15]). Similarly, Asher and others [5] mention examples of “severe post-fire weed spread and impacts” that include “Dyer’s woad at Perry, Utah”, although no additional information is given.

Timing and frequency of fire may affect Dyer’s woad postfire response; Dyer’s woad may be more susceptible to mortality by fire in late spring than early spring. Clipping studies in northern Utah revealed increased mortality in Dyer’s woad clipped on either 23 May or 11 June, and no increase in mortality if clipped before 23 May unless clipped on 3 sequential dates (see Physical or mechanical control).

No information was available regarding heat or fire effects on Dyer’s woad seed. However, buried seed is likely to survive fire and Dyer’s woad may establish from the seed bank after fire.

FUELS AND FIRE REGIMES
Fuels: No information is available on this topic.

Fire regimes: As of this writing (2009), no information was available regarding native fire regimes in which Dyer’s woad evolved. Based on information regarding its vegetative regeneration and response to clipping (see Physical or mechanical control), Dyer’s woad is likely adapted to survive and persist under a regime of frequent and/or severe fire. It is also likely to persist in the absence of fire or with long fire-free intervals (see Establishment and persistence in late succession). In North America, Dyer’s woad most commonly occurs in big sagebrush and mountain grassland communities, where fire regimes are characterized by mixed-severity or stand-replacement types with varying fire frequencies (reviews by [60,65]).

Presettlement fire regimes in big sagebrush shrublands are not well understood, and a high degree of variability is assumed among communities with different sagebrush dominants and plant associates. These fire regimes are typically characterized as mixed-severity or stand-replacement types, with fire-return interval estimates ranging from 10 to 70 years (review by [65]). Fire regimes in many big sagebrush communities have been altered by annual grass invasion, especially cheatgrass, in areas where Dyer’s woad is most invasive. See the FEIS review on cheatgrass and the review by Rice and others [65] for more information on fire regime change in Interior West shrublands. It is unlikely that Dyer’s woad invasion would further alter fire regimes unless it reduced cheatgrass dominance. A study from northern California in 1971 [87] suggests that Dyer’s woad may replace cheatgrass on some sites (see Establishment in late succession). See FEIS reviews on basin big sagebrush (A. tridentata subsp. tridentata), mountain big sagebrush (A. tridentata subsp. vaseyana), and Wyoming big sagebrush (A. tridentata subsp. wyomingensis) for more information on fire regimes in communities dominated by these species.

Presettlement fire regimes in mountain grasslands, where Dyer’s woad commonly occurs and may be invasive, are classified as stand-replacement fires at intervals of about 10 to 35 years [60]. Rice and others [65] suggest that nonnative perennial forbs (e.g., spotted knapweed (Centaurea maculosa)) that displace native grasses in mountain grasslands may reduce fire frequency and spread due to their coarser stems and higher moisture content compared to dominant native bunchgrasses. Large infestations of Dyer’s woad may have a similar effect, although this was not documented in available literature as of 2009. See FEIS reviews on bluebunch wheatgrass and Idaho fescue (Festuca idahoensis) and the review by Rice and others [65] for more information on fire regimes in Interior West mountain grassland communities.

See the Fire Regime Table for further information on fire regimes of vegetation communities in which Dyer’s woad may occur. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under “Find Fire Regimes”.

FIRE MANAGEMENT CONSIDERATIONS:
Preventing postfire establishment and spread: Preventing invasive plants such as Dyer’s woad from establishing in weed-free burned areas is more effective and less costly than managing established populations. This may be accomplished through early detection and eradication, careful monitoring and followup, and limiting dispersal of invasive plant seed into burned areas. General recommendations include:

  • Incorporate cost of weed prevention and management into fire rehabilitation plans
  • Acquire restoration funding
  • Include weed prevention education in fire training
  • Minimize soil disturbance and vegetation removal during fire suppression and rehabilitation activities
  • Minimize the use of retardants containing nitrogen and phosphorus
  • Avoid areas dominated by high priority invasive plants when locating firelines, monitoring camps, staging areas, and helibases
  • Clean equipment and vehicles prior to entering burned areas
  • Regulate or prevent human and livestock entry into burned areas until desirable site vegetation has recovered sufficiently to resist invasion by undesirable vegetation
  • Monitor burned areas and areas of significant disturbance or traffic from management activity
  • Detect weeds early and eradicate before vegetative spread and/or seed dispersal
  • Dradicate small patches and contain or control large infestations within or adjacent to the burned area
  • Reestablish vegetation on bare ground as soon as possible
  • Avoid use of fertilizers in postfire rehabilitation and restoration
  • Use only certified weed-free seed mixes when revegetation is necessary

For more detailed information on these topics see the following publications: [5,8,29,77].

Use of prescribed fire as a control agent: Fire is not likely to be useful in controlling populations of Dyer’s woad because Dyer’s woad would likely only be top-killed by fire, and may then sprout and produce seed in the same or following year after fire.

MANAGEMENT CONSIDERATIONS

OTHER USES:
As the common name implies, Dyer’s woad was cultivated for textile dye in Europe since ancient times, and ancient Britons and Celts colored their faces and bodies with the blue dye extracted from Dyer’s woad in order to frighten their enemies in war. Its importance as a dye crop began to decline when a cheaper blue dye derived from true indigo (Indigofera tinctoria) was imported from the Far East during the 16th century. Dyer’s woad’s cultivation was practically abandoned in the 19th century, when synthetic dyes were developed (reviews by [26,72]); however, its cultivation as a dye crop is increasing as demand for natural products grows. Dyer’s woad is also attractive for this purpose because it grows well under marginal conditions (review by [72]). A search of the scientific literature revealed several studies regarding natural indigo dye production from Dyer’s woad. This literature is not included here because it is outside the scope of this review.

A review by Galletti and others [26] indicates that Dyer’s woad is a source of indolic compounds that can be degraded into bioactive molecules effective against phytopathogenic fungi, nematodes, weeds, and human tumoral cell lines. Among these compounds, glucobrassicin and its derivatives seem to play an antitumoral role, and research suggests a possible protective effect against human breast cancer associated with the consumption of glucobrassicin-containing vegetables like broccoli and cauliflower. Extraction of glucobrassicin in pure form is complicated because in most vegetable sources it is usually present at low concentrations and always mixed with other compounds ([26] and references therein). Screening among different accessions of Dyer’s woad in Italy found a cultivar with relatively abundant leaf content of glucobrassicin and a lack of other indolic compounds, leading researchers to explore methods for increasing glucobrassicin production in this cultivar in order to set up a model of low-cost, large-scale production of this compound. See Galletti and others [26] for details of this study.

A search of the literature revealed several other studies of medicinal properties of Dyer’s woad (specifically anti-inflammatory, antiallergic, and anticancer properties), which are not covered in this review.

IMPACTS AND CONTROL:
Impacts: As of this writing (2009), research regarding impacts of Dyer’s woad invasion on native communities was limited. Several reviews suggest that Dyer’s woad spreads rapidly and may reduce growth and abundance of desirable plants in both croplands and rangelands, thus imposing an economic impact (e.g., [2,12,54,80]).

Dyer’s woad seems to be most invasive in the Intermountain West, where it can spread rapidly and form dense infestations that may reduce forage grass production (review by [12]). Field studies and a review of the literature by Farah [20] suggest that the invasiveness and rapid spread of Dyer’s woad in northern Utah may be due to its efficient utilization of environmental resources. Specifically, because Dyer’s woad germinates both in the fall and spring, overwinters as a rosette, initiates early spring growth, has deep taproots, and possesses summer dormancy mechanisms, it may escape many restrictions by which growth and spread of associated native species are regulated [20]. Laboratory studies from California that demonstrate an allelopathic potential in Dyer’s woad [87] (see Successional Status for details) are cited as evidence that Dyer’s woad may chemically inhibit germination and root elongation of some competing species [20].

Several reports indicate that Dyer’s woad may spread rapidly once established. A 1985 report estimated an annual spread rate of 14% for Dyer’s woad on rangelands in the northwestern United States, reducing grazing capacity by an average of approximately 38% [79]. A review by Dewey and others [14] indicates that the number of hectares occupied by Dyer’s woad in the Intermountain West increased more than 35-fold between 1969 and 1985. On the Cache National Forest of northern Utah, a 1988 study of nearly 373,000 acres (150,000 hectares) suggested that there was potential for a 124-fold increase in the number of Dyer’s woad populations [14]. One infestation south of Dillon, Montana, reportedly increased from 2 to more than 100 acres (0.8-40 ha) in 2 years [6]. Observations by weed specialists in the Great Basin describe Dyer’s woad as increasing from initial infestion sizes of 12,000 to 150,000 acres (4,856-60,704 ha) in 8 years; from 35 to 1,774 acres (14.2-718 ha) in 16 years; and from a “first report” of unknown size to an infestation of 24,000 acres (9,713 ha) over a period of 51 years [71]. Locations of these populations were not given, nor were specific sources for the observations.

As of 2000, Dyer’s woad was on 10 noxious weed lists in the continental United States and southern provinces of Canada [70]. Noxious weeds in Colorado are nonnative plants that meet at least one of several criteria regarding negative impacts on agricultural systems, livestock, or native plant communities; and in Utah a noxious weed is any plant determined to be especially injurious to public health, crops, livestock, land or other property (review by [69]). In other areas where Dyer’s woad occurs, it does not seem to be particularly invasive, or its invasiveness is unknown. For example, Dyer’s woad is a “Class A” invasive species in the Southwest: species with limited distribution within a management unit, or not present in a management unit but in adjacent areas and therefore posing an invasive threat. Preventing new outbreaks and eliminating existing populations is the primary focus of management for Class A species [24]. The California Invasive Plant Council classifies Dyer’s woad as a plant for which more information is needed to determine its potential threat to wildlands there [11].

Control: This review of control methods for Dyer’s woad is not intended to be either comprehensive or prescriptive in nature, but focused on control studies that may illuminate aspects of Dyer’s woad’s fire ecology or the potential for its management with prescribed fire. More information on control methods can be found in the literature cited in the following sections and in these reviews: [6,19,37,54]. Control of biotic invasions is most effective when it employs a long-term, ecosystem-wide strategy rather than a tactical approach focused on battling individual invaders [53]. In all cases where invasive species are targeted for control, the potential for other invasive species to fill their void must be considered [9].

As with most invasive plants, early detection and removal of Dyer’s woad plants is important for successful control. According to reviews [6,54], surveys for Dyer’s woad should be conducted when it is flowering or fruiting: from April or May through July or August in most areas. Its bright yellow flower clusters and dark brown to black seed capsules that hang down like an umbrella make Dyer’s woad easy to recognize (reviews by [6,54]).

Dyer’s woad populations may be reduced substantially if seed production can be prevented for a few years and soil seed reserves exhausted. It is best to remove Dyer’s woad plants as soon as possible after flowering to prevent further seed production, and it is important to remove entire plants because even when plants have been uprooted, Dyer’s woad seeds from green fruits may be germinable or may continue to develop and reach maturity if left on site [13]. Flowering of Dyer’s woad can be delayed by continual defoliation [84].

For large, known infestations control efforts could also be focused at the young rosette stage in Dyer’s woad. This stage is at greatest risk of mortality, probably due to the lack of development of the root system, and may be most susceptible to control efforts [20]. Plants at this stage are unlikely to sprout from buds on the root crown (see Vegetative regeneration). Farah and others [21] suggest that plants at this stage be targeted for biological control.

Control efforts may be best focused on areas with the best potential returns. For example, it is important to remove Dyer’s woad from roadsides, railways, and trails because these areas are effective avenues for seed dispersal. Dyer’s woad can be controlled more easily in croplands than in rangelands and forests, where control efforts are limited by inaccessible terrain, undesirable impacts of control efforts on native plants, and questionable economic returns (review by [54]). On exceptionally steep hillsides where few other plants are present to reduce or prevent erosion, total elimination of Dyer’s woad may be ill-advised (review by [80]).

Prevention: Prevention and early detection are critical in managing Dyer’s woad invasions (review by [54]). It is commonly argued that the most cost-efficient and effective method of managing invasive species is to prevent their establishment and spread by maintaining “healthy” natural communities [53,68] and by conducting monitoring several times each year [38]. Managing to maintain the integrity of native plant communities and to mitigate the factors enhancing ecosystem invasibility is likely to be more effective than managing solely to control the invader [34].

Weed prevention and control can be incorporated into many types of management plans, including those for fire management, logging and site preparation, grazing allotments, recreation management, research projects, and road building and maintenance [77]. See the “Guide to noxious weed prevention practices” [77] for specific guidelines in preventing the spread of weed seeds and propagules under different management conditions.

Cultural control: No information is available on this topic.

Physical or mechanical control: A review by McConnell and others [54] suggests that hand-pulling is one of the most important methods of Dyer’s woad containment or control. This approach is most convenient and effective when Dyer’s woad is in flower, because the distinct yellow flowers and umbrella-shaped stalk make it easy to locate and identify. Volunteer groups and/or seasonal employees can clear large tracts of land in a relatively short time with basic tools (review by [54]). Hand-pulling may be an important part of an integrated management approach for controlling Dyer’s woad. Cutting or mowing Dyer’s woad does not usually kill it [20,84] but may be useful to prevent or delay flowering.

Physically removing Dyer’s woad plants by hand-pulling or digging is probably most effective for small infestations, sensitive areas, and hard-to-reach spots. Fay [22] recommends hand-pulling Dyer’s woad when its density is around 1 plant/10,000 feet², or in areas where herbicides are not desired, such as areas frequented by recreationalists (e.g., Mt Sentinel in Missoula, Montana). Hand-pulling Dyer’s woad plants from ditch banks, rock piles, fence lines, and ravines may prevent seed dispersal onto adjacent areas (reviews by [19,54]).

Important considerations for physical control of Dyer’s woad are preventing seed set or dispersal and removing as much of the root as possible. Dyer’s woad seeds mature within 4 to 6 weeks from the time of flowering (see Seasonal Development), so it is essential that the plants be removed as soon as possible after flowering to prevent seed dispersal. Removing rosettes in early spring may prevent seed production; however, Dyer’s woad plants may be difficult to locate at this phenological stage. The thick, fleshy taproot of Dyer’s woad must be removed well below the root crown to prevent sprouting (see Vegetative regeneration). Plants are easily pulled or dug with a hoe or shovel if the ground is wet (review by [54]).

Dorst and others [18] describe successful control of Dyer’s woad on a heavily infested rangeland in northern Utah using hand-pulling, hoeing, or digging flowered plants, rosettes, and seedlings. Mature fruits were stripped into buckets or plastic bags and removed from the site. Infested areas were visited by volunteer work crews, often consisting of troops of 12- and 13-year-old Boy Scouts, an average of twice per season: once in mid- to late May when Dyer’s woad plants were approaching full bloom, and again about 3 to 4 weeks later. Once low Dyer’s woad densities were achieved, they were maintained by annual monitoring and pulling remaining scattered plants. Eradication was not achieved on any of the land units studied, but control equaled or exceeding 95% on the majority of units retained in the program for 8 or more years [18].

Clipping Dyer’s woad plants when they are rapidly growing in spring (see Seasonal Development) can increase mortality and decrease fruit production. Dyer’s woad response to clipping was affected more by timing of clipping than by frequency, when effects of clipping on mortality, percent flowering, fruit production, and fruit weights of Dyer’s woad were investigated on a Utah rangeland site. Clipping treatments were either low intensity (60% removal of aboveground phytomass) or high intensity (90% removal of aboveground phytomass), and were conducted on one or more of the following dates: 16 April, 7 May, 23 May, and 11 June. Significant mortality and reduction in reproductive performance occurred when at least 60% of the aboveground phytomass was removed on or after 23 May (P<0.05). Clipping once or twice, to remove as much as 90% of aboveground tissue before 23 May, did not significantly affect woad mortality, percent flowering, or fruit production. At least 3 sequential clippings at either 60% intensity or 90% intensity were required to significantly increase mortality rates or reduce flowering over controls. Plants clipped once on 11 June had mortality rates and flowering response similar to plants clipped on 16 April, 7 May, 23 May, and 11 June. Mean fruit production per plant did not differ between the various clipping treatments. Total fruit production as a percent of control production was reduced in plants clipped twice at either intensity, with those clipped at low intensity producing 49% of the control, and those clipped at high intensity producing 38% of the control [84]. Clipping after 7 May delayed flowering 1 year [20].

Similarly, Fuller (1985 as cited by [20]) found that greater mortality and less flowering were attained when clipping occurred on 25 June than on 1 May in 1982. Fuller demonstrated that to substantially reduce flowering capacity and cause adequate mortality before 23 May, Dyer’s woad had to be clipped 2 inches (5 cm) below the ground. “This suggests that regeneration of Dyer’s woad, following clipping damage, results from activation of (root) crown buds and those located on the roots just beneath ground level” [20].

Response to clipping in spring may be related to available soil moisture. A combination of diminished root-absorbing capacity and insufficient soil water would likely impede Dyer’s woad regrowth from basal meristems following clipping. It would also explain the minimal impact of clipping before 23 May and the dramatic effect of a single clipping after this date [20,84].

Biological control: Biological control of invasive species has a long history, and many factors must be considered before introducing biological controls. Refer to the Weed control methods handbook [76] for background information and important considerations for developing and implementing biological control programs. Two types of biological control for Dyer’s woad are discussed in the literature: grazing by domestic sheep and dissemination and enhancement of a naturally occurring rust pathogen, Puccinia thlaspeos. Neither shows much promise for substantial control of Dyer’s woad.

Grazing by domestic sheep does not seem a feasible control method for Dyer’s woad, because grazing occurs during midspring, a time when little damage is done to the plants. A foothill rangeland pasture on the west slope of the Wellsville Mountains was selected for monitoring the utilization of Dyer’s woad by domestic sheep. The site was dominated by big sagebrush, Dyer’s woad, cheatgrass, bulbous bluegrass, broom snakeweed, and curlycup gumweed. Soil and site information suggest a potential natural vegetation comprised mostly of bluebunch wheatgrass; therefore, the pasture was considered in poor condition relative to both its ecological and livestock grazing potential. Grazing began on 27 April, and the researchers speculate, based on a change in palatability of Dyer’s woad during flowering and increased availability of better forage, that domestic sheep switched to forage other than Dyer’s woad by 18 May. During this period, 48 out of 300 marked Dyer’s woad plants (16%) had some utilization. Utilization of individual grazed Dyer’s woad plants ranged from 18% to 92% with an average of 39% of the aboveground tissue removed. This level of utilization did not have any effect on mortality, percent flowering, mean fruit production, or fruit weight. Basal diameter and rosette diameter were greater on grazed than on ungrazed Dyer’s woad plants, suggesting that animals were selecting larger plants, which are also more likely to survive defoliation [84]. Stocking and timing required to impact Dyer’s woad would result in range deterioration, as important native species such as bluebunch wheatgrass and arrowleaf balsamroot (Balsamorhiza sagittata) are susceptible to heavy grazing at the same time of year as Dyer’s woad ([20] and references therein).

A rust fungus, Puccinia thlaspeos, was discovered on Dyer’s woad in an isolated foothill canyon in southern Idaho in 1978. Puccinia thlaspeos is reported to be a “naturally occurring” rust throughout Europe and much of North America [49]. Dyer’s woad plants infected with the rust were “stunted, severely malformed, and failed to produce seed”. Over a period of 9 years, rust incidence at this site increased from less than 1% of Dyer’s woad plants infected in 1978 to an average of 44% of Dyer’s woad plants infected in the spring of 1987. Distribution of the rust had also spread throughout an 1,125 square mile area in southeastern Idaho and western Wyoming during that period, and it was purposefully introduced to Dyer’s woad plants in a field study at Logan, Utah, in 1987 [49]. It now occurs in most populations of Dyer’s woad in northern Utah (review by [42]).

Puccinia thlaspeos causes systemic infection in Dyer’s woad. These infections are usually asymptomatic during the first year of Dyer’s woad’s life cycle, with the fungus overwintering in the tissue of infected plants. Symptoms typically appear during the second season (review by [42]), and Dyer’s woad plants infected with the rust typically appear chlorotic in the rosette stage, while bolted plants are covered with rust sori, severely stunted, and often epinastic (curved downward) [49]. Kropp and others [43] provide additional details on how rust infection proceeds over time and on the effects of dew on infection.

Silicle development and/or seed production were initially thought to be prevented on Dyer’s woad plants infected with Puccinia thlaspeos [49]; however, later studies found seed production on both symptomatic and asymptomatic branches of diseased plants. No differences in germination were found between seed with rust sori and those lacking sori. Plants grown from infected seed did not show symptoms of infection, suggesting that the rust is not spread via Dyer’s woad seed even when sori are present on the seed [42].

Studies have been conducted to help determine methods for using the rust as a biological control agent on Dyer’s woad in large populations, whether the level of inoculation obtained in the field is maintained over time, and the dispersal rates and mechanisms of spread subsequent to inoculation [42]. Inoculum can be prepared from dry leaf material collected from infected Dyer’s woad plants in the spring. The Dyer’s woad rust can be successfully established in populations of Dyer’s woad that lack naturally occurring rust using relatively low doses of inoculum applied in the spring. Once established, the rust is able to reproduce and disperse on its own, although dispersal is slow and augmentation may be needed to maintain high rates of disease incidence in Dyer’s woad stands. More details on field applications of Puccinia thlaspeos for biological control of Dyer’s woad are available from Kropp and others [42].

A substantial amount of additional literature is available that addresses various aspects of the biology of the rust and its use as a biological control agent for Dyer’s woad. This literature is not included here, as it is outside the scope of this review.

Chemical control: Herbicides are effective in gaining initial control of a new invasion or a severe infestation, but they are rarely a complete or long-term solution to weed management [10]. See the Weed control methods handbook [76] for considerations on the use of herbicides in wildlands and detailed information on specific chemicals. For information on particular chemicals, rates, timing of applications, techniques, safety, and other considerations, for control of Dyer’s woad, see these sources: [22,54,79].

Dyer’s woad is most sensitive to herbicides such as 2,4-D, metsulfuron, and chlorsulfuron during the early rosette to early blossom stage [6,19]. Research at Montana State University has shown effective control of Dyer’s woad when 2,4-D is applied to rosettes annually in spring for “a number of years”. Dyer’s woad is relatively tolerant to 2,4-D after it begins to set seed [6], and application at that time is likely to damage nontarget vegetation because many of Dyer’s woad’s associates are susceptible at that time of year (review by [80]). Research at Utah State University found that chlorsulfuron and metsulfuron prevented seed production in Dyer’s woad when applied as late as the flowering and seed-set stages [6]. Extremely low applications of metsulfuron at any phenological stage interfere with normal seed development and inhibit fruit formation and viable seed production. Treating plants in flower inhibits viable seed production by reducing fruit formation, seed development, and germinability [2,4]. Dyer’s woad tolerance to metsulfuron increases as flowering stages progress. Preanthesis stages were the most sensitive to metsulfuron application. Dyer’s woad plants treated in the midblossom stage with >5g/ha of metsulfuron produced no viable seeds [4].

Integrated management: The Montana Dyer’s Woad Cooperative Project was a program that incorporated early detection, treatment, repeated monitoring, and education for control and eradication of Dyer’s woad in Montana. Treatments included hand-pulling, digging, and spot-spraying with metsulfuron. Bolting and rosette plants were pulled and left on site, while flowering and fruiting plants were removed from the site in double-lined plastic bags. Cutting and removing the flowering or fruiting stems combined with spot-spraying of the remaining basal leaves was thought to be the most effective treatment because it killed root fragments inadvertently left in the soil. Monitoring data indicate that during the course of the project (20 years), Dyer’s woad was eradicated from 9 of 13 Montana counties and that infestation sizes decreased in the remaining infested counties. In some counties, containment, repeated inventories, and treatment applications were needed to prevent spread of Dyer’s woad while depleting the seed bank to the point where eradication was possible. See Pokorny and Krueger-Mangold [62] for further details.

Because accessible populations of Dyer’s woad are routinely treated with herbicides, Kropp and Darrow [41] suggested that it may be possible to integrate applications of herbicides and inoculum of Puccinia thlaspeos, provided that herbicides have no negative impact on rust viability. Researchers found that neither chlorsulfuron nor metsulfuron-methyl had a significant impact on Puccinia thlaspeos spore germination when used without surfactants, while 2,4-D with an added surfactant significantly decreased spore viability (P=0.05). Whether the decreased viability was due to the 2,4-D or the surfactant is unknown; however, when 3 different surfactants were tested alone or added to chlorsulfuron and metsulfuron-methyl, spore viability was reduced in some combinations. See Kropp and Darrow [41] for details.

APPENDIX: FIRE REGIME TABLE

SPECIES: Isatis tinctoria The following table provides fire regime information that may be relevant to Dyer’s woad habitats. Dyer’s woad likely occurs in plant communities other than those listed here; for example, fire regimes of plant communities in the eastern United States are not included because no information was found regarding native plant communities in which Dyer’s woad occurs in that part of the United States. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under “Find Fire Regimes”.

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