Troublesome weed seeds present


Peppers are a warm-season crop that need a long growing season. They are produced in a number of regions in the state. The earliest production district is in the Coachella Valley. There is also significant production in the Central Valley and the inland areas of the coastal production districts. Peppers are planted in a number of bed sizes that range from 40 to 60 inches wide. On narrower beds they are planted in one to two seed lines per bed and on wider beds they are planted in two seed lines.

Peppers in California are nearly all transplanted, which gives the crop some advantage over the weeds. Unfortunately pepper transplants are slow to establish and initially compete poorly with weeds, which can reduce pepper yields by competing for light, water, and nutrients as well as delaying maturity. Weeds germinating during the first 6 to 8 weeks after transplanting can have serious consequences. After 8 to 10 weeks, yield is less affected by late-emerging weeds; however, weeds can interfere with harvest and produce weed seeds that can be troublesome in rotational crops. In some growing areas such as Southern California, fresh market peppers are grown on beds covered with black plastic mulch, which prevents germination of most weed species.

Effective weed management in peppers begins with proper field selection and identification of potential weed problems. It involves preirrigation and cultivation, proper land and planting bed preparation, sanitation, and proper selection of herbicides.

When combined with good cultural practices, available herbicides can control many of the weed species that are found in pepper fields. The choice of herbicide depends upon the weed species that are present and the cultural practices followed by the grower.

Preemergence herbicides are soil applied and mechanically mixed with the soil or are irrigated into the soil before weeds emerge. They are effective against germinating seeds before they emerge from the soil; these materials usually give residual control. Postemergence herbicides are sprayed onto the foliage of the weeds after they have emerged. They are absorbed by the leaves and stems of the weeds and translocated to the site of action; thus they work best when sprayed on unstressed plants. Some herbicides have both preemergence and postemergence activity.

Herbicides work best if they are applied when soil moisture is adequate for plant growth. Preemergence herbicides are effective against germinating seeds, not dry seeds. Do not apply these materials to wet soils, however, as compaction can occur. Postemergence herbicides work best on unstressed plants, which absorb and translocate the material more readily than stressed plants.

In pepper production, many growers establish their beds in fall, but in Southern California beds are created in April. To keep beds weed-free during winter, use either a rolling cultivator on rough beds or apply oxyfluorfen before weeds germinate. After weed seedlings emerge, glyphosate or paraquat can be used. In spring the beds can be worked and then fumigated with metam sodium. Alternately, shaped beds may be flamed or treated with preemergence herbicides; both methods require subsequent hand weeding.

Herbicides may also be applied after planting but before crop emergence. Generally after peppers emerge, growers rely on hand hoeing to remove weeds from the seed line and cultivation to keep the rest of the bed tops and furrows weed-free. Herbicides may be needed for grass weed problems after pepper transplanting or after the crop emerges.

In coastal production areas peppers are sometimes rotated with strawberry. Strawberry fields receive pre-plant fumigation and remaining weeds are managed with opaque mulch and herbicides; the weed population in the following crop is reduced.


To plan a weed management program, it is essential to know which weed species are present and the relative abundance of each. Conduct weed surveys of each field at least twice a year: the first before planting and the second just before harvest. Identify which weeds are present to determine appropiate management tactics. Records from previous crops will indicate which weeds escaped control and will likely infest the pepper crop. Also examine fence rows and ditch banks, as these are other sources for weed invasion. Pay special attention to where perennial weeds such as field bindweed and yellow nutsedge occur so that follow-up control measures can be taken.

Field selection and preparation

Many major weed problems can be reduced by avoiding fields that are severely infested with weeds such as nightshade, little mallow or cheeseweed, field bindweed, nutsedge, and parasitic dodder, which compete effectively with peppers. Irrigation water can also be a source of weeds; keep canal banks free of weeds or install a weed screen on the inlets from canals. Avoid moving weed seed into fields on equipment. When equipment has been used in a weedy field, clean it before entering other fields.

Deep plowing (9-10 inches) with a moldboard plow before listing the beds can bury nutsedge tubers to sufficient depth to prevent their emergence. Preirrigation coupled with light cultivation or flaming can also greatly reduce weed problems by helping to reduce the initial flush of weeds such as nightshades. Herbicides, such as Roundup or Gramoxone, can also be used in this manner.

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For direct-seeded crops, a soil cap (2- to 4-inch mound of soil) over the seed line at planting can reduce the first flush of weeds competing with the crop seedling. The dirt mound also reduces soil moisture loss and precludes soil crusting after periods of rainfall. Disks or other implements form the soil cap just after planting. The cap is removed just after pepper seedlings germinate and before rapid elongation of the hypocotyl. Rains could delay cap removal, causing potential pepper stand loss. High temperatures during cap removal can lead to scalding of the emerging pepper seedlings, leading to stand loss. However, under good conditions, weed seeds that germinate in the soil cap are destroyed when the cap is removed and fast-growing weeds that germinate in the original bed are scraped off by the cap removal operation.

Proper land and planting bed preparation allow for more rapid and uniform germination of pepper seeds or for more rapid growth of transplants.

Crop rotations

Crop rotations help to reduce weed problems, as well as disease such as Phytophthora root rot. Corn is a good rotational crop for peppers because herbicides available for use in corn control nightshade and field bindweed. Also, corn is not a host for dodder. Alfalfa is a good choice for a rotational crop because the frequent cutting cycle reduces many weeds and available herbicides eliminate most other weeds. Other crops considered to be useful rotational crops with peppers include beans, cereals, cotton, garlic, rice, onions, carrots, lettuce, cole crops and safflower. Avoid crops such as tomatoes, potatoes and eggplant; they are in the same family as peppers (Solanaceae) and similar herbicides are used in their production, resulting in similar uncontrolled weeds.


Oxyfluorfen (GoalTender), paraquat (Gramoxone SL 2.0), and glyphosate (Roundup Powermax) can be used to control weeds on winter beds. They help keep the beds weed-free during winter.

Preplant herbicides

In spring apply preemergence herbicides just before planting. They are incorporated into the soil either mechanically or by sprinkler irrigation. The entire bed top may be treated or band treatments applied over the seedling row. Band treatments proportionally reduce the herbicide cost and may reduce the risk of herbicide carryover into the next crop. If weeds have already emerged, a postemergence treatment may also be necessary. When band treatments are used, the area between crop rows requires cultivation or some other method of weed control.

In southern California, treat bare beds, before applying a plastic mulch for yellow nutsedge control. For direct-seeded peppers, soil-applied preemergence herbicides such as napropamide (Devrinol) and bensulide (Prefar) can be applied before planting to control grasses and many broadleaf weed species (except nightshades). For transplanted peppers, trifluralin (Treflan) may be incorporated into the bed before transplanting. Pendimethalin (Prowl H2O) is also registered for use before transplanting.

In some areas, especially desert valleys, growers may choose to fumigate the soil with metam sodium, which greatly reduces weeds as well as disease and nematodes. Contact your Farm Advisor for advice on the most effective application method (flood, sprinkler, shank, or drip) for a particular situation.


Transplanting peppers is the dominant method of planting peppers statewide; a much smaller portion of pepper acreage is direct seeded. Weed control following planting between the two planting systems varies to some degree. Initially, using transplants provides the crop a growth advantage over the weeds. However, the slow initial emergence of direct-seeded peppers provides an opportunity to burn off the first flush of germinating weeds with herbicides or flaming before the pepper plants germinate.

Cultivation and Hand Weeding

After transplanting, mechanical cultivation close to the seed line removes weeds on the bed top and furrow, except for those in the seed line. To avoid excessive competition with the peppers and to make removal easier, cultivate when weeds are small. Removing weeds when they are in the seedling stage permits a shallow cultivator setting, which helps to avoid bringing more weed seeds near the surface where they might germinate.

Close cultivation effectively controls many weeds in a pepper field with the exception of dodder and weeds growing in the seed line. Dodder does not require connection with the soil once it has attached to the pepper plant; therefore cultivation will not destroy it after attachment has occurred. Control weeds that can act as alternate hosts for dodder, allowing it to spread onto pepper plants. At thinning, eliminate pepper plants that have dodder attached to reduce dodder problems. When pepper seedlings are tall enough, cultivation tools can be adjusted to throw soil to the base of the plants to create a dust mulch and smother young weed seedlings.

Hand weeding controls weeds in the seed line that are not removed by cultivation. The expense of hand hoeing can be reduced by close cultivation before thinning, which allows the hand crew to move more quickly through the field. The nightshades closely resemble peppers and can at times be difficult to detect by weeding crews. Transplanted peppers are not thinned, but for direct-seeded peppers, escaped weeds in the seed line can be removed at thinning. Generally, one to three hand weedings are needed to maintain peppers free of weeds through harvest.

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Cultural Practices

Preventing weeds from setting seed helps reduce the population of weeds in the following crop; this also applies to areas adjacent to cropped fields. A program of roguing out nightshade plants late in the season can dramatically reduce the amount of seed that is deposited in the field at the end of the cropping cycle.

Buried drip irrigation can help reduce weed problems by keeping the tops of the beds dry. If peppers are direct seeded, a second irrigation system (furrow or sprinkler) is needed to germinate the pepper seed. This process also favors germination of weed seeds, which can be flamed once they have emerged. The crop should be well established and free of emerged weeds before the subsurface drip irrigation is used. If conditions permit and water from the buried drip tape is sufficient to meet the needs of the crop but not ample enough to wet the bed tops, weed control can be satisfactory. However, if it is necessary to sprinkler-irrigate after peppers have emerged, subsequent weed growth will occur and require hand weeding. Perennial weeds are likely to remain a problem with this system.

With furrow-irrigation systems, maintaining deep furrows keeps the bed tops from becoming overly wet while providing adequate moisture for the crop. By keeping the bed tops drier, fewer weeds are likely to germinate in the soil surface.

Opaque Mulch

Black, brown, or green plastic mulch is used on transplanted peppers and can inhibit most weed growth on pepper beds. However, plastic mulch does not control yellow nutsedge, which can germinate and emerge through the plastic. Weeds can also establish in planting holes immediately around the pepper plant and need to be removed by hand.


The slow emergence of direct-seeded peppers can be used as an advantage for weed control purposes. Weeds generally germinate earlier than peppers and these weeds can be controlled by flaming the beds with propane burners. In order to kill the maximum amount of emerged weeds, flame when 1 to 3% of the pepper plants begin to emerge. Timing is critical: only a 1- or 2-day window exists for flaming once the pepper seedlings are ready to emerge because any emerged seedlings will be killed by the process. Flaming will control most broadleaf weeds when they are in the two- to four-true leaf stage; however, many grasses and volunteer cereals will not be effectively controlled. Pepper seed that has been primed germinates too fast; as a result, fields planted with primed seed cannot safely be flamed.


In direct-seeded peppers, paraquat (Gramoxone Inteon), carfentrazone (Shark), and pelargonic acid (Scythe) can be applied to the flush of weeds that emerge before the emergence of pepper seedlings, in much the same manner as the use of flaming described above. There is only one foliar-applied herbicide (halosulfuron-Sandea) that selectively controls some broadleaf weeds in peppers, but clethodim (Select Max) and sethoxydim (Poast) can be applied to control most annual and perennial grasses.

Clethodim and sethoxydim have good selectivity on peppers; however, they may cause some phytotoxic symptoms if the temperatures are above 85° to 90°F. Trifluralin (Treflan) and pendimethalin (Prowl H2O) are only labeled for use on transplanted peppers and must be incorporated into the soil; they do not control nightshade. Napropamide (Devrinol) can be applied over the top of transplants and then immediately sprinkler-irrigated into the soil.

Pendimethalin can be applied at layby when the plants are 5 to 7 inches tall. Treatments are usually applied as directed or shielded sprays on each side of the plant row and immediately incorporated by sprinkler irrigation or mechanical incorporation.


UC IPM Pest Management Guidelines: Peppers
UC ANR Publication 3460


R. F. Smith, UC Cooperative Extension, Monterey County
O. Daugovish, UC Cooperative Extension, Ventura County
Acknowledgment for contributions to Weeds:
C. E. Bell, UC Cooperative Extension, Imperial County
W. T. Lanini, Weed Science/Plant Sciences, UC Davis

Statewide IPM Program, Agriculture and Natural Resources, University of California
All contents copyright © 2017 The Regents of the University of California. All rights reserved.

For noncommercial purposes only, any Web site may link directly to this page. FOR ALL OTHER USES or more information, read Legal Notices. Unfortunately, we cannot provide individual solutions to specific pest problems. See our Home page, or in the U.S., contact your local Cooperative Extension office for assistance.

North Dakota State University

Noxious, Troublesome Weeds Spread Through Manure Weed seeds pass unharmed through the digestive tracts of animals such as cattle and sheep.

Noxious, Troublesome Weeds Spread Through Manure

(Click the image below to view a high-resolution image that can be downloaded)

  • Manure is being spread at the NDSU Carrington Research Extension Center. (NDSU photo)

Using manure as fertilizer can lead to the spread of noxious and troublesome weeds.

“It is a known fact that weed seeds pass unharmed through the digestive tracts of ruminant animals (cattle, sheep),” says Mary Keena, livestock environmental management specialist based at North Dakota State University’s Carrington Research Extension Center. “This means that whatever weed seeds are in the feed or bedding you’re using are still viable when they exit the animal as manure.

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“There is also a line of thought that says there is an extensive weed seed bank in most fields already and applying manure gives them the nutrients they need to grow,” she adds. “Either way, manure does promote plant growth.”

Producers have tools to minimize the amount of viable weed seeds in fresh manure, one of which is composting. Information about composting is available in a self-paced online workshop at To learn the reasons behind these composting operations, check out

Another tool more commonly used is herbicide control. Applying a pre-emergence herbicide will help reduce competition between weeds and newly seeded crops.

But what happens when those herbicides don’t work on specific noxious and troublesome weeds? How do you keep noxious and troublesome weeds at bay when you need to spread manure but know hard-to-control seeds such as Palmer amaranth and waterhemp are present?

“Even in direct competition with a crop, these plants can still produce up to 100,000 seeds in a year,” warns Joe Ikley, NDSU Extension weed specialist.

Due to this extensive seed production, the ability of the weeds to germinate throughout the growing season, and widespread resistance to glyphosate and Group 2 herbicides, herbicide programs for control of severe infestations of waterhemp and Palmer amaranth often will cost two to three times the amount of money spent on a weed control program in fields without these two weeds, he says.

In addition to the added cost of controlling these weeds, weed scientists in the U.S. have documented herbicide resistance in Palmer amaranth to every herbicide mode of action that can be used in row crop production.

“This is why it is important to scout fields for these two pigweeds before they become established,” Ikley says. “In many cases where the weeds are spread in contaminated manure, the infestation starts with a manageable level of plants and the population can be managed by hand pulling if correctly identified.”

Producers have a few steps they can take to help mitigate and monitor the potential impacts of these weeds. One is to keep records of where they spread manure so they can monitor that field throughout the growing season.

Another step is to clean spreading equipment before moving to a new field.

“This is probably one of the most important and least popular steps you can take,” Keena says. “If you are doing custom work for someone, this is especially important as you do not want to take one client’s issue to the next client.

“Clean the spreader with an air hose for dry manure or a pressure washer for wet manure,” she advises. “This takes time and can be messy but can save years of headaches down the road. Make sure to record where cleanout occurs so you can monitor that spot during the growing season.”

She recommends spreading weed seed-heavy manure on tame grass pastures or grass hayfields because more options are available for herbicide control on them.

“It is never recommended to spread manure on native rangeland,” says Miranda Meehan, NDSU Extension livestock environmental stewardship specialist. “Adding additional nutrients can benefit invasive grass species such as Kentucky bluegrass and smooth brome.”

Examination of commercially available bird feed for weed seed contaminants

In 2016 and 2017, 98 separate commercially available bird feed mixes were examined for the presence of weed seed. All weed seed contaminants were counted and identified by species. Amaranthus species were present in 94 of the 98 bags of bird feed. Amaranthus species present in bird feed mixes included waterhemp [ Amaranthus tuberculatus (Moq.) Sauer], redroot pigweed ( Amaranthus retroflexus L.), Palmer amaranth ( Amaranthus palmeri S. Watson), smooth pigweed ( Amaranthus hybridus L.), and tumble pigweed ( Amaranthus albus L.). Amaranthus palmeri was present in 27 of the 98 mixes. Seed of common ragweed ( Ambrosia artemisiifolia L.), kochia [ Bassia scoparia (L.) A.J. Scott], grain sorghum [ Sorghum bicolor (L.) Moench], wild buckwheat ( Fallopia convolvulus L., syn: Polygonum convolvulus ), common lambsquarters ( Chenopodium album L.), large crabgrass [ Digitaria sanguinalis (L.) Scop.], and Setaria species were also present in bird feed mixes. A greenhouse assay to determine Amaranthus species seed germinability and resistance to glyphosate revealed that approximately 19% of Amaranthus seed in bird feed mixes are readily germinable, and five mixes contained A. tuberculatus and A. palmeri seed that were resistant to glyphosate. Results from linear regression and t -test analysis indicate that when proso millet ( Panicum miliaceum L.), grain sorghum, and corn ( Zea mays L.) were present in feed mixes, Amaranthus seed contamination was increased. The presence of proso millet and grain sorghum also increased contamination of grass weed species, while sunflower ( Helianthus annuus L.) increased A. artemisiifolia contamination and safflower ( Carthamus tinctorius L.) increased contamination of Bassia scoparia .


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