Alligator weed seeds

Alligator weed

Alligator weed is most commonly found spread across the surface of a body of water described in a sprawling fashion. It can be found in terrestrial areas around gardens or in between rows of crops with sufficient moisture present. Stems are pink and hollow and can reach lengths of 1 m with opposite narrow elliptical leaves. Flowers are reduced and white in color, have thin petals, and are on stems that extend 4-5 inches away from the plant.

Ecological Threat

Alligator weed grows in thick dense mats along the shoreline of lakes and streams creating difficulty for wildlife to access the edge of the water. Alligator weed doesn’t provide a sufficient food source or shelter for aquatic wildlife. By preventing native plants from growing, alligator weed removes necessary food sources and shelter for native animals.


Alligator weed is able to spread and reproduce rapidly through stems or leaf cuttings making it difficult to eradicate in areas once established because it can grow from small portions of the plant left behind. Alligator weed propagates most commonly from stolons vegatatively with each individual node capable of propagating allowing for rapid spread and propagation of the plant.


Alligator grass originated in South America, but was transferred to the United States through water ways accidentally. The exact date when the weed was transferred to the United States is not known. It became noticeable on a pest status in 1959 when the U.S. Army Corps of Engineers investigated the damage potential caused by propagation of alligator weed. Investigation into damage potential of alligator weed resulted in placement on the federally noxious weed list for several states preventing further propagation and distribution.

Native Origin

Current Location

U.S. Present: Alligator weed is found in the southwest United States from San Joaquin Valley south to Los Angeles and across the south and east portions of the continent south to Central America.

U.S. Habitat: Alligator weed can grow in a variety of habitats from dry to immersed in water, but the preferred habitat is aquatic. In the United States alligator weed is most often found growing along the surface of streams and ponds at the shores edge.


Physical removal of alligator weed is possible, but not usually 100% successful in eradicating the weed because the plant is able to re-grow and propagate from stem fragments alone. There are currently no biological control methods of eradication rather than goats which can keep the plant under control by feeding on the weed. Chemical control has been found to be the most successful when containing fluridone or imazapyr. Other chemical treatments have been found slightly less successful, but still effective when containing: 2,4-D, glyphosate, triclopyr, and imazamox. Systematic herbicides such as Navigate and Weedar 64 are successful chemical treatments as well.


Andres, L. A. 1977. The economics of biological control of weeds. Aquatic Botany 3: 111-123.

Barreto, R., R. Charudattan, A. Pomella, and R. Hanada. 2000. Biological control of neotropical aquatic weeds with fungi. Crop Protection 19: 697-703.

Buckingham, G. R. 1996. Biological control of alligatorweed, Alternanthera philoxeroides, the world’s first aquatic weed success story. Castanea 61: 231-243.

Holcomb, G. E. 1978. Alternaria alternantherae from alligatotorweed is also pathogenic on ornamental Amaranthaceae species. Phytopathology 68: 265-266.

Pemberton, R. W. 2000. Predictable risk to native plants in weed biological control. Oecologia 125: 489-494.

Zeiger, C. F. 1967. Biological control of alligatorweed with Agasicles n. sp. in Florida. Hyacinth Control J. 6: 31-34.

Alligator Weed

Thank two tiny bugs that alligator weed, Alternanthera philoxeroides, isn’t a bigger problem than it already is.

Alligator weed is a plant of South American origin that made its way to Mobile, Ala., in 1897, most likely as a stowaway in a ship’s ballast accidentally dumped in state waters. It’s now found throughout the Southeast and, surprisingly, as far north Illinois and as far west as California. It’s also spread to Australia, Asia and parts of the Pacific, where it invariably becomes a herbaceous pest.

It can form sprawling mats over rivers or along shore lines, but it can also grow on dry land. Its leaves are elliptical to oblong in shape and grow opposite each other along its fleshy stem. The outer edges of the leaves are smooth, what botanists call complete. The stems themselves can exceed 30 feet in length. It blooms during summer, the flowers tiny, white-and-yellow, clustered on a head the size of a marble. Alligator weed apparently does not reproduce via seed in North America but rather vegetatively. It has nodes along the stems where roots and new stems grow. Break off a piece and you’ve got a new plant. By contrast, according to the authoritative Flora of North America, neither the fruit of the plant or its seed has been observed.

Alabama. Arizona, Arkansas, California, South Carolina and Texas all considered it to be a noxious weed.

Alligator weed is listed on Florida’s prohibited aquatic plant list, and as a Category II invasive. That means it growing in abundance but not enough to have altered habitats. Yet. Which brings us to the heroes of our story: the alligatorweed flea beetle, Agasicles hygrophila, and the alligatorweed thrip, Amynothrips andersonii, both released in the U.S. back in the 1960s.

Both bugs are native to southern Brazil and northern Argentina and both feed exclusively on the leaves of alligator weed. Left uncheck, alligator weed will form large, dense mats that can choke navigation, clog drain and intake pipes, limit light penetration and block out native species. Waterways can become unuseable for boating, fishing and swimming. By decreasing water flow, it increases the amount sediment in the water and provides breeding places for mosquitos. When it grow on land, it can invade farm fields and become and agricultural pest (and potentially clogging irrigation ditches).

Australia considers alligator weed to be a “high risk” invasive despite an aggressive effort to limit its spread. In China, it’s ranked as the country’s 12th worst invasive; in some parts of China, it’s reduced rice production by 45 percent and sweet potato production by 63 percent.

Part of the reason why alligator weed is such a problem is that it is one tough, adaptable plant. It will grow in water, it will grow on land. It will tolerate a fair amount of salt. It will also tolerate cold to a degree, dying back come winter in colder climates. By the way, the alligatorweed flea beetle is ineffective on alligator weed when it grows on land.

It’s also nutritious when cooked as a green, full of minerals and a fair share of protein. However, it also tends to pick up any toxins that might be present in the water, including heavy metals.

Alligator weed, also spelled alligatorweed, is a member of Amaranthaceae, the amaranth family.

alligator weed

A semi-aquatic, aquatic, or terrestrial herbaceous plant that produces roots at its stem joints. These stems are often hollow when growing in water, and form dense mats of vegetation out over the water surface. Its oppositely arranged leaves are almost stalkless and elongated in shape (2-14 cm long and 1-4 cm wide). Its flowers are borne in dense globular clusters (1-2 cm across) on stalks 2-9 cm long in the forks of the upper leaves.These small flowers have five white ‘petals’ that acquire a papery appearance as the fruit mature.

Category 3 – Must not be distributed or disposed. This means it must not be released into the environment unless the distribution or disposal is authorised in a regulation or under a permit.

Though this species is quite widespread, it is not yet very common in Australia. It is mainly found in the coastal regions of central New South Wales (i.e. near Sydney) and in southern Victoria (i.e. near Melbourne). It is present in other parts of New South Wales and Victoria and has occasionally been recorded in the ACT and eastern Queensland. Also widely naturalised in other parts of the world including in the USA, the Caribbean, southern Europe (i.e. France and Italy), tropical Asia (e.g. India, Sri Lanka, China, Taiwan, Indonesia, Myanmar, Singapore, Thailand and Papua New Guinea) and New Zealand.

This species has become a weed in the warmer temperate, sub-tropical and tropical regions of Australia. It usually grows in aquatic habitats (e.g. canals, rivers, swamps, lakes, dams, ditches, etc.), being rooted to the ground and emerging above the water surface. However, it can also be found free-floating in dense mats on the water surface or growing in terrestrial habitats. It is also a potential weed of wetter pastures and irrigated crops (e.g. rice) in Australia.

A long-lived (i.e. perennial) aquatic, semi-aquatic or terrestrial herbaceous plant with a creeping (i.e. prostrate), semi-upright (i.e. ascending) or free-floating growth habit.

Alligator weed (Alternanthera philoxeroides) is already an important environmental weed in New South Wales, Victoria and Queensland, and is seen as a potentially significant environmental weed in many other parts of Australia. It is regarded as one of the worst weeds in Australia because of its invasiveness, potential for spread, and economic and environmental impacts. In fact, this species has been named as one of Australia’s 20 Weeds of National Significance (WoNS). It is thought that the potential range of alligator weed (Alternanthera philoxeroides) includes waterways throughout most of southern and eastern Australia.It is an especially troublesome weed because it invades both terrestrial and aquatic habitats, and is very difficult to eradicate. Alligator weed (Alternanthera philoxeroides) can totally disrupt natural aquatic ecosystems by blanketing the surface of the water with thick mats of vegetation that impede the penetration of light below the water surface, shading out any submerged native plant species. These mats also promote sedimentation and flooding and prevent gaseous exchange, leading to a reduction in water quality (i.e. reduced oxygen levels in the water). Such changes to aquatic ecosystems can have significant negative impacts on the native plants and animals growing in them (e.g. reduce water bird and fish activity, cause the death of native fish and replace native wetland plants). When growing on land it also grows into a dense mat of vegetation with a mass of creeping underground stems (i.e. rhizomes) and is capable of out-competing all but the most robust plant species. It quickly displaces native plants and can be harmful to the native animals that rely on them.Alligator weed (Alternanthera philoxeroides) is currently having the greatest impact in New South Wales, where the total infested area is now estimated at 3,950 hectares. It has spread from initial sites in the Newcastle area to invade aquatic habitats and seasonally flooded land in the Fullerton Cove, Williamtown and Raymond Terrace areas in the Lower Hunter region. In the Sydney region, alligator weed (Alternanthera philoxeroides) has spread throughout the Parramatta and Georges River catchments. It has also spread to many other locations in the Sydney basin including the Botany wetlands, several northern suburbs and the Hurstville area. However, its continual spread downstream in the Hawkesbury/Nepean catchment, which now has approximately 70 km of infested waterways, in of most concern in this region. Large infestations have also bee located in Barren Box Swamp, near Griffith, and in a tributary of the Richmond River, on the far north coast.It should also be noted that alteration to the natural flow regimes of rivers, streams, floodplains and wetlands has been listed as a “key threatening process” to natural ecosystems in New South Wales. Exotic plant species such as alligator weed (Alternanthera philoxeroides) have reduced stream flows and substantially contributed to this worsening environmental problem (i.e. by impeding water flow and increasing water loss through transpiration). Alligator weed (Alternanthera philoxeroides) has also been found at several hundred sites in Victoria, though most of these sites are in backyards in suburban Melbourne. It is listed among the top 50 most invasive plant species in south-eastern Queensland, even though it is not yet widely naturalised in this region. This species is also regarded as a potentially invasive garden plant in the Greater Adelaide region and is considered to to pose a significant threat to Adelaide’s biodiversity.Alligator weed (Alternanthera philoxeroides) is also very invasive overseas, and is considered to be one of the worst aquatic weeds in the world. In the USA, it forms dense tangled mats of vegetation that overtop native aquatic plants and out-compete them for sunlight. It eventually replaces desirable native species and can significantly alter the aquatic and riverine ecology of heavily infested areas. This species also invades drains, streams, swamps and similar wet habitats in New Zealand and is noted to be harmful to native biodiversity in China. It is also a major problem parts of southern Asia (i.e. in Burma, Thailand, Indonesia and India). For more information from the Queensland Government

The stems of this weed often grow as runners along the ground (i.e. stolons) or creeping below the ground surface (i.e. rhizomes). They may also spread out over the surface of water bodies and tend to form dense mats of vegetation (up to 1 m thick). These aquatic stems usually become hollow as they mature, which aids in floatation. The production of roots (i.e. adventitious roots) from the joints (i.e. nodes) of these stems is quite common. Stems can be up to 10 m long and mats of vegetation can be formed up to 15 m out over the water surface. Younger stems are light green to reddish in colour, hairless (i.e glabrous), and have slightly swollen joints (i.e. nodes). The dark green leaves are borne in pairs along the stems and usually do not have any leaf stalks (i.e. they are sessile or sub-sessile). They are elongated in shape (i.e. narrowly elliptic to lanceolate) with entire margins and pointed tips (i.e. acute apices). These leaves (2-14 cm long and 1-4 cm wide) are also hairless (i.e. glabrous) and have a somewhat waxy appearance.

The whitish flowers are borne in dense globular clusters (1-2 cm across) at the top of stalks (i.e. peduncles) 2-9 cm long. These flower clusters are usually produced in the forks (i.e. axils) of the upper leaves. Each flower has five small white ‘petals’ (i.e. perianth segments or tepals) and five yellow stamens. The ‘petals’ (5-7 mm long) tend to develop a papery appearance and may turn straw-coloured as they mature. Flowering occurs from late spring through to early autumn. The small fruiting ‘capsules’ (i.e. utricles) are brownish in colour, bladder-like in appearance, and contain a single seed. These seeds are smooth in texture and oval (i.e. elliptic) in shape, but are rarely produced in Australia.

This plant can reproduce by seed and vegetatively by stem fragments. However, because seeds are generally not produced in Australia, most reproduction here is vegetative.Stem segments, which have the ability to float, are easily dispersed by floods and water currents. They may also be spread by boats and other water craft, in dumped garden waste, and by animals or vehicles.

Alligator weed (Alternanthera philoxeroides) is similar to several native and introduced species of joyweeds (Alternanthera spp.), including lesser joyweed (Alternanthera denticulata), hairy joyweed (Alternanthera nana), calico plant (Alternanthera ficoidea), purple joyweed (Alternanthera brasiliana), khaki weed (Alternanthera pungens) and gomphrena weed (Gomphrena celosioides). However, it is the only one of these species that has hollow floating stems or forms dense mats over the water surface.The native joyweeds (e.g. Alternanthera denticulata and Alternanthera nana) can also be easily distinguished from terrestrial alligator weed (Alternanthera philoxeroides) plants by the fact that their whitish flower clusters are stalkless. The other species can be separated by the following differences: alligator weed (Alternanthera philoxeroides) is a large and robust plant with relatively large green leaves (2-14 cm long) that are almost hairless (i.e. glabrous). Its flowers are borne in relatively large, rounded, clusters (1-2 cm across) at the end of a flower stalk (i.e. peduncle) 2-9 cm long that grows from the upper leaf forks (i.e. axils).calico plant (Alternanthera ficoidea) is a relatively small plant with relatively small leaves (1-6 cm long) that are somewhat hairy (i.e. pubescent). Its flowers are borne in moderately large, dense, stalkless (i.e. sessile), clusters (5-12 mm across) in the upper leaf forks (i.e. axils).purple joyweed (Alternanthera brasiliana) is a large and robust plant with relatively large purplish coloured leaves (about 10-15 cm long) that are almost hairless (i.e. glabrous). Its flowers are borne in relatively large, dense, clusters (10-20 mm across) at the end of a flower stalk (i.e. peduncle) 2-10 cm long that grows from the upper leaf forks (i.e. axils).khaki weed (Alternanthera pungens) is a small creeping plant with relatively small leaves (0.5-5 cm long) that are almost hairless (i.e. glabrous). Its flowers are borne in relatively small, dense, stalkless (i.e. sessile), clusters (8-12 mm across) in the upper leaf forks (i.e. axils) and produce spiny fruit.gomphrena weed (Gomphrena celosioides) is a small plant with relatively small leaves (2-5 cm long) that are somewhat hairy (i.e. pubescent). Its flowers are borne in relatively large, dense, stalkless (i.e. sessile), clusters (up to 4 cm long) at the tips of its stems. The water primroses (Ludwigia adscendens and Ludwigia peploides subsp. montevidensis) often form similar dense mats of vegetation out over the water surface. However, these species can be distinguished by their alternately arranged leaves, larger four-petalled flowers (about 25 mm across) and elongated fruit.