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alligator weed seeds

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.

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.


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.

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.

Alligatorweed is listed as prohibited by the IFAS Assessment and is listed as a Category II by FLEPPC

Thick mats of alligatorweed prevent drainage canals, ditches, streams, and other small waterways from emptying rapidly during periods of heavy water load, thus causing flooding. If mats break loose, they create obstructions by piling up against bridges, dams, and sharp bends in waterways. Thick mats also increase mosquito habitat. Navigation of small waterways is obstructed, as is shoreline navigation in large waterways. Fishing and swimming can be affected, although a small fringe of alligatorweed probably benefits fishing.

Species Characteristics

Alligatorweed was accidentally introduced into Florida in 1894 in ballast water of ships and can now be found growing throughout the state. This rooted perenial plant can grow in a variety of habitats, although it is usually found in water. It forms sprawling mats over deep rivers or along shorelinesand can be a pest on land.

Because alligatorweed spreads easily by fragmentation, attempts at physical control usually only serve to spread the weed.

Alligatorweed should be removed and disposed of properly to prevent spread.

Freshwater and moderately brackish sites, estuaries, damp habitats, dune lakes and hollows and wetlands, and can also grow in dry pastures, crops and urban areas.

Perennial aquatic or terrestrial herb with long, fibrous roots. Pink, soft, hollow stems (<10 m long) root at nodes, creep along ground, or float on water with tips standing upright and forming dense stands or rafts. Dark green, waxy leaves (3-13 x 1-4 cm) are opposite. Clusters (1-2 cm diameter) of white clover-like flowers appear from December to February but no seed is produced in New Zealand.

Reproduces from stem sections only. Water flow, contaminated diggers, soil movement, dumped vegetation, machinery, eel nets, livestock, boats and trailers all spread fragments into new catchments, pastures, cropping land, waste places and drains. Also potentially spread by ethnic groups mistaking alligator weed for mukunu-wenna (Alternanthera sessilis), which they use as a vegetable.

Which habitats is it likely to invade?

Report all sites to your regional council.
1. Dig out small patches: either dispose of all pieces at a refuse transfer station, or dry them out and burn them – don't leave pieces of alligator weed on the top of soil or attempt to compost them as each piece can regrow.
2. Spray terrestrial sites (spring to autumn): glyphosate (20ml/L) or metsulfuron-methyl 600g/kg (5g/10L). Use penetrant in all herbicide mixes.
3. Spray aquatic sites (spring to autumn): glyphosate (20ml/L + penetrant).
4. Weedmat: cover site for 6-12 months to keep out light, checking edges for creeping stems.

Rapidly forms dense mats over water and margins, with roots down to 2 m deep. Stem sections break and root readily. Tolerant of 30% sea water, high temperatures, high pollutant levels, grazing, and other damage, but intolerant of frost.

Mukunu-wenna (Alternanthera sessilis), nahui (Alternanthera denticulate), Ludwigia species, and willow weed all look similar.

Replaces most other herbaceous species on water and dry land, causes silt accumulation, obstructs water usage, and causes flooding. Rotting vegetation degrades habitats for aquatic fauna and flora.