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A. philoxeroides is commonly encountered as a shallow-water aquatic plant. Photo courtesy UF/IFAS Center for Aquatic and Invasive Plants. Photographer Ann Murray.

Species Name: 
Alternanthera philoxeroides (Mart.) Gris.

Common Name(s):
Alligatorweed, Alligatorweed, Pig Weed

Synonymy:
Achyranthes philoxeroides (Mart.) Standl.
Alternanthera paludosa Bunbury
Alternanthera philoxerina Suess.
Bucholzia philoxeroides Mart.
Telanthera philoxeroides (Mart.) Moq.

Species Description:
Alligator weed, Alternanthera philoxeroides, is a non-woody perennial aquatic/shoreline plant found in Florida but non-native to the U.S.

Leaves and stems vary greatly in size and shape. Fleshy, succulent stems can grow horizontally and float on the surface of the water, forming rafts, or form matted clumps which grow onto banks. The horizontal stems (called stolons) may reach a length of 10 m. The leaves are opposite in pairs or whorls, with a distinctive midrib, and range in size from 5-10 cm (Environment Waikato undated, Virginia Cooperative Extension undated).

Fibrous roots arising at the stem nodes may hang free in water or penetrate into the sediment/soil. Flowers, which appear from December to April, are thin and clover-like in shape. The white flowers grow on stalks and are approximately 1.25-7.6 cm in length and 13 mm in diameter (Virginia Cooperative Extension undated).

Alligatorweed occurs in a range of habitats ranging from dry terrestrial to aquatic. To facilitate buoyancy, plants growing in aquatic habitats tend to have stems that are hollow and larger than those of plants growing on land (Julien et al. 1992).

Potentially Misidentified Species:
A number of non-native Alternanthera species occur in the United States, including sessile joyweed Alternanthera sessilis which is similar in appearance to A. philoxeroides and co-occurs with it in the IRL region of Florida. Mis-identification of these two species has had negative ecological ramifications elsewhere. A. philoxeroides was accidentally spread in Australia by members of the Sri Lankan immigrant community because it was mistaken for its congener A. sessilis which is the Indian herb mukunawanna (Hosking, et al 1996).

Several species of aquatic primrose (genus Ludwigia) and spiderwort (genus Tradescantia) that occur in Florida also have appearances and/or growth habits somewhat similar to those of A. philoxeroides.

II.  HABITAT AND DISTRIBUTION 

Regional Occurrence:
The native range of Alternanthera philoxeroides is reportedly the Parana River region of South America (Maddox 1968, Vogt et al. 1979). It can now be found elsewhere in South America, and also on the continents of North America, Australia, and Asia, and on a number of adjacent islands.

In the United States, A. philoxeroides occurs throughout the southeast from Virginia south to Florida and west to Texas. It has been reported as far north as Illinois, and now also occurs in California (Virginia Cooperative Extension undated, USDA).

IRL Distribution:
Distribution records indicate Alternanthera philoxeroides occurs in the northern IRL counties of Volusia and Brevard, and also at the far southern end of the IRL region in Palm Beach County (Wunderlin and Hansen 2004).

III. LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:
Alternanthera philoxeroides Alligator weed is a summer perennial that typically grows to around 1-2 m and whose horizontal stolons can reach 10 m

Abundance:
Alternanthera philoxeroides was substantially more abundant in the southeastern U.S. four decades ago than it is now. In 1963, over 65,000 ha of land in 8 southern states were overrun with invasive alligatorweed. Within two decades, however, the infested land in those states was brought down to only about 1% of that total, primarily through successful biological control (Coulson 1977). During this same period, however, the amount of infested land in Texas and Louisiana increased (Cofrancesco 1988). Biocontrol of A. philoxeroides is further discussed below.

Reproduction:
Reproduction in Alternanthera philoxeroides is predominantly through vegetative means; individuals rarely produce seeds, and when they do the seeds are typically non-viable. Vegetative growth occurs at the apical stem buds and axillary stem and root buds and the plant is spread through fragmentation (Julien et al. 1992, Virginia Cooperative Extension undated).

IV.  PHYSICAL TOLERANCES

Temperature:
Alligatorweed thrives in subtropical to cool climates; optimum growth temperature of alligatorweed has been experimentally determined to occur between approximately 15 and 20°C. The plant is susceptible to winter die-back of exposed portions at higher latitudes (Julien et al. 1992, 1995).

Salinity:
Ensbey (2004) reports that Alternanthera philoxeroides can withstand salinities of approximately 10% full strength seawater in static aquatic environments and up to 30% full strength seawater in flowing brackish water.

Hydrology:
The ability of this plant to grow in aquatic as well as terrestrial habitats allows it to persist in regions with pronounced wet and dry seasons, including Florida.

V.  COMMUNITY ECOLOGY

Trophic Mode:
Autotrophic (photosynthetic).