Practical information to identify and manage non-native, invasive plants and animals
The Quiet Invasion:
A Guide to Invasive Species of the Galveston Bay Area
Alligatorweed, pigweed
Alternanthera philoxeroides

USDA Symbol:ALPH
ITIS TSN:20770
Presence:Current Invaders
Habitat:Aquatic
Native Range:South America
Toxic Characteristics
This species is not known to be toxic.
Geographic Distribution
Most common in coastal areas from the Atlantic Coast in Virginia south to Florida, and west to the Texas; isolated populations also exist in California. Found in Chambers, Galveston, and Harris counties in the Lower Galveston Bay watershed.
Introduction Pathways
Most likely introduced to the United States from the ballast of ships around 1900.
Specific Primary Habitats
Grows in high nutrient freshwater aquatic habitats. Prefers to root in shallow water or saturated soils, later extending to open water. Once established, tolerates brackish water, water drawdown, and subsequent dry periods.
Identifying Characteristics
Stems are hairless, often pink, and may be several meters long. Submerged nodes of stems bear roots. Leaves are opposite, simple, sessile, thick, linear-elliptic, up to 9 cm long and 1.5 cm wide, narrowing at the base. Flowers lack petals, are made of small white sepals, and occur in small clusters at the end of a stalk.
Reproduction Characteristics
Blooms in early Spring and fruits soon afterward. Reproduction, however, is vegetative; United States populations do not produce viable seeds. When fragments containing one node detach, they travel downstream and root.
Growth Characteristics
Demonstrates extremely vigorous growth both in aquatic and terrestrial warm, moist environments. Between 1963 and 1973, the alligatorweed invasion expanded from 1,200 acres to 11,200 acres in Texas alone.
Ecological, Economic, or Social Impact
Alligatorweed forms thick mats that crowd out native aquatic vegetation, retard water flow, lower dissolved oxygen levels, and increase sedimentation. Flooding may result from impeded drainage. Can restrict water flow for irrigation. Inhibits fishing.
Control
Avoid introducing this plant to local waterways. Biological control has been very successful using Agasicles hygrophila, an aquatic flea beetle; used exclusively to curb Alligatorweed populations in most states. Mechanical removal results in numerous fragments which may lead to a population explosion.
General Notes
Agasicles hygrophila, the aquatic flea beetle used to control Alligatorweed, was first introduced to Texas in 1967. By 1972, populations were established in Brazoria, Harris, and Liberty county.

Native Species Alternatives
Below is a list of alternative plants that are native to the area:
  • Pickerelweed (Pontederia cordata)
  • Delta arrowhead (Sagittaria platyphylla)
  • Lizard's tail (Saururus cernuus)
This species belongs to the following lists:
Images
To view a larger version of an image, click on the thumbnail.
Flower stalk and flower of the alligatorweed plant. Photo courtesy of Robert H. Mohlenbrock, USDA NRCS PLANTS Database, www.forestryimages.org; Image Number 1237068.
Flower stalk and flower of the alligatorweed plant. Photo courtesy of Robert H. Mohlenbrock, USDA NRCS PLANTS Database, www.forestryimages.org; Image Number 1237068.
Flower stalk and flower of the alligatorweed plant. Photo courtesy of John D. Byrd, Mississippi State University, www.forestryimages.org; Image Number 1391180.
Flower stalk and flower of the alligatorweed plant. Photo courtesy of John D. Byrd, Mississippi State University, www.forestryimages.org; Image Number 1391180.
Alligatorweed growing from shore to shore of a waterway. Photo courtesy of Gary Buckingham, USDA Agricultural Research Service, www.forestryimages.org; Image Number 4723001.
Alligatorweed growing from shore to shore of a waterway. Photo courtesy of Gary Buckingham, USDA Agricultural Research Service, www.forestryimages.org; Image Number 4723001.
Foliage of the alligator weed plant; note opposite, non-succulent leaves. Photo courtesy of USDA ARS Archives, USDA Agricultural Research Service, www.forestryimages.org; Image Number 4723002.
Foliage of the alligator weed plant; note opposite, non-succulent leaves. Photo courtesy of USDA ARS Archives, USDA Agricultural Research Service, www.forestryimages.org; Image Number 4723002.
Flower of the alligatorweed plant showing stalked, short, head-like spikes, with a style. Photo courtesy of Gary Buckingham, USDA Agricultural Research Service, www.forestryimages.org; Image Number 4723003.
Flower of the alligatorweed plant showing stalked, short, head-like spikes, with a style. Photo courtesy of Gary Buckingham, USDA Agricultural Research Service, www.forestryimages.org; Image Number 4723003.
Page Updated/Reviewed: 07/14/2010 8:33 AM
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