Download Common Name: Alligatorweed Scientific Name

Common Name: Alligatorweed
Scientific Name:
Alternanthera philoxeroides
Order: Caryophyllales
Family: Amaranthaceae
Wetland Plant Status: Obligatory
Ecology & Description
The stems of alligatorweed are long (up to 4
ft), hollow, and branched to allow the plant
to float. The leaves are opposite, elongated, and elliptical with smooth edges. Leaves have a
defined midrib with small pinnate veins. The plant produces a small cluster of white flowers
during the warm parts of the year. The flowers are fragrant and consist of 6-10 florets and
produce one small seed.
Habitat
The plant roots in shallow water (less than 6 ½ ft) and then begins to grow out from the anchor.
This can be problematic as it can choke off entire waterways. The plant grows in segments and
can grow roots or stems out of the nodes that separate each segmented piece.
Distribution
In the United States, alligatorweed is found from the southern marshes of Virginia to southern
Florida and westward to Texas and is found in some parts of California.
Native/Invasive Status
Alligatorweed is a perennial non-native species of plant from South America that was
accidentally introduced in the state of Florida. It is considered invasive in the United States, New
Zealand, China, Australia, and Thailand. Alligatorweed is also considered to be a noxious plant
because it disrupts water flow and aeration when it becomes thick. In times of high rain fall it
can lead to flooding due to its clogging of the waterways.
Wildlife Uses
Mats of alligatorweed can be good habitat for many aquatic invertebrates and small fish that may
serve as a food source for wildlife. However, these mats can also serve as a breeding ground for
mosquitos. Alligatorweed has no direct food value for wildlife.
Management & Control Techniques
The U.S. has introduced several non-native insect species to combat the growth of alligatorweed
such as the alligatorweed flea beetle (Agasicles hygrophila),alligatorweed thrip (Amynothrips
andersoni), and alligatorweed stem borer (Arcola malloi). These insects have had varying
results. The flea beetle eats the leaves off of the plant and has been known to defoliate entire
mats in weeks. The stem borer is a moth that lays its eggs on the stems of the plant and, upon
hatching, the young bore into the plant. They move through the stems feeding on the inside of
the plant’s hollow body, quickly causing it to wilt and die. Thrips deform the leaves of
alligatorweed, but the damage is relatively minor and scattered.
Alligatorweed can run native plant species out of an area due to its fast growth rate and hardy
disposition. It can regenerate itself from fragmented stems or nodes that have been severed.
Therefore, cutting the plant can do more harm than good because the plant could then root
downstream and start a new mat. While some chemical treatments can kill alligatorweed, they
also cause harm to native plants and retreatment is necessary to kill the plant.
Reference
Natural Resources Conservation Service (NRCS). 2015. PLANTS Database. Plants Data Team,
United States Department of Agriculture, Greensboro, North Carolina.
<http://plants.usda.gov/core/profile?symbol=ALPH> Accessed 28 July 2015.
This document was authored by Andrew Hill, School of Forestry, Louisiana Tech
University, April 2015.