Alligator Weed Alternanthera philoxeroides (Martius) GrisebachNon-native  -  Invasive    Synonyms: Alligator-weed, Alligatorweed.
Kingdom Plantae   >   Division Tracheophyta   >   Class Magnoliopsida   >   Order Caryophyllales   >   Family Amaranthaceae   >   Genus Alternanthera   


Alligator Weed is a recent non-native invader of Maryland's piedmont waterways. First found in Maryland near College park and along the Potomac River near Great Falls, Alligator Weed should be looked for in aquatic environments throughout Maryland. Alligator Weed is native to the Parana River system in South America. It was first introduced to the United States in the late 1890's in Mobile, Alabama and New Orleans by the emptying of ship ballasts. Alligator Weed quickly spread through the southeast and is now recorded in all southeastern states and California. The Maryland populations represent the farthest north this invasive plant has been recorded in the United States. Alligator Weed can form dense floating mats that block sunlight and can cause changes in the chemical and biological composition of infected waterways resulting in death of native organisms. Alligator Weed may also grow along the banks of waterways creating dense stands that crowd out native plants thus altering the makeup of the eco-system.


From the Texas A&M AgriLife wesbsite: Aquatic stems are hollow and can be single or branched. Leaves are opposite, long, elliptical or lance-shaped up to 3/4 inch wide and 5 inches long with a prominent midrib. Roots often develop at leaf nodes. Soft, whitish hairs are found in the leaf axis. Flowers are singular, small (about 1/2 inch in diameter), white, fragrant clusters of 6 to 10 florets, borne on long branches (to 3 inches). The flowers resemble those of white clover.

Where to find:

Thankfully only two populations of this highly invasive plant are known to have been discovered in Maryland. It is not known if these populations have become eradicated or are persisting. If you find a population of Alligator Weed in Maryland please alert us right away and we will let the appropriate authorities know of its existence.

There are 3 records in the project database.

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Alligator Weed in Prince George's Co., Maryland (9/17/2014). Photo by John Hall. (MBP list)

Alligator Weed in Prince George's Co., Maryland (9/17/2014). Photo by John Hall. (MBP list)

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