Alligatorweed

What is alligatorweed?

Alligatorweed is a South American plant that arrived in the Southeast in the late 1800s, probably in ballast water, and has been working its way north ever since. It roots along the bank and then sprawls out across the water in floating mats, and it also invades wet fields. Good alligatorweed identification starts with three features: hollow stems, leaves in opposite pairs and small white flower heads on stalks.

Its mats can stretch more than 30 feet from shore and stand a few feet thick. They shade the water, crowd out native plants and keep new seedlings from starting in spring, cut off shoreline access for anglers and wildlife, harbor mosquitoes, and can pile up against dams, adding to flooding and erosion. On land it can compete with row crops.

Alligatorweed identification: what to look for

Snap or slice a stem and look inside, then check how the leaves attach. On land the stems can be more solid, so look at plants growing in or near water for the hollow-stem test, and confirm with the leaf arrangement and flower heads.

• Stems: Round and hollow in water plants, with a thin wall across the inside at each joint. Pale green with swollen nodes that turn pink, purple or brown with age; stressed stems can redden.
• Leaves: Opposite pairs from the swollen joints, glossy and slightly succulent, narrow and pointed, about 1 to 3 inches long, attached directly to the stem.
• Flowers: Rounded white heads about 1/2 to 3/4 inch wide, papery like dried clover, each held on a stalk that rises from an upper leaf joint.
• Runners: Thick, jointed horizontal stems root at each node and can stretch tens of feet.
• Mats: Tangled stems form floating islands that start at the bank and push out over open water.

Where it grows in the United States

Alligatorweed is most common across the Southeast and Gulf Coast, where it forms scattered dense patches in wetlands, and it is spreading northward. In North Carolina it is widespread on the Coastal Plain but rare in the mountains, and it also grows in California. It suits roughly USDA hardiness zones 7 through 10.

Look for it along pond and lake edges, in drainage canals and roadside ditches, in marshes and slow streams, and in low, wet farm fields. It handles fresh or slightly salty water and sandy to clay soils, and it moves downstream every time a piece breaks loose.

Alligatorweed look-alikes

A few plants get mixed up with it:

• Water primrose (Ludwigia spp.): Grows in the same wet edges, but its stems are solid rather than hollow.
• Smartweeds and knotweeds (Polygonum spp.): Common on wet soils and shorelines, but their leaves alternate along the stem instead of growing in opposite pairs.
• Other joyweeds (Alternanthera spp.): Related species with similar white flower clusters, but their flower heads sit tight against the stem without a stalk.

How to get rid of alligatorweed

Pulling, digging and mowing: Pulling, raking and cutting usually make things worse, because every broken piece can root and start a new mat. If you remove a small patch by hand, bag every fragment and keep disturbance to a minimum, and check the area often for regrowth. In reservoirs where water levels can be managed, raise or lower water in step with treatments so loose mats do not float off and spread.

Cultural control matters most, because the right conditions keep it from coming back.

• Disturb infested areas as little as possible, since breaking stems spreads the plant.
• In the warmest parts of the South, the alligatorweed flea beetle, a released biological control insect, can keep it in check; it does not reliably survive colder winters.
• After clearing a pond, sterile grass carp (where permitted) can help keep other weeds from filling the gap.
• Never move plants or soil from an infested ditch or pond to a clean one.

Herbicides: Mississippi State University Extension lists glyphosate, imazamox, imazapyr and triclopyr for alligatorweed. A treatment typically holds it for two to three months, so repeat applications are common in long growing seasons and dense stands, and pairing imazapyr or imazamox with triclopyr or glyphosate may work better than any one product alone. Spray when air and water are at least 60 degrees F and plants are actively growing, and wet all exposed foliage. Aquatic formulations of glyphosate and triclopyr are easier on nearby plants, while imazapyr and imazamox can injure desirable plants through their roots. With imazamox or imazapyr in a pond, treat about a third of the pond at a time with at least a week between rounds. Use only products labeled for the site, wear the protective equipment the label lists and follow it exactly; the label is the law. Check with your state natural resources agency about any permit needed before treating water. If you hire someone to spray, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Keeping it from coming back

Treat a new patch as an emergency: Mississippi State Extension advises wiping it out as soon as it appears. Clean mud and plant bits off boats, mowers and ditch equipment before moving to another site, and never plant or share it.

Questions we get about alligatorweed

Similar weeds

Common DuckweedLemna minorCurly-Leaf PondweedPotamogeton crispusEurasian WatermilfoilMyriophyllum spicatum