Alligator weed

Alligator weed (Alternanthera philoxeroides) forms dense mats that grow either rooted in mud or floating on water. Leaves are opposite, lance-shaped, smooth-edged, 2–4 inches long; the diagnostic feature is the single small white clover-like flower at each leaf axil and the hollow jointed stems. Highly resistant to most aquatic herbicides. Biological control with the alligator weed flea beetle (Agasicles hygrophila) is the only consistently effective method across the Southeast.