Alligatorweed is a perennial plant with two growth forms: aquatic and terrestrial. Both forms often create dense mats; those of the aquatic form may be floating. Both forms are stoloniferous and have opposite, stalkless leaves typically up to 5½ in (14 cm) long. The aquatic form produces hollow stems that emerge above water, and its leaves are long, lance-shaped, and smooth. Stems of the terrestrial form grow much shorter than the aquatic form and are less hollow with shorter, rounder leaves. The flowers of both forms appear in ball-like clusters on long stalks. There are 6–20 flowers in each cluster, and each flower has five white, papery sepals that resemble petals. Seeds are smooth and lens-shaped, but are not typically produced in North America.