American water willow is a shrublike, but not woody perennial, with creeping rhizomes, often covering large areas along the edges of waterways. Flowers are clustered into headlike groups on stems arising from the upper branches; about ¾ inch long with a notched upper lip and a 3-lobed lower lip. The upper lip is light purple, rarely white; the lower lip is white or pale purple with purple markings. Blooms May–October. Leaves willowlike, narrow, opposite, sessile (stemless), 3–6 inches long.
Similar species: Lance-leaved water willow (J. ovata var. lanceolata), is uncommon and grows in swamps, bottomland forests, ditches, and other wet places in the Mississippi Lowlands (Bootheel swamps). Its flowers are loosely spaced along one side of an elongated, spikelike flowering stalk; each flower is light purple with darker purple markings on the lower lip.