Sium suave is an herbaceous wetland plant with compound alternate leaves and one or more large inflorescences of numerous small white flowers. There are several other species of roughly similar appearance and the reader is urged to use a reliable key to identify plants in this group. Leaves are highly variable, ranging from once pinnately compound on drier sites to several times divided into very narrow segments on wetter sites, and many intermediate forms between those extremes. Two or more of the different forms can sometimes be found on the same plant if water levels changed significantly during development of the plant.