Sium is a genus of flowering plants in the family Apiaceae. It is widely distributed across Europe, Asia, Africa, and North America. Plants of this genus are commonly called water parsnips. These are perennial herbs, including some fully aquatic plants and some species that grow near water. The branching stem roots at the nodes. The lower leaves are pinnate, divided into leaflets. They are borne on petioles that sheath the stem at their bases. The inflorescence is a compound umbel of flowers with white petals. There are about 12 species in the genus. Species include: