Heads usually 2.5-3 cm. Cypselae oblong, ca. 5 mm. 2n = 32. Flowering spring-summer (May-Sep). Tidal marshes; of conservation concern; 0-5 m; Calif. Suisun thistle is known only from the Suisun Marsh (Solano County) in the Sacramento River delta.

Variety hydrophilum is listed by the U.S. Fish and Wildlife Service as endangered.