Ptilimnium costatum, commonly called big bishopweed,[1] is a species of flowering plant in the carrot family (Apiaceae).

[1] Its natural habitat in wetlands, such as swamps, marshes, and wet prairies.

[1][2] Ptilimnium costatum is a robust perennial, growing to 150 cm tall.

It flowers and fruits from June to October, which is generally later in the season that other Ptilimnium in its range.

In addition, it can be distinguished from other nearby Ptilimnium by its longer fruit styles (1–2 mm) and perennial habit from a corm base.