Cup plant is a tall perennial herb with square stems. Flowerheads numerous, to about 3 inches wide, yellow, with 18–35 ray flowers to 2 inches long. Blooms July–September. Leaves mostly opposite, to 1 foot long, with wavy, coarsely toothed margins (not lobed), rough on both sides, oval to triangular, covered with dots. Bases of opposite leaves are joined around the stem, forming a leafy cup that holds water.
Similar species: There are 6 Silphium species recorded for Missouri. Of these, cup plant, starry rosinweed (S. asteriscus), rosinweed (S. integrifolium), compass plant (S. laciniatum), and prairie dock (S. terebinthinaceum) are relatively common. Cup plant is identified by its square stems and unlobed leaves that join to form cups around the stem.