Silphium terebinthinaceum aka Prairie Dock

Silphium terebinthinaceum is a long-lived perennial in the aster family. Gardeners know it as prairie dock or prairie rosinweed. Its native range spans southeastern Canada and central and eastern America. It grows in prairies, dry woods, and open glades. It also grows beside roads and rail lines. Its large leaves and tall flower stems give it a coarse form.

Garden plants reach about 4–10 feet (1.2–3 metres) tall. RHS gives a higher limit of 13 feet (4 metres). Plants spread about 1–3 feet (0.3–0.9 metres). RHS allows a spread of 3.3 feet (1 metre). The leaves form a rosette close to the ground. Each leaf can reach 1.5 feet (0.46 metres) long. They can measure 1 foot (0.3 metres) across. These green leaves have rough faces and toothed edges. Fine hairs cover their pale undersides. Their shape ranges from oval to heart-shaped. Leaves often stand upright along a north-south line. Smooth green or reddish stems carry few leaves. They branch near their tops, and the foliage dies back seasonally.

Yellow flower heads form loose groups above the leaves. Each head has yellow rays around a yellow centre. The heads measure about 0.17–0.25 feet (0.05–0.08 metres) across. Bloom can run from July through October. Native bees pollinate the flowers, and hummingbirds also visit. Bees use the plant for nest materials and shelter. The small, flat fruits are achenes that mature in autumn. Birds, including goldfinches, eat the seeds. The plant also draws butterflies.

Prairie dock flowers best in full sun but accepts partial shade. It prefers deep soil that drains well. Loam suits it, though it accepts clay and rocky soil. RHS favours moist, heavy soil with neutral to alkaline pH. Other sources note tolerance of dry soil and drought. Chicago lists USDA zones 3–8; RHS rates it H7. Its deep taproot helps explain its slow start. Plants may wait two or three years before flowering.

Use prairie dock behind shorter plants in borders or meadows. It also suits prairie, native, and cottage garden plantings. Care needs are low, with few serious pests or diseases. Stems may lean, though they seldom need support. Grow it from seed or divide plants. Leave old stems until new growth starts, or cut after bloom. PFAF gives it no food rating and reports no known hazards. It records stem resin chewing and past medicinal uses. Native Americans used root tea and plant smoke as remedies.