Prairie dock is a tall perennial herb with woody taproots, the flower stalk very slender, with reduced stem leaves. Flowerheads one to few in an open inflorescence on a long stem; yellow, with 15–21 rays. Blooms July–October. Leaves almost all basal, very large, to 16 inches long, heart- or spade-shaped, with coarse teeth, on a long petiole, thick, leathery, and rough like sandpaper. The leaves develop all summer and are present at flowering time. The flower stalk rises in early fall. The species name means “with turpentine” and refers to the rosin, which gives this plant a pleasant scent.
Similar species: There are 6 Silphium species recorded for Missouri. Of these, prairie dock, starry rosinweed (S. asteriscus), rosinweed (S. integrifolium), compass plant (S. laciniatum), and cup plant (S. perfoliatum) are relatively common. Prairie dock is identified by its large, basal, unlobed leaves with only small, bractlike leaves on the stem.
Height: to 10 feet.
Scattered, mostly in the Ozark and Ozark Border Divisions, in central and northeastern Missouri.
Habitat and Conservation
Occurs in glades, upland or rocky prairies, tops of bluffs, savannas, openings of dry upland forests, and rarely banks of streams; also old fields, railroads, and roadsides. Look for it in areas with limestone or dolomite rock. Like its relative compass plant, the leaves of prairie dock often are oriented north and south, maximizing morning and afternoon sun for photosynthesis and minimizing water loss from midday heat.
It’s not an accident that rosinweeds are in their own genus, Silphium, and not in the genus Helianthus (sunflowers). Yet some of them look much alike. How can you tell the difference? The disk florets in rosinweeds are essentially staminate (male) and therefore don’t create seeds, just pollen; but the disk florets in sunflowers, as most of us know, create seeds. The petal-like ray florets in rosinweeds are pistillate (female) and turn into seeds, while those in sunflowers are sterile.
Although the resinous sap has a turpentine-like odor, grazing animals find this plant palatable. As with other rosinweeds, the gummy sap that it exudes was used by Native Americans and pioneers as a kind of chewing gum. It is a tough, showy native flower for the back of a garden.
Birds, including goldfinches, eat the seeds. Many insects visit and pollinate the flowers. If you inspect a rosinweed plant closely, you will find that many insects live, hunt, eat, and mate among the leaves and flowers. Also, the larvae of some wasps grow with in the stems, forming galls.