Prairie Dock (Prairie Rosinweed)

Scientific Name

Silphium terebinthinaceum

Family

Asteraceae (daisies, sunflowers)

Description

Of Missouri’s six rosinweeds, prairie dock is identified by its huge, leathery, toothed, unlobed leaves, which are nearly all in a basal whorl. Only a few small leaves grow on the stem.

Prairie dock is a tall perennial herb with woody taproots. The flower stalk is very slender, with reduced stem leaves. The foliage has a pleasant scent, somewhat like turpentine.

The flowerheads are one to few in an open inflorescence on a long stem; yellow, with 15–21 rays.

Blooms July–October.

The leaves are almost all basal, very large, to 16 inches long, heart- or spade-shaped, with coarse teeth, on a long petiole (leaf stem), thick, leathery, and rough like sandpaper. The leaves develop all summer and are present at flowering time.

Sometimes, you might encounter big colonies of plants having only leafy rosettes, with few or no flowering stems developing.

Similar species: Six Silphium species are recorded for Missouri. Of these, prairie dock, starry rosinweed (S. asteriscus), wholeleaf rosinweed (S. integrifolium), compass plant (S. laciniatum), and cup plant (S. perfoliatum) are relatively common.

Prairie dock is identified by its whorl of very large, wavy-edged, unlobed basal leaves (which look something like those of the unrelated curly dock), and by its having only small, bractlike leaves on the stem.

To separate rosinweeds from the similar-looking sunflowers (genus Helianthus), see Ecosystem Connections.

Size

Height: to 10 feet.

image of Prairie Dock (Prairie Rosinweed)

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.

Especially, look for it in areas with limestone or dolomite rock.

Status

Native Missouri wildflower.

Human Connections

Prairie dock is a tough, showy native flower that's great for the back of a garden. With its dramatically large whorl of upright basal leaves, and its six-foot tall flowering stems, it can also be grown as a specimen plant in a prominent place.

Although the resinous sap has an odor somewhat like turpentine, grazing animals find this plant palatable.

As with other rosinweeds, the gummy sap that this species exudes was used by Native Americans and pioneers as a kind of chewing gum.

The species name, terebinthinaceum, means “with turpentine” and refers to the rosin, which gives this plant its pleasant, spicy scent. Extra points for pronunciation: TEAR-uh-BINTH-uh-NAY-see-um.

Terebinth trees, native to the Mediterranean region, are unrelated (in the cashew family); their sap was the original source for turpentine. (The word turpentine was derived from the word terebinth.)

Ecosystem Connections

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.

Like its relative the 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. This is an adaptation for life in hot, dry prairie and glade habitats.

Prairie dock, compass plant, and many other native prairie perennials grow tough, deep taproots that extend 10 to 15 feet or more into the soil.

Over the centuries, the living and dying of these deep-rooted plants caused the deep soils of the Great Plains to become some of the richest in the world.
The Dust Bowl in the 1930s occurred after the deep-rooted native plants were plowed up and replaced with shallow-rooted row crops.
American farmers still benefit from the vast prairies that once dominated the center of the continent.

Prairie dock and other rosinweeds are in their own genus, Silphium, and not in genus Helianthus (sunflowers). Yet the two groups look so much alike! How can you tell the difference? It has to do with which parts of the compound flowerheads produce seeds:

The disk (center) florets in rosinweeds are staminate (male, producing only pollen) and therefore don’t create seeds; meanwhile, the disk florets in sunflowers are pistillate and create seeds. Then, in rosinweeds, it’s the petal-like ray florets that are pistillate (female) and turn into seeds, while those in sunflowers produce only pollen.
Due to the above fact, the disks in rosinweeds tend to be smaller in diameter than the disks of sunflowers.
Looking beneath the flowerhead, the green, leafy outer involucral bracts in rosinweeds are comparatively large and broad compared to those in sunflowers.