Prairie Dock
Silphium terebinthinaceum
Aster family (Asteraceae)

Description: This plant has a vase-like rosette of large basal leaves that are spade-shaped (cordate). Each leaf has a thick sandpapery texture, particularly on the underside, and is up to 18" long and 12" wide. On young leaves, the upper surface is relatively hairless and shiny, while older leaves become dull and rough. The lower surface is light green and pubescent. The leaf margins are coarsely serrated or dentate. The petioles are about 6" long, rather narrow and wiry, and light green or brown. The overall appearance is similar to a rhubarb plant, except the petioles are more slender. A naked flowering stalk emerges from the base of the plant, ranging in height from 3' to 10' in height. This stalk is green or red, and largely hairless. The upper part of the stalk divides gracefully into a panicle of yellow composite flowers and spherical green buds. Each compound flower is about 2-3" across, and consists of 15-30 ray florets with rather pointed tips surrounding numerous disk florets. There is no noticeable floral scent. The blooming period usually occurs from late summer to early fall, and lasts about a month for an individual plant. Prairie Dock usually blooms later than other Silphium spp. It has a stout taproot that can penetrate the soil to about 12' deep, and may form offsets only a short distance away from the mother plant. The rather light, flattened achenes can be carried several feet by the wind; they are without tufts of hair.

Cultivation: The preference is full sun, a deep loamy soil, and moist to slightly dry conditions. Rocky or gravelly soil is tolerated. Drought tolerance is very good. Prairie Dock is rather slow to develop, but very reliable and nearly indestructible when mature. Patches of brown may develop when the leaves are damaged by drought, windstorms, or passing animals. It is a long-lived plant.

Range & Habitat: Prairie Dock occurs in most of Illinois, except for a few counties in the south and NW. It is a common plant. Habitats include moist to dry black soil prairies, gravel prairies, shrub prairies, hill prairies, savannas, seeps, limestone glades, and prairie remnants along roadsides and railroads. The exceptional height of the flowering stalk is probably useful in keeping the flowers above the taller grasses, such as Andropogon gerardii (Big Bluestem). Recovery from occasional wildfires is very good.

Faunal Associations: The composite flowers attract long-tongued bees primarily, including honeybees, bumblebees, and Miner bees. Other flower visitors include Halictine bees, bee flies, and the Ruby-Throated Hummingbird. The grubs of Okanagana balli (Prairie Cicada) feed on the large taproot; this is an uncommon prairie insect that emerges during mid-summer. Both the larvae and adults of the Silphium Beetle (Rynchites sp.) feed on the flowerheads and stems of this and other Silphium spp. The larvae of Antistrophus rufus and Antistrophus minor (Gall Wasp spp.) feed on the flowering stem of Prairie Dock, forming invisible galls. The larvae of these species attract the hyperparasitic wasp Eurytoma lutea, whose larvae feed on these gall-formers. The larvae of Mordellistena aethiops (Tumbling Flower Beetle sp.) also feed within the stem, while the adults may feed on the flowerheads. The oligolectic Iowana frisoni (Aphid sp.) sucks juices from the flowering stem. Goldfinches eat the seeds, and may help to disperse them. Large mammalian herbivores, such as cattle and the American Bison, readily eat the foliage and stems of Prairie Dock; however, the coarse sandpapery leaves of mature plants are not attractive to the Cottontail Rabbit, which prefers vegetation that is shorter and more tender.

Comments: This is a typical plant of mesic black soil prairies that competes successfully with tall prairie grasses. It has an impressive and unusual appearance, like something out of the Stone Age when Columbian mastodons and other mega-fauna inhabited the prairies of North America.