Silphiums are classic prairie plants, once blossoming across almost two-thirds of the American continent. Robust and showy, the different species of silphium can be best distinguished by their leaves.
Compass plant (Silphium laciniatum) has deeply lobed leaves that tend to align in a north-south direction. Prairie dock (S. terebinthinaceum) has huge, coarse, unlobed basal leaves. Cup Plant (S.perfoliatum)’s leaves unite to form a cup around the stem.
“What a thousand acres of silphiums looked like when they tickled the bellies of the buffalo is a question never again to be answered, and perhaps not even asked.“
-Aldo Leopold, A Sand County Almanac.
Rosinweeds, of which there are a number of species, have oval mostly opposite leaves that are closely spaced along their stems. Among them, Silphium integrifolium has the widest range, and is found from Ontario to Texas, and west to Wyoming and New Mexico.
Value for Birds
Silphiums offer birds all three of their basic needs – food, cover, and water. In summer, the nectar-filled flowers of cup plant feed hummingbirds. Later as summer wanes into autumn, goldfinches flock to the silphiums to feast on their nutritious seeds, which are also eaten by other songbirds and by sharp-tailed grouse. The dense foliage of silphiums, particularly cup plant and rosinweed, provide protection for the birds from predators and harsh weather. The “cups” of cup plant retain water after rainfalls, and are popular bathing pools for small birds.
Other Wildlife Values
Cup plant nectar not only feeds hummingbirds, but also bees and butterflies. Tree frogs have been seen sitting on their “cups,” and butterflies, other insects, and small mammals come for a drink. Silphium weevils feed and lay their eggs on the plants. The seeds of silphiums are eaten by mice, and deer sometimes graze on the young plants.
Landscape Notes
The three taller silphiums – compass plant (three to ten feet), prairie dock (two to ten feet), and cup plant (three to eight feet) – can be used as bright, bold backdrops for sunny perennial beds. They are also useful for borders or screens. The bright blossoms and dense foliage of rosinweeds (two to four feet) make them excellent additions within flower beds. Cup plant grows well in both full and partial sun, but the other three Silphiums need full sun.
Cup plant can be aggressive, but this may be desirable in some situations. For example, it is one of the few plants that compete well with the invasive reed canary grass.
All silphiums are choice species for prairie and meadow plantings. Cup plant does the best in moist to wet conditions, so is also well suited for the background of rain gardens and for wet prairies. The other silphiums do well at the edge of rain gardens or in more mesic prairies.
I think of the taller silphiums as the American equivalent of the hollyhock. Although the blossoms of silphiums last only a short time, new flowers open in succession for up to three weeks. Their distinctive foliage will add interest to your landscape throughout the year.
Also of Interest
Silphiums are distinguished from sunflowers (Helianthusspecies) by their flowers. Like most plants in the composite family, both have ray and disk flowers in their flower heads. In silphiums, the conspicuous yellow ray flowers are fertile and produce seed, while the central disk flowers are sterile. The opposite is true for sunflowers – the central disk flowers form the seeds.
The resin that forms after the flower heads of a compass plant have been removed has been used as chewing gum by Indians and pioneers. Several other silphiums also have edible sap.
Grazing has been an important factor in the loss of silphiums (and other species) from prairies. According to John Curtis in his classic book, Vegetation of Wisconsin, cattle and horses seek out compass plant and prairie dock “like hidden candy at a child’s birthday party,” and can wipe them out within two years.