Having lived in Illinois for 32 years, I became very aware of prairies and prairie plants. Illinois is synonymous with the prairie, which is about the last thing that comes to mind when you think of Wisconsin vegetation. Historically, however, prairies and oak savannas (widely spaced trees with prairie between) were found in the Southwest half of the state, essentially south of a line from Ozaukee County to Polk County. Vast prairies have long ago gone the way of the plow and urbanization, but remnants still exist along fence lines and railroad rights-of-way. The oldest, and perhaps the finest restored prairie in the country can be seen at the University of Wisconsin Arboretum in Madison. Built through the visions of Aldo Leopold and John Curtis and with the help of the Civilian Conservation Corps, it has been a living laboratory for over 80 years.
Synonymous with the prairie are plants of the genus Silphium, commonly called Rosinweeds. Rising above the grasses and lesser forbs, their bright yellow composite flowers wave in the breezes and their impressive leaves soak up the often oppressive prairie sun. Though roughly 20 species have been identified in the Eastern US, only four are native to Wisconsin.
Silphium integrifolium (Prairie Rosinweed) is found in southern Wisconsin but not in Ozaukee County. It flowers from June to early September. Mature height is 4 to 6 feet. Silphium laciniatum (Compass Plant) is also a southern Wisconsin species with its range touching on southern Ozaukee County. The flowers appear in July and August on top of 6 to 12 foot stalks. The thick, dissected leaves are up to 15 inches long. This plant can live up to 100 years. Silphium perfoliatum (Cup Plant) is native to the Southwest half of Wisconsin. It flowers from July through September and is a prolific seeder. It is called cup plant because the leaves are opposite and joined around the stem, resulting in a cuplike attachment. This precocious plant reached 8 feet in my garden in just its second summer, and now after 7 years it is over 12 feet tall.
The fourth native species is Silphium terbinthinaceum (Prairie Dock). In Wisconsin it is found only in the Southeast corner of the state, not in the driftless area of Southwest Wisconsin. Flowers appear from July to early September, later than other Silphiums. The large basal leaves are thick and leathery and have a sandpaper-like feel. As the plant gets older, the leaves get larger until the mature size is reached. The morphology of this and other Silphium species make them highly resistant to hot dry conditions, like are often found on the prairie. Along with Compass plant, the leaves of Prairie Dock align their edges in a north-south direction (blades east-west). It is said that settlers when lost on the prairies on dark nights would find their way by feeling the direction of the leaves. Prairie Dock can be 8 feet or more tall. Its stout taproot can knife through the heavy prairie soil to a 12 foot depth. The height of the plant usually keeps the flowers above taller grasses such as big bluestem and Indian grass. Resistant to fire and grazing, this plant can outlive the most geriatric human.
Aldo Leopold, in the classic book A Sand County Almanac said, “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.” He continues, “Silphium first became a personality to me when I tried to dig one up to move to my farm. It was like digging an oak sapling. After half an hour of hot grimy labor the root was still enlarging, like a great vertical sweet-potato. As far as I know, that Silphium root went clear through to bedrock. I got no Silphium, but I learned by what elaborate underground stratagems it contrives to weather the prairie drouths (sic).”
Carolus Linnaeus named the genus Silphium because the plants have abundant resinous juices (the word Silphium is from the ancient Greek name of some currently unknown resinous plant.) Compass plant is sometimes called Rosin Weed because of the resin. The resin of another Silphium was noted by the early naturalist William Bartram as being used by the indigenous Creeks and traders in Alabama for a chewing gum to “cleanse their teeth and mouth and sweeten their breath.” Native American children chewed gum gathered from the upper parts of the stem of the Compass plant. The gum is produced during flowering, and tastes somewhat like tar.
Allan Armitage in Herbaceous Perennial Plants said that Silphiums “are not used more often in gardens mainly because they are so big (some growing 7 to 8 feet tall) and rather weedy looking. They look much better with a prairie sky in the background than a two-story bungalow, anyway. However, what is too big and tall to one person is bold and impressive to another.” Impressive, they certainly are. Use them with discretion and they can provide an exclamation point in your garden.
The Cottage Gardener 