A giant on the prairies, the rosin weed grows in the blazing hot sunshine of the Illinois summer. It is part of that picture which remains to tell of the old prairie when plants like this one grew for miles and their resinous odor filled the late summer air.
Silphium laciniatum L.
July - September Prairie roadsides.
Rosin weed is one of the giant Silphiums which still are guide-posts to prairie conditions. Like the finger-posts of Spartina grass and Sullivant's milkweed, the Silphiums, in particular the rosin weed and prairie dock, indicate the presence of that dense black soil, unplowed and undisturbed since the days when the prairie itself was unplowed and undisturbed. The rosin weed grows tall. It is seldom less than five feet tall and often reaches twice that height. Its stout, densely hairy stem is tough and resinous; globules of white resin ooze out and are sticky along the stalk. The upper half of the stalk contains bright yellow sunflowers, two to three inches broad with a calyx of overlapping hairy, resinous bracts, the flowers set alternately and spirally around the stem. There may be a dozen or two flowers all in bloom at once. Below them are the remarkable leaves. They are thick, resinous, tough, so deeply lobed that they almost appear compound. On the wide, open prairies the leaves of this plant are often disposed so that they present their edges to north and south, hence the name of compass plant by which it is commonly called.