These are strikingly large plants, with big heads and usually very large leaves. Both surfaces of the leaves (or chiefly the upper in S. integrifolium) are strongly roughened with small, pale, hard papillae, these sometimes with a tiny sharp tip.
Three of our species, Silphium integrifolium, S. laciniatum, and S. terebinthinaceum, may occur along railways, though usually in "prairie remnants." It is therefore possible that some outlier populations are recent disperals along railway rights-of-ways.
1. Principal leaves all opposite (occasionally whorled).
2. Leaf bases sessile (or even slightly clasping) but not connate, on a ± terete stem.
2. Leaf bases connate around a strongly 4-angled stem.
1. Principal leaves alternate or basal.
3. Leaves alternate, deeply pinnately lobed.
3. Leaves basal, unlobed (occasionally one or more much-reduced cauline leaves present).
All species found in Silphium
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. June 23, 2017. http://michiganflora.net/genus.aspx?id=Silphium.