Contributed by Mason Brock and Alan Weakley
A genus of 20-30 species, herbs, of e. North America.
ID notes: The number of ray flowers per head is a useful taxonomic character in Silphium; since only ray flowers are fertile, the number of ray flowers can also be estimated by the number of achenes in freshly fruiting material. The leaves of Silphium in the "Asteriscus group" are typically opposite at the lowermost nodes and alternate at the uppermost nodes subtending the inflorescence. However, mid-stem leaf arrangement remains a useful diagnostic character. Specimens of predominately opposite-leafed taxa that have regrown from damage earlier in the year will often develop alternate leaves at mid-stem. The key and taxonomic treatment is provisional.
Ref: Clevinger (2004); Clevinger (2006) In Flora of North America Editorial Committee (2006c); Cronquist (1980); Cruden (1962); Medley (1989); Perry (1937); Steyermark (1951); Sweeney (1970). Show full citations.