This is a widespread, largely midwestern species, but apparently adventive in New England (K). It is usually associated with open riparian woods or edges, and it prospers locally in derived farmland, where its tall stems can push through thickets and vines along weedy gullies, ditches and fencerows. It can also form large dense patches when cultivated in full sun, and may be suitable for biogas production (Gansberger et al. 2015). Like most Silphium species and other perennial Heliantheae of better soils (e.g. Anderson et al. 2001), perfoliatum appears to have been much reduced in some areas by livestock and dense populations of deer, which can prevent plants from flowering (R. Seymour, pers. comm.; D. Dourson, pers. comm.). Also, stem boring insects (perhaps the moth Eucosma giganteana) often kill flowering tops in Ky. (pers. obs.). There is subtle variation across the range of this species, which needs further analysis (Y). Some authors (e.g. Cr, FNA 21, W) have distinguished plants named S. connatum L. or S. perfoliatum var. connatum (L.) Cronq. These have more pubescence (especially on the stems), leaves less narrowed to a petiolar base, and heads usually bearing only ca. 8 or 13 rays (versus 21 or 34). S. connatum is known from central Appalachian regions (W.Va., Va., N.C.), including the Kanahwa Rv. and New Rv. valleys of s. W.Va. (SC), and it may be expected along the Big Sandy Rv. in Ky. (Michaux 1803; M). An unusually robust variant from Arkansas (?) has been cultivated near Knoxville, Tenn., up to 3.75 m tall, with leaves less connate and more lobed, and with more rays in flowering heads (T. Avent, Juniper Level Botanic Garden, pers. comm.). Hybrids with other species have not been documented in the wild, but introgression into integrifolium is suspected in Mo. (J. Thomas, pers. comm.); and Settle & Fischer (1972) did make some artificial hybrids.