Although species status for glabrum can be justified (Sm; G. Nesom, pers. comm.), there has been much confusion among integrifolium, glabrum, trifoliatum and asteriscus, which appear to form a somewhat intergrading series from west to east. Uncertain records mapped here (open dots) are mostly based on possible transitions to (or confusion with) trifoliatum. S. glabrum occurs mostly in submesic, thinly wooded to brushy openings, mostly on base-rich soils in the southern Appalachians (occasionally along river banks), southern Interior Low Plateaus, and central Coastal Plain (Miss. to Ga.), extending north to s. Ind. (D, GH), c. Ky., w. Va. and perhaps W.Va. (FNA 21). It is generally uncommon in Ky. but locally abundant in some western localities, e.g. along Rt 171 in northern TODD, and along the Trace in Land-Between-the-Lakes (LYON, TRIG). Compared to typical trifoliatum, larger mid-stem leaves of typical glabrum are mostly 4-7 cm wide (versus 1-4 cm) with L/W = 2-3.5 (versus 4-6), broadly cuneate to truncate (versus tapering more gradually to a less distinct petiole), usually subentire (versus serrate), and strictly opposite (versus opposite or in whorls of 3). Lower leaf surfaces and stems tend to be less scabrous-hispid to glabrate, but this is not diagnostic. Plants tend to be shorter (ca. 1-1.5 m versus 1.5-2 m), with more compact inflorescences, the longest peduncles at each node mostly ca. 2-5 cm (versus 5-10 cm). Also, glabrum may tend to form larger clonal patches, with rhizomes; in contrast, trifoliatum usually occurs as isolated stems or small clumps.