This poorly understood species appears largely restricted to open woods on xeric limestone blufftops in c. Ky. and c. Tenn. There may also be outlying Appalachian populations, perhaps with independant hybrid origin, in Lee Co. of w. Va. (TENN) and even Wyoming Co, of e. Penn. (PH). In Ky., svensonii forms distinct populations along the Palisades of the Kentucky Rv., especially in the downstream section near Frankfort (FRAN). But it also appears to hybridize extensively with hystrix in some localities, and occasionally with virginicus. An outlying coll. from the knobs of ESTI (KY) appears hybridized with virginicus. The morphology and distribution of svensonii suggests origin from hybridization between hystrix and the more western species, canadensis, perhaps during a cooler or drier period when canadensis extended east into the Interior Low Plateaus and central Appalachians (Church 1967, Campbell 2002a). The name E. interruptus Buckl. was formerly applied to plants that later became described as svensonii; see FNA 24 for notes on these and other taxa in the hystrix group. E. churchii J.J.N. Campb. is particularly close to svensonii; it occurs mostly in the Ozark-Ouachita region; similar plants have been recently discovered along the Tennessee Rv. in n. Ala. (Schotz & Datillo 2012) and perhaps s. Tenn. (D. Estes, pers. comm.), but their status remains somewhat uncertain.