This weedy species is the only Asclepias in Ky. that has widely running roots (Stevens 1966, Wilbur 1976). In 1914, Gm stated: "common plant of pastures, meadows and roadsides... not difficult to eradicate; the occasional use of a hoe is sufficient in all ordinary cases." Although generally toxic to mammals, there are reports of browsing by deer (e.g. Hochwener et al. 2000); and for livestock "During droughts it may prove tempting" (Gm). A. syriaca is common across northeastern states, but it has a rather abrupt southern limit from Okl. to N.Car. (K). The species was present in Ky. during the early 19th Century (Short et al. 1833), but it has been spreading south during recent decades, especially along major highways (W). In the Mississippian Embayment, and even in the southern Interior Low Plateaus of Tenn., Ala. and Ga., it remains rare to absent; see also Cirsium arvense. Although apparently native in most of its range, syriaca is rarely recorded as a frequent species within supposedly "natural" vegetation. Within remnants of native grassland, it is generally associated with disturbed areas that have concentrations of alien plants (Taft et al. 2006). The species is concentrated along roads and in cropland, especially when "conservation reserve programs" allow plants to increase (Hartlzer & Buhler 2000). There is strong evidence that the recent decline in monarch butterflies has been caused by more effective chemical control of A. syriaca in or near cropland (Pleasants & Oberhauser 2012). The species can cross with some congeners (as reviewed by Y); 2n = 22 in most or all Asclepias species of Ky. However, no hybrids have been documented in Ky.