In Ky. this shrub ("usually not rhizomatous" according to FNA 8) is restricted to dry acid soils of Appalachian regions (K, W). See notes under cumberlandense, which has been confused. Both species have yellowish to red "xanthic" flowers of varied hue (versus pinkish red or whitish in other Ky. azaleas), which smell acrid (versus musky or spicy). R. calendulaceum is considered to be an allotetraploid derived from ancestors of cumberlandense and prinophyllum (FNA 8). Epps et al. (2015) have shown that butterfly wings, especially those of eastern tiger swallowtails, are the major means of pollination for calendulaceum; such effects are rarely reported across the globe, but probably widespread.