A genus of about 100 species, herbs (mostly perennial) and shrubs, temperate and tropical, of North and Central America (Endress et al. 2018).
ID notes:The flowers of Asclepias have an unusual structure and associated terminology that is necessary for successful interpretation of the key. From the bottom of each flower, there is first a whorl of 5 sepals, almost always green or greenish. Next is a whorl of separate petals, in most species recurved, and variously colored (brightly, or in quite a few species green, greenish white, yellowish green, or green flushed with rose or purple). Next inmost is the corona, a pentamerous structure of 5 hoods and 5 horns (these are petaloid appendages attached to the stamens). At the center is the gynostegium, a fusion of the stigmatic disc (innermost of all) and the 5 anthers. An insect leg passes through the stigmatic slit between two adjacent anthers, and can pick up the two pollinia, attached to one another by two translater arms, fused at the corpusculum.