The generic boundaries of Asclepias are still somewhat obscure. As currently circumscribed (Fishbein & al. 2011) it is a genus of ca. 130-140 species, all native to North, Central and South America and the Caribbean. Up to 250 additional species from Africa could potentially be included in Asclepias but this is not supported by phylogenetic studies (Fishbein & al. l.c.). Several species are cultivated as ornamentals in Europe (Knees 2000a) but only few are rather widespread in cultivation. Beside the species treated beneath Asclepias curassavica L. is increasingly grown as a pot plant in Belgium.
1. Leaves (narrowly) lanceolate, in whorls of 3-6. Stem glabrous === 1. Asclepias incarnata
1. Leaves broadly ovate, opposite. Stem densely hairy === 2. A. syriaca
Literature
Fishbein M., Chuba D., Ellison C., Mason-Gamer R.J. & Lynch S.P. (2011) Phylogenetic relationships of Asclepias (Apocynaceae) inferred from non-coding chloroplast DNA sequences. Syst. Bot. 36(4): 1008-1023.
Knees S.G. (2000a) Asclepias. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 6. Cambridge University Press, Cambridge: 51-53.
Woodson R.E. (1954) The North American species of Asclepias L. Ann. Missouri Bot. Gard. 41: 1-211.