Belongs within: Asclepiadeae.
Got milk?
Published 11 September 2025
Among the characteristic plants of open country in North America are the milkweeds, an assemblage of more than a hundred species included in the genus Asclepias. Asclepias species are mostly erect perennials, more rarely annuals or twiggy shrubs. Most authors would currently restrict Asclepias to species found in the Americas, with the placement of African species in separate genera supported by phylogenetic analysis (Fishbein et al. 2011).
Milkweeds have received particular attention for two reasons, their production of toxic compounds and sticky latex (from which, of course, the name ‘milkweed’ derives) and their unusual floral structures. Despite their overall protective function, milkweed toxins have made them the target foodplant of insects that sequester the compounds for their own defense. The most famous of these, of course, is the monarch butterfly Danaus plexippus, but monarchs are by no means unique in this regard (Eumolpus beetles came up on this site just recently).
The complex flowers of milkweeds are small (albeit sometimes brightly coloured) and typically borne in dense inflorescences (cymes). Within the five-lobed corolla, the anthers and stigma are fused to form a complex structure, the gynostegium. Five petal-like hoods (the corona) arise from the base of the gymnostegium column and surround the inner structures. The stigma is largely covered by corneous wings formed by the sides of the anthers, accessible only through glandular chambers with slit-like openings between the anthers. Pollen grains, rather than being released individually, are contained within packages (pollinia) with an adhesive stalk. As large insects such as wasps walk across the flower while collecting nectar, one of their legs may end up becoming caught in an anther opening. When the insect frees the caught appendage, it will carry an attached pollinium. The process is reversed when the insect visits another flower and gets similarly caught in the opening of a stigmatic chamber. This transfer may be facilitated by the adhesive stalk which shrivels while the insect is in flight, changing the angle of the pollinium to one better suited to finding a chamber (Woodson 1954).
The North American Asclepias species were revised by Woodson (1954) who divided them between numerous subgenera and series, largely on the basis of floral anatomy. It is worth noting, however, that Woodson himself expressed skepticism whether his divisions would prove to be ‘natural’, and later phylogenetic studies would indeed largely refute their monophyly (Fishbein et al. 2011). Instead, geography and (to some extent) vegetative characters are often more informative. The basalmost clade within the genus (according to Fishbein et al. 2018) appears to be a small group of narrow-leaved shrubs and subshrubs found in the Sonoran desert region of southwestern North America. In some species of this clade (themselves forming a subclade), the filiform leaves are actually lost when the inflorescence develops, with photosynthesis being conducted in the green stems.
Next diverging is the Incarnatae clade, 25 species of mostly herbaceous and similarly narrow-leaved species found the length of the Americas. This clade includes all Asclepias species found in South America, seemingly the descendents of a single colonisation event from the north. Basal relationships within the remaining Asclepias species, many of which have broader leaves, are less resolved. However, the greater number of species may be assigned to two main subclades, one found in the highlands of Mexico, one found in northern temperate habitats. Floral characters appear to be prone to conversion across all clades, as the milkweeds have explored different ways to best ensnare their hapless pollinators.
Systematics of Asclepias
<==Asclepias L. 1753L95
|--A. albicansH93
|--A. asperula [incl. A. capricornu ssp. occidentalis]H93
|--A. californicaH93
|--A. cordifoliaH93
|--A. cornutiC55
|--A. cryptoceras [incl. A. cryptoceras ssp. davisii]H93
|--A. curassavica L. 1753L95
|--A. eriocarpaH93
|--A. erosaH93
|--A. fascicularisH93
|--A. incarnataS72
|--A. linariaH93
|--A. nivea L. 1753L95
|--A. nyctaginifoliaH93
|--A. obtusifoliaJ23
|--A. pubescensBR65
|--A. rotundifolia Miller 1768 [incl. A. arborescens Linnaeus 1771, Gomphocarpus arborescens]BR65
|--A. solanoanaH93
|--A. speciosa [incl. A. giffordii]H93
|--A. subulata [incl. A. subulata var. parishii]H93
|--A. syriacaW69
|--A. tuberosaD51
|--A. verticillataC01
`--A. viridifloraJ23
*Type species of generic name indicated
References
[BR65] Black, J. M., & E. L. Robertson. 1965. Flora of South Australia. Part IV. Oleaceae–Compositae. W. L. Hawes, Government Printer: Adelaide.
[C55] Candolle, A. de. 1855. Géographie Botanique Raisonée: Ou exposition des faits principaux et des lois concernant la distribution géographique des plantes de l’époque actuelle vol. 2. Librairie de Victor Masson: Paris.
[C01] Cockerell, T. D. A. 1901. Contributions from the New Mexico Biological Station.—X. Observations on bees collected at Las Vegas, New Mexico, and in the adjacent mountains. Annals and Magazine of Natural History, series 7, 7: 125–131.
[D51] Dobzhansky, T. 1951. Genetics and the Origin of Species 3rd ed. Columbia University Press: New York.
Fishbein, M., D. Chuba, C. Ellison, R. J. Mason-Gamer & S. P. Lynch. 2011. Phylogenetic relationships of Asclepias (Apocynaceae) inferred from non-coding chloroplast DNA sequences. Systematic Botany 36 (4): 1008–1023.
Fishbein, M., S. C. K. Straub, J. Boutte, K. Hansen, R. C. Cronn & A. Liston. 2018. Evolution at the tips: Asclepias phylogenomics and new perspectives on leaf surfaces. American Journal of Botany 105 (3): 514–524.
[H93] Hickman, J. C. (ed.) 1993. The Jepson Manual: Higher Plants of California. University of California Press: Berkeley (California).
[J23] James, E. 1823. Account of an Expedition from Pittsburgh to the Rocky Mountains, performed in the years 1819 and ’20, by order of the Hon. J. C. Calhoun, sec’y of war: under the command of Major Stephen H. Long. From the notes of Major Long, Mr. T. Say, and other gentlemen of the exploring party vol. 1. H. C. Carey & I. Lea: Philadelphia.
[L95] Liogier, H. A. 1995. Descriptive Flora of Puerto Rico and Adjacent Islands: Spermatophyta-Dicotyledoneae vol. 4. Melastomataceae to Lentibulariaceae. Editorial de la Universidad de Puerto Rico.
[S72] Shields, O. 1972. Flower visitation records for butterflies (Lepidoptera). Pan-Pacific Entomologist 48 (3): 189–203.
[W69] Wiens, J. A. 1969. An approach to the study of ecological relationships among grassland birds. Ornithological Monographs 8: 1–93.
Woodson, R. E., Jr. 1954. The North American species of Asclepias L. Annals of the Missouri Botanical Garden 41 (1): 1–211.