Common names: milkweed (Eng.); melkbos (Afr.)
Members of the genus Asclepias (as most of the taxa in the family Apocynaceae in the broad sense, subfamily Asclepiadoideae), are an interesting group because of their extra-ordinary fusion of the male and female parts of the flowers and the development of interesting pollination mechanisms and development of fascinating flowers.
Figure 1. Asclepias woodii. (Photo A. Shuttleworth)
Perennial, re-sprouting herbs from a deep-seated underground tuber or woody rootstock. All parts exude milky latex when bruised. Plants usually small (less than 400 mm tall), with their stems and spreading along the soil surface (prostrate) or erect; branched or not.
Opposite, and decussate, leaves in Asclepias. (A) Asclepias albens; (B) Asclepias cucullata. (Photos SP Bester)
Leaves opposite; blade with various shapes but usually linear, lanceolate or broadly ovate; base cordate, truncate or cuneate with an acute tip; margin usually entire and mostly rolled in on the underside.
The flowers are usually aggregated into an inflorescence, known as an umbel.
Figure 3. Inflorescences in Asclepias species. (A) Asclepias densiflora (Photo SP Bester); (B) Asclepias vicaria (Photo SP Bester); (C) Asclepias crassinervis (Photo J Kirkel); (D) Asclepias macropus (Photo A Shuttleworth); (E) Asclepias crispa. (Photo A Shuttleworth)
Figure 4. Some parts of Asclepias adscendens, a typical Asclepias flower. (Photo SP Bester)
The flower has 2 carpels, but after pollination one is usually aborted in favour of the other that fully develops into a mature fruit called a follicle.
Figure 5. Fruit of some Asclepias species. (A) Asclepias albens (Photo SP Bester); (B) Asclepias densiflora (Photo SP Bester); (C) Asclepias vicaria (Photo HM Steyn); (D) Asclepias aurea (Photo SP Bester); (E) Asclepias gibba (Photo SP Bester)
Of the 45 species (48 taxa) of this genus that occur in the Flora of southern African Region, most of the taxa were assessed with conservation status of Least Concern (LC). There are, however, 19 taxa (40%) of conservation concern: 4 Rare; 2 Near Threatened (NT); 5 Data Deficient (DD); 1 Vulnerable (VU); 3 Critically Rare (CR) and 3 Endangered (EN). Most of these taxa occur in grassland and savanna habitats and are threatened because of destruction of their natural environment by large-scale agriculture, mining and urban development. It is also interesting that in the southern African region, most of the members of this genus occur in areas of summer rainfall.
Distribution and habitat
In the current circumscription of the genus it is native to the Americas and sub-Saharan Africa. It has been indicated that the species from the Americas and those from Africa are only of ancient affiliation and as the first member was described from America, the African species should be assigned a new generic name(s). These new combinations have not yet been made and for now they all are placed in the genus Asclepias. In Africa the genera are closely related. Gomphocarpus is separated from Asclepias mainly based on having a rootstock instead of a deep-seated tuber.
In southern Africa it is found in various habitats within grasslands, savanna and wetlands, mainly in the summer rainfall areas with very few species from the winter rainfall areas.
Derivation of name and historical aspects
The generic name Asclepias is derived from the Greek doctor Aesculapius who, according to the ancient mythology, was immortalised as the god of medicine.
The fusion of the male and female parts of the flowers renders the flowers complicated, with pollination usually performed by specific pollinators, which need to remove the pollen that is aggregated in pollinia consisting of 2 masses of pollen joined by caudicles to a clip. The clip attaches to the mouth parts, or even a leg, of a possible pollinator and needs to be deposited in the guiderail of another flower for successful pollination.
Figure 6. Some parts of Asclepias brevicuspis flower. (Photo SP Bester)
The follicles (fruits) burst longitudinally to release seeds that have a tuft of spreading hairs on one side (coma), which helps the seeds get airborne for dispersal.
Figure 7. Asclepias gibba fruit (follicle) splitting open and releasing seed with coma. (Photo SP Bester)
Many of the species are used as medicine or entire plants or parts thereof as famine food plants.
African species are not cultivated, nor are they available from any nurseries.
Growing Asclepias genus
Flowers and fruits of members of the genus are very attractive, but relatively short lived and these plants are mostly vegetative. Indigenous species are not at all grown in gardens nor available from nurseries in South Africa (to the author’s knowledge). Species that are cultivated are those like Asclepias currasicava which is introduced from Mexico to South Africa.
Figure 8. Asclepias curassavica, a species native to Central and Northern parts of South America, is not uncommon in South African gardens and sometimes available from nurseries
If plants are cultivated, it is usually by interested connoisseur growers and collectors of exotic plants, rather than as ornamental garden subjects. Plants could be grown from wild-collected seed, which germinate relatively easily, but it is difficult to maintain a plant in cultivation until it flowers. Fresh seed should be sown in a light sandy to humic soil substrate and kept moist until germinated (usually about a week to sixteen days).
See photos and descriptions for a selection of species in the genus Asclepias.
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Stoffel Petrus Bester
Biosystematics and Collections Division: National Herbarium