Asclepias L. (1753), commonly known as milkweed, is a genus of herbaceous perennial flowering plants in the family Apocynaceae (subfamily Asclepiadoideae). Linnaeus established the genus in his Species Plantarum of 1753, naming it after ancient Greek and Latin plant names used by Dioscorides and Pliny the Elder in the first century AD. The genus contains more than 200 species (with approximately 307 taxa recorded in GBIF), distributed across North America from Canada south through the United States and Mexico, South America, and sub-Saharan Africa. In southern Africa alone, 45 species are documented.
The plants are immediately recognisable by the milky latex that exudes from any damaged tissue. This latex, along with the leaves and roots, contains cardiac glycosides called cardenolides that serve as chemical defences against herbivores and are toxic to livestock. Stems are typically erect or prostrate, arising from deep-seated tubers or woody rootstocks, and most species reach under 400 mm in height, though some North American species grow taller.
The flowers of Asclepias are among the most structurally complex in the plant kingdom, often compared to orchids in their intricacy. Five reflexed petals expose a central gynostegium surrounded by a five-part corona of hoods and, in many species, horns. Pollen is aggregated into paired pollinia joined by caudicles; these clip mechanically onto the legs or mouthparts of visiting insects. Large-bodied bees and wasps are the most effective pollinators, accounting for more than half of all documented pollination events. After fertilisation, typically one of two carpels develops into a dry follicle. Seeds are fitted with long, silky hair filaments (the "coma") that catch the wind and carry seeds over considerable distances.
The ecological relationship between Asclepias and the monarch butterfly (Danaus plexippus) is one of the most studied examples of coevolution in North America. Monarch larvae feed exclusively on milkweed foliage; in the process they sequester cardenolides, rendering themselves unpalatable to predators. The plants' cardenolides also reduce the burden of the protozoan parasite Ophryocystis elektroscirrha in monarch populations. Loss and fragmentation of native milkweed stands is considered a leading driver of monarch population decline, and widespread planting of the non-native Asclepias curassavica year-round may disrupt migration timing and elevate parasite loads — conservationists recommend native milkweed species instead.
Etymology
The genus name Asclepias traces to ancient Greek and Latin plant names recorded by the physician Dioscorides and the naturalist Pliny the Elder in the first century AD. Linnaeus adopted the name when he formally described the genus in Species Plantarum in 1753, with Asclepias syriaca as the type species. A popular association links the name to Asclepius, the Greek god of medicine, though this connection is regarded as uncertain by botanical historians. The English vernacular "milkweed" refers directly to the white latex that oozes from cut stems, while the German "Seidenpflanze" (silk plant) reflects the silky seed-hair filaments. Indigenous names include the Navajo Ch'ilabe'é and the Ojibwe Zhaaboziganan.
Distribution
Asclepias has a disjunct distribution spanning the Americas and sub-Saharan Africa. In North America the genus ranges from southern Canada through the continental United States into Mexico; a diverse centre of diversity lies in the arid southwest (Kansas, Nebraska, Oklahoma, Colorado, New Mexico, Texas, Arizona, California, Nevada, Utah), where species occupy sandy or rocky calcareous soils, open plains, deserts, and grasslands. At least 12 species occur in South America. In Africa the genus is centred in sub-Saharan regions, with 45 species (48 taxa) documented in southern Africa alone; most occupy summer-rainfall zones in grasslands, savannas, and wetlands, with only a few in winter-rainfall areas.
Ecology
Asclepias species produce three overlapping defensive systems: leaf trichomes (surface hairs), viscous latex, and cardenolide toxins. These deter most generalist herbivores and are toxic to livestock. The monarch butterfly (Danaus plexippus) has co-evolved to circumvent these defences: larvae sequester cardenolides from the foliage, which in turn protects adult butterflies from avian predators. Species with high cardenolide concentrations additionally suppress the protozoan parasite Ophryocystis elektroscirrha, benefiting monarch health. Pollination is specialised: pollen masses (pollinia) adhere to insect visitors and are transferred entire to the stigma. Seeds disperse by wind via long, silky "coma" filaments. In southern Africa, pollinia attach to specific pollinators' body parts, reflecting tight co-evolutionary relationships.
Cultivation
Most Asclepias species suited to temperate gardens are hardy to USDA zones 5–9. They prefer full sun to light semi-shade and well-drained soils of light to moderate fertility; many tolerate mildly acid to basic pH. Young growth is susceptible to slug damage. North American native species (e.g., A. tuberosa, A. incarnata, A. syriaca) are widely grown in pollinator gardens and "monarch waystations." Indigenous southern African species are not commercially cultivated, though the introduced A. curassavica is available in South African nurseries.
Propagation
Asclepias can be propagated by seed, division, or basal cuttings. Seeds sown in autumn or late winter germinate in one to three months at 18°C; in southern Africa, fresh wild-collected seed in sandy substrate germinates in as little as one week. Division is best done in spring, but plants resent root disturbance and should be handled carefully. Basal cuttings taken in late spring — 10 cm shoots with a portion of the underground stem — root successfully and are useful for named cultivars that do not come true from seed.
Conservation
No Asclepias species is globally threatened, but population-level concerns are significant. Planting the tropical non-native Asclepias curassavica year-round in the United States has been shown to disrupt monarch butterfly migration, prolong breeding into winter months, and elevate Ophryocystis elektroscirrha parasite loads in monarch populations; conservation organisations recommend replacing it with locally native milkweed species. In southern Africa, 40% of Asclepias taxa face conservation challenges: four are Rare, two Near Threatened, five Data Deficient, one Vulnerable, three Critically Rare, and three Endangered. The primary threats to African species are habitat destruction through large-scale agriculture, mining, and urban development.
Cultural Uses
Milkweed has been woven into human culture across multiple continents. Several Native American nations — including the Omaha, Menominee, Dakota, and Ponca — used Asclepias syriaca medicinally. The Miwok of northern California twisted stems of Asclepias cordifolia into cords and ropes. The seed-floss was pressed into large-scale industrial service during World War II, when the United States collected over 5,000 tonnes as a buoyant, hypoallergenic substitute for kapok in military life jackets; today it is commercially produced as pillow filling and coat insulation. The same floss is highly water-repellent and has been tested for oil-spill cleanup. Latex from stems was explored as a natural rubber source by both the United States and Nazi Germany during WWII with limited commercial success. Edible uses include young shoots (prepared like asparagus), unopened flower buds (cooked like broccoli or peas), young seed pods, and seeds pressed for oil — all requiring thorough cooking due to the plants' toxic glycosides. Stem bark yields a strong fibre used for twine, cloth, and paper. In southern Africa, certain species are eaten as famine foods (young leaves of Asclepias albens as spinach) or used medicinally as emetics and for urinary complaints. Cardiac glycosides from some species served historically as arrow poisons in South America and Africa.
Taxonomy Notes
Linnaeus established Asclepias in 1753 (Species Plantarum: 214) with A. syriaca as the type species. The genus sits within Apocynaceae subfamily Asclepiadoideae, formerly treated as a separate family (Asclepiadaceae) before molecular phylogenetics led to its merger with Apocynaceae. GBIF recognises approximately 307 descendant taxa under the accepted name Asclepias L.; Wikipedia cites more than 200 species. The disjunct Africa–Americas distribution has attracted considerable phylogenetic attention. Common synonyms exist at the species and section levels, particularly for taxa transferred from formerly segregate genera.
