Herbs perennial, deciduous; cryptophytes.
Roots fibrous, rhizomatous, or taprooted.
Stems erect to spreading.
Leaves opposite or whorled, simple, colleters present or absent.
Inflorescences extra-axillary umbels, may appear terminal.
Flowers bisexual, rotate; sepals 5, united; petals 5, united; corolla lobes reflexed, much longer than fused portion, valvate in bud; corona conspicuous, of 5 basally fused segments (hoods), each hood with an internal cavity, sometimes also with an exserted appendage (horn); stamens and coronas exposed; stamens connate and adnate to styles forming a gynostegium, anthers with apical scale-like appendages, pollen from each anther agglutinated into pollinia, pairs of pollinia from adjacent anthers joined by translators to a corpusculum that acts as a mechanical clip for attachment to insects; ovules many.
Fruits usually single, erect, fusiform, < 7 × as long as wide wide.
Seeds many, comose.
Africa, North America, South America. ~400 species; 4 species treated in Flora.
The plants mostly commonly referred to as “milkweeds” in North America belong to this genus. Some or all of the 250 native African species are often excluded from the genus. The species are variously familiar to many Americans as rangeland and agriculture weeds as well as colorful and interesting subjects for native flower gardens. Asclepias species are the primary host plants of the monarch butterfly; this fact, in conjunction with the large number of butterflies and other insects drawn to the flowers, have made them highly valued by gardeners as “butterfly” plants. Several species not native to Oregon are commonly grown in gardens, including butterflyweed (A. tuberosa), blood flower (A. curassavica), and swamp milkweed (A. incarnata). Much less is known about the use of Oregon native milkweeds by monarchs than that of the milkweeds of the eastern states. Several milkweeds enliven America’s roadsides in the summer with large floral displays, including A. speciosa in some parts of Oregon. In 2014, a small population of Asclepias solanoana was reported from Josephine County. Whether this represents a long-existing population, a recent migration, or a short-lived waif population may merit further investigation.