The Genus Asclepias
Family: Apocynaceae
This is an important genus of plants that has gained attention in recent years because of the enthusiasm behind the monarch butterfly, which species of this genus serve as a larval host, as well as important nectar plant. But the plant is host to other lepidoptera, like queen butterflies and tussock moths. As a nectar plant, countless pollinators are dependent on milkweed. Plants readily grow back from larval hosting, so don’t worry if the plants get eaten to the ground.
The silky floof produced to use wind disperse the seeds is used by many bird species for nest building.
There are 207 species of Asclepias distributed broadly across Africa, North America, and South America. Pollination in this genus is accomplished in an unusual manner. Pollen is grouped into complex structures called pollinia (or "pollen sacs"), rather than being individual grains or tetrads, as is typical for most plants. The feet or mouthparts of flower-visiting insects, such as bees, wasps, and butterflies, slip into one of the five slits in each flower formed by adjacent anthers. The bases of the pollinia then mechanically attach to the insect, so that a pair of pollen sacs can be pulled free when the pollinator flies off, assuming the insect is large enough to produce the necessary pulling force (if not, the insect may become trapped and die). Pollination is effected by the reverse procedure, in which one of the pollinia becomes trapped within the anther slit. Large-bodied hymenopterans (bees, wasps) are the most common and best pollinators, accounting for over 50% of all Asclepias pollination.
Milkweeds often have aphids and this is something you should ignore—let the aphids be; they attract ladybugs, lacewings, hoverflies, and a multitude of other predator insects that you want to encourage in your landscape since they eat the aphids as well as mites, whiteflies, and many other problematic (to people) insects. The aphids don’t harm the milkweed and they won’t spread to other plants (aphids are host-specific).
In Arizona alone, there are something like 30 species of milkweeds. Photo by Sue Carnahan, SEINET
Asclepias angustifolia
Arizona milkweed
Small, evergreen perennial sometimes reaching 2.5’ but usually less than a foot high. Spreads almost 3 feet. Flowers very often with white flowers, whenever the weather is warm. Flowers followed by pods that split and release downy floof with seeds attached. This plant reseeds readily in landscapes. Foliage turns purplish in winter but is usually evergreen in the low desert. May go dormant in colder regions.
Grow in full to part sun, moderate water, hardy to 15° F.
Asclepias is named for the Greek god of healing Asklepios (a reference to the long history of medicinal use of milkweeds, while angustifolia means narrow leaves.
Found in riparian woodlands, floodplain meadows, cienega edges, canyons and arroyo bottoms from 3,500-7,000 ft. in southern Arizona to Southern Texas, south through Sonora, Chihuahua and mountainous regions throughout Mexico to Chiapas.
Asclepias asperula
Antelope Horns
Small, deciduous perennial growing about 1.5’ high and spreading about 2’. Plant dies to ground in winter and reemerges in spring with vigor. Magnificent green flowers with burgandy markings followed by horn-like seed pods that burst with downy floof and seeds.
Grow in full to part sun (avoid reflected heat), moderate water with good drainage, hardy to at least 15° F.
Asclepias is named for the Greek god of healing Asklepios (a reference to the long history of medicinal use of milkweeds, while asperula means “delicately roughened” referring probably to the texture of the foliage.
In meadows, grasslands, sometimes in oak forests, and along roadsides in a wide range in the southwestern United States and Northern Mexico.
Asclepias brachystephana
Bract Milkweed
Deciduous perennial growing to about 2 feet high eventually, often much lower (about a foot) and spreading 2-3’ wide. In summer, purple and white flowers appear and are followed by pods that burst with downy floof and seeds. Plants go dormant in winter and reemerge with vigor in spring.
Plant in part sun in the low desert, moderate water with good drainage. Hardiness untested but probably hardy to at least the low teens °F, probably lower since they’re dormant in winter.
Asclepias is named for the Greek God of healing Asklepios referring to the traditional use of milkweeds medicinally; brachystepha comes from the Greek brachys, or -short,- and stephanos, meaning -crown.
Found on desert mountains, plains, mesas, grasslands, and roadsides, from 3,500-6,500 ft. in Arizona, New Mexico, and Texas, south toward Guanajato through Coahuila and Sonora.
Asclepias curassavica
Tropical Milkweed
Evergreen shrubby plants getting 2-3’ tall with reddish orange and orange flowers (there is a yellow flowered selection)—flowers almost any time of year if the weather is warm. Flowers followed by follicles with seeds accompanied by floof.
Full to part sun, even bright shade (with less flowers in shadier conditions). Avoid reflective heat. Moderate to regular water. Plants are root hardy to 15°F but will suffer damage in the upper 20s °F. Plants should be cut back in spring (as early as February) if they don’t freeze back—both for cosmetic reasons and for the health of the monarch.
It is native to the American tropics, especially Sonora, south throughout Mexico, and has a pantropical distribution as an introduced species.
Asclepias curassavica on iNaturalist
Photo by Zachery Berry, SEINET
IMPORTANT NOTES ABOUT THIS SPECIES: In colder regions of the United States, this plant is more of a problem because it is evergreen and slows down monarch migration (monarchs should be heading south, and native milkweeds going dormant is their cue to head south). But we live in a region with many evergreen milkweeds, and monarchs can be seen in Arizona in the cool season, sometimes sleeping on tree stems during the short cold spells. Tropical milkweed is almost native here, found near Hermosillo, in Sonora. Another concern is that this species host the protozoan parasite Ophryocystis elektroscirrha (OE), which effects monarchs. This problem can be avoided by letting plants freeze back in winter, or cutting them back (February is a good time) as OE thrives on tired, old growth on plants. That said, OE does not seem to be as much of a problem here. We still recommend you cut your plants back.
Often what people are attributing to OE is a response to pesticides. The nursery trade is notorious for spraying plants, especially because of the aphids. Even nurseries that claim they don’t spray their milkweeds still spray other plants in the same greenhouses as the milkweeds. It is very difficult to find clean plants in the trade. So if you get this plant, make sure it comes from a nursery that makes a special commitment to keeping these plants clean.
Aphids are often collected on the tips of the stems, and we recommend leaving them alone—as long as the plant has enough sun and water, it is relatively unaffected by the aphids. What is more, in amongst the aphids are usually eggs of butterflies, newly hatched butterfly larvae, and also larval forms of ladybugs, lacewings, hoverflies, and other beneficial insects. And those beneficial insects are usually eating the aphids. Even if you merely wash the aphids off with a get of water, you also remove all those other bugs, and aphids reproduce quickly, while those beneficial insects take longer to recover.
The white sap in the stems is used medicinally, because the sap contains steroids called cardenolides, which exhibit cardiotonic properties, meaning it stimulates the heart. This sap is applied directly to warts and corns in order to kill them in Costa Rica. A syrup made from the juice of the Mexican Milkweed is a powerful vermifuge and laxative, meaning that this plant has antiparasitic qualities and can be used in the treatment of parasitic worms. In Jamaica, a decoction of the roots is made to treat dysentery, ringworm, bleeding and to serve as an eyewash for infected eyes because the root contains emetic and purgative compounds. Skin ulcers are treated by a paste made from the crushed leaves. This plant is also considered a cicatrizant, meaning that it heals cuts by encouraging the formation of scars. Be careful using milkweeds medicinally.
Asclepias is named for the Greek god of healing Asklepios, referring to the common medicinal use of plants in this genus, while curassavica means from Curaçao, Lesser Antilles, one of the places that this species is found naturally.
Asclepias involucrata
Dwarf Milkweed
A tiny deciduous, tuberous rooted perennial that grows from 2-10’ wide, usually not more than a foot tall, usually half that. This small, low-growing milkweed is distinguished by its white-yellowish flowers and the white-hairy margins of the long-triangular leaves.
Part sun in the low desert, moderate water, good drainage. Hardy to below 0°F.
Asclepias is named for the Greek God of healing Asklepios referring to the traditional use of milkweeds medicinally; the species name, involucrata, means provided with an involucre, referring to the leafy bracts that subtend the flower clusters.
Found in sandy desert scrub, chaparral, grasslands, dry plains, mesas, and oak-pine woodlands, on flats and in arroyos from 3,500-7,000 ft. in Arizona and Utah, east through Colorado and New Mexico to Kansas, Oklahoma, and Texas. Also in adjacent Mexico, especially just past the border in New Mexico.
Asclepias fascicularis
Narrowleaf Milkweed
A 2-3’ perennial with white flower clusters through the summer— goes dormant in winter to re-emerge in spring though if the winter isn’t too cold, dormancy may be shortened. White flowers with light pink in warm weather followed by pods that burst with downy floof and seeds. Plants reseed readily in the landscape.
Full to part sun, moderate water. Hardy to below 0°F.
Asclepias is named for the Greek God of healing Asklepios referring to the traditional use of milkweeds medicinally; fascicularis mean “in bundles” perhaps referring to the whorled leaves?
The plant is a common perennial in the Western United States and Baja California. It is found in numerous habitats, including deserts, chaparral and woodlands, and montane locales in a range of soils from moist to very dry sites, from 150 to 7,500 ft.
Asclepias subverticillata
Horsetail Milkweed
Deciduous, slender perennial growing about 3x3’. Flowers May through September with white flowers followed by skinny pods that burst with downy floof and seeds.
Grow in full to part sun, but avoid reflective heat. Provide moderate water (good drainage), hardy to 15° F.
Asclepias is named for the Greek god of healing Asklepios, while subverticillata means not quite whorled.
Found in open areas, generally in woodlands, grasslands, to disturbed areas from 3,000-8,000 ft. in New Mexico, Arizona, Colorado and Utah and parts of some nearby states; south into northern Mexico.
Butterflyweed
Asclepias tuberosa
Deciduous perennial growing about 3’ tall x 2’ wide. Orange or yellow flowers in warm weather followed by pods that burst with downy floof and seeds. This species reseeds readily in the landscape. Plants go dormant in winter, re-emerging from underground tubers in spring.
Grow in full to part sun (avoid reflective heat), moderate water to regular water (depending on sun exposure), hardy to -30˚F.
Asclepias is named for the Greek god of healing Asklepios, while tuberosa means bearing tubers.
Found a variety of habitats from chaparral, to pine, to riparian woodlands, meadows and disturbed sites from 3,000-8,000 ft. across North America, north Canada and south to northern Mexico, there are four subspecies continentally in a rough east to west gradient.