The Genus Asclepias
This entry mostly concerns the shrubby, mostly evergreen species of Asclepias (the deciduous perennials can be found in the perennial section. Asclepias species are collectively known as milkweeds (named for their latex, a milky substance containing cardiac glycosides termed cardenolides, exuded where cells are damaged) and have been made famous as larval host for the popular, and threatened monarch butterfly, a species we consider as a “gateway” into wildlife gardening. Milkweeds are almost all, universally, great pollinator plants (for other than the monarch or queen butterfly). There are 207 accepted species of Asclepias distributed broadly across Africa, North America, and South America.
The species listed here are also easier to grow than the more deciduous, perennials that are generally native to higher elevations.
Asclepias albicans
Whitestem Milkweed
This succulent upright plant might remind you of desert milkweed (Asclepias subulata), but a much more giant version. On average it gets to about 5-6’ tall, but in favorable conditions it reaches an incredible 10’ tall or more! Flowers anytime between winter and spring, usually in response to winter rains, with creamy-white flowers with black anthers, sometimes with a pink blush, fading yellow in ball-like clusters near and at the ends of tall stems. It seems to do most of its growing in the cool season—it is native to the western edge of the Sonoran Desert and the Mojave Desert which get most if not all of their precipitation in the cool season.
It can be distinguished from its close kin, A. subulata, by its fewer, longer, thicker, more noticeably waxy-blue stems, the very short nectar cups of the flowers, and its preference for rocky uplands rather than fine-textured substrates on playas, dunes or low sandy hummocks. There are known natural hybrids of Asclepias albicans and A. subulata, as well as a few hybrids in cultivation.
Full sun, low water, good drainage. Established plants are hardy to 20°F but young plants should be protected from frost.
As with most species of Asclepias, it is a larval food plant for the monarch and queen butterflies, and the flowers are nectar rich, attracting a multitude of nectar-seeking insects, especially tarantula hawks.
Used as a cathartic, an emetic, an eye medicine, for stomach disorders, for many other ailments, but it is also considered poisonous.
Asclepias is named for the Greek god of healing Asklepios, referring to the common medicinal use of plants in this genus, while albicans means white, referring to the white waxy coating on the stems of this species.
This desert milkweed prefers rocky, north-facing slopes of granitic ranges, although it also occurs on the walls of volcanic craters.; below 2,500 feet in the Mojave and Sonoran Deserts of California, Arizona, and Baja California and coastal Sonora with a disjunct population in Sinaloa.
Asclepias angustifolia
Arizona Milkweed
Evergreen subshrub, usually about a foot+ tall and spreading about 2’, however it can get to about 2.5x2.5’ in favored conditions. Almost everblooming with white flowers, this species may only take short breaks in the coolest part of the winter. Plants reseed readily in the landscape.
Full to part sun (but avoid reflective heat), moderate to regular water, hardy to about 15°F. May go dormant with cold temperatures though this doesn’t happen often in the low desert.
As with most species of Asclepias, it is a larval food plant for the monarch and queen butterflies as well as the milkweed tussock moth, and the flowers are nectar rich, attracting a multitude of nectar-seeking insects, especially tarantula hawks. 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.
Asclepias is named for the Greek god of healing Asklepios, referring to the common medicinal use of plants in this genus, 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. Rare in Arizona, in canyons on the border.
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.
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 linaria
Pineleaf Milkweed
Pine-like shrub, growing 3x3’ with white flowers appearing in warm weather. Flowers followed by follicles that contain the seeds plus floof. There are a few selections that are more compact in growth habit.
Full to part sun, moderate to low water, drainage is very important to this species. Hardy to 15°F.
As with most species of Asclepias, it is a larval food plant for the monarch and queen butterflies and the flowers are nectar rich, attracting a multitude of nectar-seeking insects, especially tarantula hawks. This species is sometimes reputed to be not good for monarchs and queens, but this is false and if anything, its better because of this species’ very high cardenolide content which reduces the impact of the OE parasite. Often, butterflies may prefer other species of Asclepias with softer leaves, but occasionally this plant will be preferred. It is unknown why sometimes this species is preferred and why sometimes it is not.
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.
Asclepias is named for the Greek God of healing Asklepios, while linaria likely means linear, parallel-sided, referring to the leaf shape.
Found on dry, rocky slopes and mesas, in canyons, and oak pine juniper woodlands from 1,000-6,000 ft. in southern Arizona (with a disjunct population in the extreme the southern tip of California), south throughout the Mexican Highlands and the Transvolcanic belt of Mexico.
Asclepias subulata
Desert Milkweed
Succulent broom-like shrub, 4x4’ with a spreading habit. White and cream colored flowers followed by a fruit that is a pouchlike follicle that contains many flat, oval seeds with long, silky hairlike plumes.
Full sun, moderate to low water, hardy to 20°F.
As with most species of Asclepias, it is a larval food plant for the monarch and queen butterflies, and the flowers are nectar rich, attracting a multitude of nectar-seeking insects, especially tarantula hawks. 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.
Used as a cathartic, an emetic, an eye medicine, for stomach disorders, for many other ailments, but it is also considered poisonous. Researchers in Bard, California tested the plant as a potential source of natural rubber in 1935.
Asclepias is named for the Greek god of healing Asklepios, subulata means awl or needle shaped.
Found on sandy or gravelly plains, rocky slopes, or along washes from sea level to 3,000 ft. This is a Sonoran Desert endemic, stretching across the low desert portions of the Baja, Sonora, Arizona, California, and Nevada.
Asclepias subulata on iNaturalist
Photo by Dominic Gentilcore