Asclepias, A Genus of Wonder

David pictured with poke milkweed (Asclepias exaltata) plant and seed pods near Mountain Lake Wilderness, Giles County, VA

By David Bellangue, Center for Urban Habitats

Hello! I’m David, the newest member of the CUH team, and I’m here to tell you about one of my favorite groups of native plants, the milkweeds (Asclepias spp.). I have always been fascinated with the complex pollination system unique to the genus, its ecological importance, and its pleasing flower displays. My interest only grew when, in 2018, I had the opportunity to go to the Missouri Botanical Garden in St. Louis to assist with a project to help conserve Mead’s milkweed (Asclepias meadii), a federally threatened milkweed from the tallgrass prairies of the Midwest. I’ll give a brief overview of the genus, its importance, and highlight some species found in the Piedmont region of Virginia. By the end of this article, I hope you’ll become a milkweed fanatic too!

Milkweed (or silkweed as it was called by early European explorers) is a genus composed of around 100 species of perennial herbaceous plants found across temperate and tropical regions of North and Central America. 72 species of milkweed are native to the contiguous United States with Arizona and Texas having the highest species diversity. The genus name, Asclepias, is derived from the Greek god of medicine Asklepios due to the medicinal uses of several species. Their most recognized common name derives from the milky sap that many species possess. This sap contains chemicals called cardenolides which are used to defend against herbivores as well as to ward off parasites and pathogens. Formerly in their own plant family, milkweeds have recently joined the Dogbane family as Apocynaceae subfam. Asclepiadoideae. Key characteristics for identification include the white milky sap that many species possess, opposite leaf arrangement, and unique and beautifully intricate 5 petaled flowers borne in clusters.

Owing to their adaptability and broad range, milkweeds have become an incredibly important group utilized by numerous organisms. The most famous interaction is between milkweeds and the monarch butterfly (Danaus plexippus) as well as the queen butterfly (Danaus gilippus).

Monarch nectaring at a goldenrod (Solidago sp.)

These two species rely solely on milkweeds (and a few other closely related genera) as host plants for their larva, while adults frequently utilize milkweed flowers for their nectar. The decline of migratory monarch populations in North America has alerted the general public to the decline of milkweeds and increased the awareness of this genus and the important ecological roles it plays.

Aside from monarchs, several other species of insect specialize in feeding on milkweeds including the large and small milkweed bugs (Oncopeltus fasciatus & Lygaeus kalmia), the swamp milkweed leaf beetle (Labidomera clivicollis), and milkweed tussock moth (Euchaetes egle). Milkweed flowers provide nectar and pollen to numerous insects with native bees being frequent visitors and essential pollinators of milkweeds including many species of bumblebees (Bombus spp.). Because milkweed flowers have shallow, easily accessible nectar reservoirs, they are also visited by a host of predatory and parasitic insects that possess short tongues and mouth parts such as wasps, hover flies, and tachinid flies. These species are beneficial to people as they prey on insects that feed on garden vegetables and agricultural crops, making milkweeds an ally in the movement towards sustainable pest management practices.

According to the Flora of Virginia, Virginia is home to 14 species of milkweed that grow from swamps and wet flats on the coast to high mountain peaks. These species include:

  • Clasping Milkweed (Asclepias amplexicaulis)
  • Poke Milkweed (A. exaltata) [primarily in mountains]
  • Northern Swamp Milkweed (A. incarnata)
  • Southern Swamp Milkweed (A. pulchra)
  • Few-flower Milkweed (A. lanceolata) [primarily coastal plain]
  • Long-leaf Milkweed (A. longifolia) [primarily coastal plain]
  • Purple Milkweed (A. purpurascens)
  • Four-leaf Milkweed (A. quadrifolia) [primarily in mountains]
  • Red Milkweed (A. rubra) [primarily coastal plain]
  • Common Milkweed (A. syriaca)
  • Butterflyweed (A. tuberosa)
  • Red-ringed Milkweed (A. variegata)
  • Whorled Milkweed (A. verticillata)
  • Green Milkweed (A. viridiflora)

Here in the Piedmont, I’ve come across a fair number of these wonderful plants including abundant species such as common milkweed, butterflyweed, and Southern swamp milkweed, as well as uncommon to rare species such as four-leaf milkweed, green milkweed, purple milkweed, clasping milkweed, and red-ringed Milkweed. While a few species such as four-leaf and poke milkweed grow in shadier forest settings, most of the Piedmont milkweeds are heliophilic (meaning sun-loving) and were an important component of the now much diminished grasslands that once covered a significant portion of the Virginia Piedmont. Like other heliophilic plants, many milkweeds are uncommon due to the overgrowth of forests as a result of fire suppression, and are now restricted to areas with abundant sunlight, such as roadsides and powerline right-of-ways.

Redring Milkweed (Asclepias variegata) pictured in a roadside grassland remnant, Prince Edward County, VA

Owing to their important place in Piedmont grasslands and their current rarity in the landscape (aside from common milkweed which is very tolerant of human-caused disturbance), it is a priority of CUH to produce and include the less common milkweeds in our restorations, as many of the species that are found in Virginia are not available commercially. As the resident nursery manager and plant propagator, I plan on increasing the number of local-genotype milkweeds we produce (especially clasping milkweed and green milkweed) with the potential to have in-house seed production in the future. My goal is to increase the availability of milkweeds for landscape restoration and promote the use of a greater diversity of milkweed as currently only common, butterfly-weed, whorled, and northern swamp milkweeds are commonly available. These plants should be available in their full diversity for landscape restorations throughout Virginia. By raising awareness and planting a diverse suite of milkweeds in public places, we at CUH hope to highlight the fascinating native plant communities these plants belong to and ensure their survival.

A vigorous seedling of Southern Swamp milkweed (Asclepias pulchra) from in-house nursery at CUH field station, Albemarle County, VA

If you’d like to learn more, additional information on milkweed can be found at the following sources:

Milkweeds: A Conservation Practitioner’s Guide 
Plant of the Week from the US Forest Service 
Native Milkweeds from the Xerces Society for Invertebrate Conservation 
Digital Atlas of the Virginia Flora