The Desert Southwest harbors at least 41 of the 76 milkweed (Asclepias spp.) species known to exist in the lower 48 states. The species richness of milkweeds in this region is influenced by the tremendous diversity and range of vegetation types, soils, topography, climate, and the exposure of unusual rock types that occur over more than a 9,000 foot elevation range. The nectar of milkweed flowers is attractive to dozens of insects including bees, wasps, butterflies, moths, and hummingbirds. The bees that milkweed flowers attract to agricultural landscapes are important for pollinating a wide variety of vegetable forage and fruit crops.
Milkweeds are named for their milky, latex sap, which contains alkaloids and cardenolides, complex chemicals that make the plants unpalatable to most animals. The plants have fleshy, pod-like fruits (follicles) that split when mature, releasing the seeds. While the size and shape of the fruit is variable depending on the species, every milkweed seed has fluffy white hairs (coma) attached to it to aid in wind dispersal.
Like many native plant species, milkweed populations are being lost at a rapid rate due to habitat loss. Milkweeds occur in diverse habitats including along roadsides, in agricultural fields and pastures, and on abandoned agricultural lands. Habitat loss drivers include land development, agricultural intensification, and the widespread adoption of herbicide resistant crops (Brower et al., 2012; Pleasants and Oberhauser, 2012).
Gary Paul Nabhan (University of Arizona), Steve Buckley (National Park Service), and Heather Dial (USDA-NRCS Arizona)
Gail Marie Morris (Southwest Monarch Study), Mace Vaughan (Xerces Society), Brianna Borders (Xerces Society), John Pleasants (Iowa State University), Mark Fishbein (Oklahoma State University), Chip Taylor (University of Kansas), and Wendy Caldwell (Monarch Joint Venture)
Margaret Smither-Kopperl (USDA-NRCS California), Bernadette Cooney (USDA-NRCS New Mexico), Eric Eldredge (USDA-NRCS Nevada), Ramona Garner (USDA-NRCS North Carolina), Linda Kennedy (Audubon Appleton-Whittell Research Ranch), Stu Tuttle (USDA-NRCS Arizona), Emilio Carrillo (USDA-NRCS Arizona), Alison Hatch (USDA-NRCS Arizona)
Editing and layout
Heather Dial (USDA-NRCS Arizona)
Cover: Asclepias incarnata with Monarch butterfly caterpillars. Photo by Mark Fishbein
Nabhan, G., S. Buckley, and H. Dial. 2015. Pollinator Plants of the Desert Southwest: Native Milkweeds (Asclepias spp.). USDA-Natural Resources Conservation Service, Tucson Plant Materials Center, Tucson, AZ. TN-PM-16-1-AZ.