Species Spotlight: Clasping Milkweed (Asclepias amplexicaulis)

by Graham Williams, rising high school senior and Volunteer-in-Chief at Shoal Creek Sanctuary

photo by David Tilson, President of the Oconee Rivers Audubon Society

Clasping Milkweed (Asclepias amplexicaulis) is a perennial wildflower native throughout Georgia, and its presence usually indicates undisturbed, high quality native ecosystems since it takes years to establish and mature before flowering. It has a single thick taproot that can plunge multiple feet underground that stores nutrients and energy. They grow up to three or four feet tall on a single unbranched stem, but one taproot can support up to six stems. It prefers sandy or rocky soils in full sun to partial shade, and is dependent on very well-drained soil. The summer-blooming flowers atop each stem are a globe of fifteen to eighty flowers of a rosy-pink and green to a deep wine red color. They are said to smell like roses and cloves. Like other milkweed species, the flowers are made of a unique structure called a corona, which is composed of five hood shaped cups that contain the nectar and pollen. They hold their pollen in pouch-like structures called pollinia instead of having loose pollen grains, so when bees or wasps visit the flowers for nectar, their legs slip into the cups of the corona, attaching the pollinia to the pollinator to be carried to the next flower. Because the mechanical entry system is so complex, only about 1% to 5% of blossoms ever successfully produce a seed pod.

Asclepias amplexicaulis is also a top-tier supporter of Monarch caterpillars in that it has exceptionally high cardiac glycoside concentrations in its latex sap, which is what Monarch caterpillars consume and sequester to make them unpalatable to predators. Other native milkweeds such as Swamp Milkweed (Asclepias incarnata) or Butterfly Milkweed (Asclepias tuberosa) have low to zero cardiac glycoside concentrations, providing little chemical protection to Monarch caterpillars. However, species with lower concentrations also have less toxic nectar, meaning they are highly preferred by pollinators. Incredibly, the sequestered glycosides are transferred through metamorphosis into the adult butterfly, where the highest concentrations of the toxins are packed into the wings and abdomen, so Monarchs are dependent on these chemicals for predator defense throughout their lives.

Clasping Milkweed gets its common name because the oppositely arranged leaves lack petioles, which means the leaves emerge directly off of the stem and wrap around or "clasp" the stem. The leaves are also evolved to be very drought tolerant, being noticeably waxy to seal in water that would otherwise be lost through transpiration in the leaves, and the leaf margins are wavy to disturb wind passing over its surface to cool down the leaves. Its range stretches from Texas to Florida, up to New Hampshire, where it's listed as endangered, and Minnesota, where it's considered a threatened species. In Georgia, it’s considered common in some places, but is always an exciting find nonetheless. In Athens, wild documented plants are only found in power line cuts where woody plants and trees are suppressed through mowing or herbicide use, which both helps and hurts plants in these strips reminiscent or remnant of the nearly extirpated Piedmont Prairie ecosystem, historically maintained through occasional fires. The absence of open spaces maintained by fire in our modern environment means less spaces are hospitable to this specialized species.