Jeff Nelson, KNPS President
Over the last several years, we have seen an explosion of interest in the growing of native milkweed plants (Asclepias genus), largely driven by the plight of the iconic monarch butterfly (Danaus plexippus). As most of us are aware, monarch caterpillars only host on milkweeds. Without milkweed leaves to feed on, monarch caterpillars cannot not survive.
Asclepias is a genus of herbaceous, perennial, flowering plants known as milkweeds, named for their latex, a milky substance containing cardiac glycosides termed cardenolides, exuded where cells are damaged. Most species are toxic to humans and many other species, primarily due to the presence of cardenolides.
The Asclepias genus contains over 200 species distributed broadly across Africa, North America, and South America. It previously belonged to the family Asclepiadaceae, which is now classified as the subfamily Asclepiadoideae of the dogbane family, Apocynaceae. The genus was formally described by Carl Linnaeus in 1753, who named it after Asclepius, the Greek god of healing.
Members of the genus produce some of the most complex flowers in the plant kingdom, comparable to orchids in complexity. Five petals reflex backwards revealing a gynostegium surrounded by a five-membrane corona. The corona is composed of a five-paired hood-and-horn structure with the hood acting as a sheath for the inner horn. Glands holding pollinia are found between the hoods. The size, shape and color of the horns and hoods are often important identifying characteristics for species in the genus Asclepias.
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, as is typical for most plants. The feet or mouth parts 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, whereas monarch butterflies are poor pollinators of milkweed
Aquatic milkweed (Asclepias perennis)
Kentucky is home to 13 species of Asclepias. Every county in the state has at least one species present. Common milkweed (A. syriaca) and butterfly milkweed (A. tuberosa) are the most widely distributed. Two of the state’s milkweeds are classed as rare by the Office of Kentucky Nature Preserves, prairie milkweed (A. hirtella) and purple milkweed (A. purpurascens).
Aquatic milkweed, a.k.a. white swamp milkweed (A. perennis), is easily identified as a milkweed by its inflorescences and the milky sap. Aquatic milkweed is found in the western third of Kentucky. It occurs naturally in floodplain swamps, cypress-gum swamps, bottomland hardwood forests, marshes and along riverbanks. It is usually found in higher quality wetlands that are semi-shaded to shaded.
Aquatic milkweed is perennial. It is a short milkweed, 1-2′ tall, that consists of a cluster of unbranched flowering stems that are ascending to erect. The stems are light green to purplish green, terete, and nearly glabrous; the upper halves of the stems are often sparsely short-pubescent in lines underneath the petioles of the leaves. Pairs of leaves occur at fairly frequent intervals along the stems; both the leaves and stems contain a milky latex. The leaves are 2-5″ long and ¼-1½” across. They are narrowly elliptic to ovate-elliptic in shape and smooth along their margins. Leaf tips taper gradually to acute points, while leaf bases are wedge-shaped. Leaf venation is pinnate with a prominent midvein and curving lateral veins. The upper leaf surface is medium to dark green, while the lower leaf surface is light to medium green. Both surfaces are glabrous or nearly so (sometimes sparse fine hairs may occur along the lower sides of the central veins).
One or more umbels of flowers about 1½-2½” across are produced from the axils of the upper leaves; each umbel has 20-50 white to pinkish white flowers. The umbels are held more or less erect on rather stout peduncles about ½-2″ long. Each individual flower is about ¼” across, with typical milkweed structure of 5 corolla lobes (or petals), 5 sepals, 5 hoods with horns, and a central reproductive column (gynostegium). The reflexed corolla lobes are generally white, although they are often pinkish along the undersides towards their tips. The white hoods are erect and scoop-shaped; the horns of these hoods are slender, curved slightly inward, and exerted. The inconspicuous sepals are pale green or pale purplish green, lanceolate-oblong in shape, and shorter than the corolla lobes. The pedicels are ¾-1¼” long, whitish green to pale purplish green, terete, and minutely pubescent. The blooming period occurs from late spring to mid-summer (or sometimes later), lasting about 1-2 months. The flowers are mildly fragrant. Afterwards, fertile flowers are replaced by lanceoloid seedpods (follicles) about 2½-3″ long.
What Makes A. perennis Unique?
So what features of the plant make A. perennis unique among the milkweeds of Kentucky? It is at the low end of milkweed size, but there are other species that are of similar size. Leaves and flowers are also similar to other milkweed species. One distinguishing feature of swamp milkweed is its habitat. A. perennis grows primarily in shaded wetland habitats. The only other milkweed within the state that also prefers wetlands, Asclepias incarnata (Swamp Milkweed), prefers wetlands that are more sunny and does not do well in the shade.
There are, however, two characteristics that make swamp milkweed unique among Kentucky’s milkweeds. The ripening seeds pods are pendulous and dangle down below the leaves as they ripen. The pods are smooth and glabrous and are about 2½-3″ long. All of the state’s other milkweeds have seed pods that point upwards and are above the leaves. When the fruits are ripe the pod splits along a single seam, allowing the ripe seeds to be distributed.
It is the seeds of white swamp milkweed that makes this species unique in Kentucky. The seeds are similar to other milkweeds in size and shape, but have expanded seed coats that make a collar around the seeds. In addition to the large collars around the seeds, the seeds lack the comas (tufts of hair) that all other milkweeds in Kentucky have.
All of the other milkweeds in Kentucky, distribute their seeds by wind. The comas on these seeds allow the seeds to travel long distances with the wind. Without the comas on their seeds, how then do A. perennis seeds get distributed? Growing in floodplains and other habitats with periods of flowing water, A. perennis has adapted for dispersal of seeds by water. The dangling seed pods get the seeds down close to any flowing water. The seeds with large collars and without comas are adapted to float on the water surface. A study done in 1994, “Seed Buoyancy and Viability of the Wetland Milkweed Asclepias perennis” found that A. perennis seeds can remain floating for up to 6 months. The authors of the study say “Milkweed seeds can remain viable in water for long periods of time, but the remarkable ability of A. perennis to remain floating for more than 6 months may provide a “floating seed bank” that contributes to high levels of gene flow among populations of this floodplain species.”
I first saw aquatic milkweed several years ago at Metropolis Lake SNP in McCracken Co. When I first saw it in flower, I knew it was a milkweed. It wasn’t until I came back at the end of the season and saw the seed pods and the seeds, that I knew A. perennis was a unique example of the milkweed genus. I love this species and have started some on our place in McCracken county. I hope you have a chance to see this lovely native in the wild.