Harsen's Island Milkweeds

During the past two weeks, I had the opportunity to spend five days botanizing and doing rare plant surveys with several coworkers in the globally and state imperiled to critically imperiled Lakeplain Wet Prairie and Lakeplain Wet-Mesic Prairie communities on Harsen's Island in St. Clair County, Michigan. These communities have developed on glacial lakeplains and have an impermeable clay layer several feet below the sandy surface layer. This results in inundation in the winter and spring and very dry conditions later in the summer. One of the greatest threats to the remaining lakeplain prairies is invasive species... you can see this impending threat in the photograph below, where there is a dense wall of Common Reed (Phragmites australis) in the background, below the tree line.


Two of the plants of conservation concern that we were looking for on this site were milkweeds (Asclepias spp.). We saw numerous milkweeds on our site, including several common species and both of the rare species.


Swamp Milkweed (Asclepias incarnata) was one of the common species that we saw. As seen above the blossoms of this milkweed are bubblegum pink... and coincidentally they also smell like bubblegum! With its primary geographic distibution centered in the upper Midwest and New England states, Swamp Milkweed is known from most of the Lower 48, with the exceptions being Mississippi, Arizona, California, Oregon, and Washington. It grows in many different wet plant communities, including wet prairies, emergent marshes, swamp forests, bogs, and along streams and pond margins.


The milkweed above, Butterfly Milkweed (Asclepias tuberosa), grows in much drier conditions and often in well-drained sandy soils. You can find this handsome orange-flowered species growing in prairies and savannas as well as along roadsides and on glades throughout the eastern half of the United States and adjacent Canada, as well as in the southwestern United States. Unlike the other milkweeds, which have a white, milky latex, Butterfly Milkweed has clear sap.


Common Milkweed (Asclepias syriaca, above) was abundant on the site we investigated on Harsen's Island. Although it is an opportunistic native species that grows in heavily degraded areas, Common Milkweed also grows in various prairie types. The corolla of this species can be pink, as in the photograph above, or more cream-colored. Although its North American distribution includes 39 of the states in the contiguous United States, Common Milkweed is most abundantly distributed in the upper Midwest, the Great Lakes states, and New England.

Now for the species of conservation concern...


We saw numerous individuals and populations of Purple Milkweed (Asclepias purpurascens). As seen above, the corollas of this State Threatened species are a deeper pink than those of Swamp Milkweed or Common Milkweed, ranging even to reddish or purplish. The center of Purple Milkweed's geographical distribution is Missouri and Illinois, with peripheral populations scattered throughout much of the rest of the eastern United States and Ontario. St. Clair County, Michigan, where Harsen's Island is located, is approaching the northernmost limit of the species' distribution. Purple Milkweed is found in wet prairies, oak savannas, and glades, as well as on woodland borders and in thickets.


We also observed some stunning populations of Sullivant's Milkweed (Asclepias sullivantii), also known as Prairie Milkweed. With glabrous, ascending leaves that have a striking pink midvein, the large, pink corollas of this State Threatened milkweed don't need to be present to make a correct identification. Of the species of milkweed discussed in this post, Sullivant's Milkweed has the most narrow geographical distribution, being found in the prairie region of the Great Plains and the Midwest, north into Ontario. As the common name suggests, this attractive milkweed is a prairie obligate.