Asclepias amplexicaulis Sm.
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Basis for Listing
The concern for Asclepias amplexicaulis (clasping milkweed) is a result of its natural rarity in Minnesota, the specificity of its habitat needs, and the land-use trends that are putting pressure on the habitat of the few remaining populations. These factors seem to be compounded by the natural demographics of this species, which may require relatively large tracts of intact habitat. This is indicated by the widely scattered spatial distribution of individuals, and the relative infrequency of seedlings and juveniles observed at known sites in Minnesota.
Asclepias amplexicaulis was listed as special concern in 1984, based primarily on historical data. Since then, extensive botanical surveys have been completed within the geographic and ecological range of this species. The results revealed a species in steep decline, with populations restricted to small habitat remnants. For this reason, the status was changed to threatened in 2013.
Asclepias amplexicaulis occasionally reaches 50 cm (1.6 ft.) in height and has large, opposite, leathery leaves. True to its common name, the leaves have no petiole (stalk) and appear to clasp the stem. The flowers are green, with a pinkish or purplish tint and, except for their large size, are similar in structure to flowers of other milkweeds. Unlike most milkweeds, however, the flowers occur in a single umbel at the top of the stem, rather than in multiple umbels in the axils of the leaves.
In Minnesota, A. amplexicaulis occurs exclusively in dry, sandy, sparsely vegetated soil in savannas, and upland prairies. It requires full sunlight and minimal competition from other perennials. In today's landscape, these are often small remnants of larger habitats that have been fragmented by human activities.
Biology / Life History
Like all milkweeds, A. amplexicaulis is a perennial, though it is not known how long individuals actually live. The thick, waxy leaves are an adaptation to the hot, dry habitat in which the species lives, allowing it to survive summer-long droughts and still produce viable seeds.
Like orchids, each milkweed flower produces a pair of pollinial sacs, rather than loose pollen. Each sac contains a waxy pollen mass of approximately 500 pollen grains. These sacs must be extracted from one flower by the leg of an insect pollinator and inserted in another. In the case of A. amplexicaulis, pollinators are usually bees, including domestic honey bees (Struven et al. 1994). Milkweeds, in general, experience low fruit-set, typically averaging 1-5% (Wyatt and Broyles 2012). One study showed A. amplexicaulis will produce, on average, about 25 flowers per stem. These flowers are likely to produce only 1 or 2 pods, each with 60 – 186 seeds (Betz and Lamp 1992). The seeds are dispersed on wind currents, potentially great distances.
Conservation / Management
Asclepias amplexicaulis is a plant of prairies and open savannas, where the trees are few and widely spaced. These conditions developed under a regime of periodic wildfire, started by lightning strikes. This fire regime favors herbaceous vegetation and discourages woody vegetation. Since wildfires are essentially a thing of the past, prairie and savanna habitats are frequently degraded by encroaching woody vegetation, which shades out herbaceous plants. The survival of A. amplexicaulis requires that the original, open conditions be maintained or recreated, preferably with a program of controlled burns conducted in early spring (before the plants have emerged from winter dormancy), on a 3 to 8 year rotation. It is also known that A. amplexicaulis does not compete well with invasive, non-native species, especially sod-forming grasses such as Bromus inermis (smooth brome). Controlling such invasives requires close monitoring and an ability to respond quickly to newly developing threats.
While its distinctive leaves can be seen and recognized all summer and fall, the best time to search for A. amplexicaulis is when the flowers or fruits are present, from early June through mid-August.
Conservation Efforts in Minnesota
A number of remnant habitats that contain small populations of A. amplexicaulis are in state ownership and are being managed to maintain natural habitat conditions. At this time, the effects of such management on this rare species are not being closely monitored, however, there is reason to believe the species is responding favorably.
Author: Welby Smith, MNDNR, 1988.
Revised: Welby Smith, MN DNR, 2008 and 2016