Asclepias sullivantii Engelm. ex Gray
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Basis for Listing
The original range of Asclepias sullivantii (Sullivant's milkweek) coincided with that of the tallgrass prairie, its primary habitat. This range now serves as the major corn-producing region of the country, and very little of the original prairie remains. Because of this habitat loss, the status of this species appears to be deteriorating. This is especially true along the northern periphery of its range in Minnesota, Wisconsin, Michigan, and Ontario. Elsewhere A. sullivantii is faring better, but throughout much of its range its future is still linked to the fate of the native prairies. In Minnesota, for example, conversion of prairie to agricultural production has been nearly complete. Of the original 7.2 million hectares (18 million acres) of prairie in Minnesota, less than 2% remains intact (Minnesota's Remaining Native Prairie). Most of the surviving A. sullivantii plants in Minnesota are confined to prairie remnants that occur on railroad rights-of-way. Railroad companies have abandoned less profitable lines in an effort to reduce losses. These abandoned rights-of-way are frequently sold to adjacent landowners who incorporate them into farms for crop production. This could eventually lead to the demise of most of the remaining populations of A. sullivantii.
Adequately protected populations do occur in the Wild Indigo Prairie Scientific and Natural Area (SNA) in Mower County and the Iron Horse SNA in Dodge County, both of which are abandoned rights-of-way acquired by the DNR for preservation purposes. Asclepias sullivantii was listed as a threatened species in Minnesota in 1984.
Asclepias sullivantii can be easily identified in the field by a few simple characters. Flowers are purplish in color and leaves are hairless, smooth, thick, and leathery. The entire plant, including the pod, is very smooth. Asclepias syriaca, the common milkweed, is similar but it has more flowers, which are about half the size, and thinner leaves with short, downy hairs. The rare A. purpurascens (purple milkweed) is also similar, but it has even smaller flowers than A. syriaca and short, downy hairs on the lower leaf surface. It is very common to see non-flowering specimens of Apocynum cannabinum (American hemp) misidentified as A. sullivantii. For this reason, only records of flowering specimens can be accepted as authentic, and only herbarium specimens serve as adequate documentation.
In Minnesota, A. sullivantii is restricted to undisturbed wet and mesic tallgrass prairies. It frequently occurs with other declining prairie species such as Arnoglossum plantagineum (tuberous Indian-plantain) and Parthenium integrifolium (wild quinine).
Biology / Life History
Asclepias sullivantii is a long-lived perennial. Flowers appear in mid-July and fruits mature in August. Flowers are modified for insect pollination, drawing a large array of pollinators to this plant including bees, wasps, flies, moths, skippers, butterflies, beetles, and plant bugs. Each greenish seedpod has numerous seeds with a tuft of white hairs that facilitate wind dispersal.
Conservation / Management
Habitat loss and degradation are the major concerns with tallgrass prairie species. Threats to this habitat include brush encroachment, invasion by non-native species such as Euphorbia esula (leafy spurge) and Bromus inermis (smooth brome), and human activities such as road construction, agriculture, housing development, and off-road vehicles. Fire suppression poses a threat as it allows the natural progression from grassland to woodland. Herbicide application from adjacent farm fields and along railroad and road rights-of-way may damage this species.
Protection of undisturbed tallgrass prairie habitat is essential. It is equally important that adjacent buffer lands be protected so as to restrict herbicide drift, support pollinators, and ease management of prescribed fire. A prescribed fire regimen should be considered on sites so as to maintain the open prairie habitat that A. sullivantii requires. Monitoring populations is advisable. All noxious weed infestations should be identified and destroyed. Surveys should be conducted on potential habitat in hopes of discovering new populations and to better understand the ecology of this species.
Several programs and resources are available to land managers and landowners to help protect and manage remaining prairie parcels including the Native Prairie Bank Program, the Native Prairie Tax Exemption Program, and a prairie restoration handbook.
Best Time to Search
The best time to search for A. sullivantii is when it is in flower from June through August, especially mid-July.
Conservation Efforts in Minnesota
Several sites supporting A. sullivantii occur on publicly owned lands that are managed for their natural qualities. Hopefully, this increases the chances of survival for this rare species.
References and Additional Information
Great Plains Flora Association. 1986. Flora of the Great Plains. University Press of Kansas, Lawrence, Kansas. 1,402 pp.
Penskar, M. R., and P. J. Higman. 2000. Special plant abstract for Asclepias sullivantii (Sullivant's milkweed). Michigan Natural Features Inventory, Lansing, Michigan. 2 pp.