Rare Species Guide

  Basis for Listing

The original range of Asclepias sullivantii (Sullivant's milkweed) 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, Ontario, and the Dakotas. 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 7.3 million ha (18 million acres) of prairie in Minnesota, less than 2% remains intact.  Most of the surviving A. sullivantii plants in Minnesota are confined to wet or mesic 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 Scientific and Natural Area (SNA) in Mower County and the Iron Horse Prairie SNA in Dodge County, both of which are abandoned rights-of-way acquired by the MN DNR for preservation purposes. Asclepias sullivantii was listed as a threatened species in Minnesota in 1984.

  Description

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 (common milkweed), is similar, but it has more flowers, which are about half the size, and thinner leaves with short, downy hairs.  Asclepias 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.

  Habitat

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-June, 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, butterflies, and beetles. It is also used as a host plant for monarch butterflies, which is an ordinarily common species thought to be in serious decline (Pocius, et al. 2018). The decline of monarchs is attributed to loss of habitat, which is concomitant with a loss of milkweed habitat in the Midwest. Only A. syriaca is believed to have stable populations, which is likely because it is the only milkweed that is not dependent on native grassland habitats. One study reported that reproductive individuals of A. sullivantii typically produce only 1-5 umbels per stem, each with 1-40 flowers, resulting in 0-5 pods per stem, with a total of 116-253 seeds per pod (Betz and Lamp 1992). Each seed has a tuft of white hairs that facilitate wind dispersal. It has been reported that A. sullivantii is capable of hybridizing with A. syriaca (Klips and Culley 2004). This poses a potential threat to the genetic integrity of small populations of A. sullivantii that are in close proximity to larger populations of A. syriaca.

  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 plant 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 can 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 controlled. 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 Asclepias sullivantii is when it is in flower or fruit from mid-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.

  Authors/Revisions

Welby R. Smith (MNDNR), 2021

(Note: all content ©MNDNR)

  References and Additional Information

Betz, R. F., and H. F. Lamp. 1990. Flower, pod, and seed production in eighteen species of milkweeds (Asclepias). Pages 25-30 in D. D. Smith and C. A. Jacobs, editors. Proceedings of the Twelfth North American Prairie Conference, 5-9 August 1990, Cedar Falls, Iowa.

Klips, R. A., and T. M. Culley. 2004. Natural hybridization between prairie milkweeds, Asclepias sullivantii and Asclepias syriaca: morphological, isozyme, and hand-pollination evidence. International Journal of Plant Sciences 165(6):1027-1037.

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.

Pocius, V. M., J. M. Pleasants, D. M. Debinski, K. G. Bidne, R. L. Hellmich, S. P. Bradbury, and S. L. Blodgett. 2018. Monarch butterflies show differential utilization of nine Midwestern milkweed species. Frontiers in Ecology and Evolution 6:169.

The Great Plains Flora Association. 1986. Flora of the Great Plains. University Press of Kansas, Lawrence. 1,402 pp.