Rare Species Guide

  Synonyms

Asclepias longifolia var. hirtella, Acerates hirtella

  Basis for Listing

Asclepias hirtella (prairie milkweed) appears to be rare or declining in several of the states along the northern periphery of its range in the east-central states as well as in Canada and some of the eastern states. There is no indication of a serious decline in the center of its range. It may, however, be vulnerable to a decline, because native habitats in that region are intensively exploited for agricultural purposes.

Woodson first reported the presence of A. hirtella in Minnesota in 1954, but he did not cite a specimen or give a location. Initial surveys of state and regional herbaria by the Minnesota DNR failed to locate a supporting specimen. Consequently, the status of this species in Minnesota remained unresolved until 1979 when a wildlife manager discovered a population at the Cartney Wildlife Management Area in Mower County Several other sight reports have been traced to confusion with A. viridiflora (green milkweed), a similar species that is not rare. Asclepias hirtella, which was listed as a threatened species in Minnesota in 1984, is quite rare in Minnesota and may be declining. This is indicated by the recent loss of major tracts of prairie habitat in southern Minnesota. Most of these tracts were not adequately inventoried for plants, but they may have sustained populations of this species before they were lost. 

  Description

The genus Asclepias has highly specialized flowers that produce five-hooded bodies that frequently bear incurved horns or crests. In A. hirtella, the hoods are present but the horns are absent. Flowers are greenish with a distinct purplish tinge and appear in lateral umbels borne on slender stalks. Leaves are alternate, linear, very numerous, and crowded. Stems are stout and covered with short soft, downy hairs. Asclepias hirtella bears a superficial resemblance to A. viridiflora but can be distinguished from it by the relatively long slender stalks of the lateral flower clusters.

  Habitat

Two populations at Cartney Wildlife Management Area (Oak Savanna Subsection) occur in an unbroken mesic prairie. It was acquired by the DNR in the 1960s and managed for wildlife habitat. Management included a prescribed burn in 1978, soon after which A. hirtella was first noticed.

  Biology / Life History

Asclepias hirtella has a large array of pollinators, including bees, wasps, butterflies, moths, and beetles. The most important pollinators are apparently bumblebees (genus Bombus [Betz et al. 1994]). Asclepias hirtella is also used as a host plant for monarch butterflies, which is a previously common species in Minnesota thought to be in serious decline (Pocius, et al. 2018). The monarch’s decline is attributed to loss of grassland habitat, which is concomitant with a loss of milkweed habitat in the Midwest. Only A. syriaca (common milkweed) is believed to have stable populations, which is likely because it is the only one that is not dependent on native grassland habitats.

One study reported that reproductive individuals of A. hirtella typically produce up to 27 umbels per stem, each with 14-101 flowers (Betz and Lamp 1992). The umbels are scattered along most of the stem, with the ones lowest on the stem opening first and the ones highest on the stem opening last. The progression happens over a period of weeks, with the lowest umbel having ripened pods and shedding seeds while the upper umbels still have receptive flowers (Betz and Lamp 1992). This allows the plant to take advantage of a greater number of pollinator species. Each seed has a tuft of white to tan hairs that facilitate wind dispersal.

  Conservation / Management

Two of the three known populations of A. hirtella in Minnesota occur on publicly owned land, so its habitat is probably safe from conversion to commercial or residential uses. Actual threats to its habitat have not been fully assessed but could include encroachment by woody species, both native and non-native, and invasion by non-native herbaceous species such as Euphorbia esula (leafy spurge) or Bromus inermis (smooth brome). Herbicides could also be a threat, either as drift from nearby agricultural fields or from the wildlife management area itself. Prescribed burns may be a useful management tool to help control encroaching brush, but the timing of a burn is critical. It should be in early spring when A. hirtella is still dormant. Once the plant has appeared above ground, a fire could do serious damage.

  Best Time to Search

The best time to search for Asclepias hirtella is when it is flowering or fruiting, from mid-June through August.

  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.

Betz, R. F., R. D. Struven, J. E. Wall, and F. B. Heitler. 1994. Insect pollinators of 12 milkweed (Asclepias) species. Pages 45-60 in R. G. Wickett et al. editors. Proceedings of the Thirteenth North American Prairie Conference, 6-9 August 1992, Windsor, Ontario, Canada.

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.

Turner, B. L. 2009. Taxonomy of Asclepias hirtella and A. longifolia (Apocynaceae). Phytologia 91(2): 308-311.