Rare Species Guide

  Basis for Listing

Asclepias stenophylla (narrow-leaved milkweed) is characteristic of dry hill prairies in the Lower Great Plains region. It was not known to occur in Minnesota until 1978 when a single population was discovered near Hokah in Houston County (The Blufflands Subsection). The Minnesota population is separated by about 500 km (310 mi.) from the main range of the species. It is believed that A. stenophylla originated in the western Ozarks and spread into the plains states, where it was already adapted to a somewhat dry habitat. If it is true that this species is in the process of migrating from its ancestral home, the population in Houston County is not a remnant of former populations but the result of long-range dispersion. Such pioneering populations are of special biological significance and are believed to play an important role in the development of unique species. Asclepias stenophylla was listed as an endangered species in Minnesota in 1984.

  Description

Asclepias stenophylla is one of a few milkweeds in the region with long narrow leaves and white to greenish flowers. It may be distinguished from other Asclepias species by its opposite, widely spaced leaves and its large short-stalked flowers that have hoods with an internal crest ending in a tiny horn (Great Plains Flora Association 1986; Larson and Johnson 1999).

  Habitat

The Minnesota population of A. stenophylla occurs in gravelly soil near the base of a southwest-facing hill prairie. This habitat is very similar to the limestone glades and hill prairies that the species occupies in the main part of its range.

  Biology / Life History

Milkweeds as a genus (the genus Asclepias) are sometimes described as the dicot counterpart to orchids (Wyatt 1994) in that they have complex insect-pollinated flowers that produce pollen grains in coherent packets called pollinia. Unlike orchids, milkweeds are generally not pollinator-specific. Any number of insects of the appropriate size can pollinate milkweeds, though the most common pollinators are bees, wasps, butterflies, and moths. In general, milkweed species are self-incompatible, meaning that a flower cannot be pollinated by pollen from the same plant.

A few species of Asclepias can spread by growth of underground rhizomes, but A. stenophylla reproduces only by seed. Each seedpod produces a number of seeds, each with a tuft of tan hairs that facilitate wind dispersal. One published study (Betz and Lamp 1990) reported on flower, pod, and seed production in 18 species of Midwestern prairie milkweeds. It reported that A. stenophylla produced a mean of 8 seeds per stem, which is quite low. By comparison, A. syriaca (common milkweed) produced a mean of 1,123 seeds per stem. Seed dormancy is known to be widespread in the genus Asclepias, but dormancy and germination requirements for A. stenophylla could not be found in a search of published literature.

  Conservation / Management

Habitat loss is a major concern with all prairie species (Minnesota’s Remaining Native Prairie). Threats to this habitat include woodland encroachment, invasion by noxious weeds such as Euphorbia esula (leafy spurge), 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. Protection of suitable habitat is essential for the conservation of this species. 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 regimen of dormant-season prescribed fire at intervals of 3-5 years should be considered so as to maintain the open grassland habitat preferred by A. stenophylla. Population monitoring is advised. Such monitoring may include a population census, changes to the habitat, status of non-native invasive species, and encroachment of human activities from adjacent lands. All noxious weed infestations should be identified and destroyed quickly, but herbicides should be used very carefully, if at all.

  Best Time to Search

The best time to search for Asclepias stenophylla is during flowering from June through August.

  Conservation Efforts in Minnesota

The only known site of A. stenophylla in Minnesota is currently protected in a state Scientific and Natural Area.

  Authors/Revisions

Welby R. Smith (MNDNR), 2020

(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.

Larson, G. E. and J. R. Johnson. 1999. Plants of the Black Hills and Bear Lodge Mountains. South Dakota State University, Brookings, South Dakota. 608 pp.

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

Wyatt, R., and S. B. Broyles. 1994. Ecology and evolution of reproduction in milkweeds. Annual Review of Ecology and Systematics 25:423-441.