Asclepias stenophylla Gray
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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 Minnesota population is separated by about 500 km (311 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.
Asclepias stenophylla is one of a few milkweeds in the region with thin 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).
The Minnesota population of A. stenophylla population 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
Asclepias stenophylla is an herbaceous perennial that reproduces only by seed. Milkweeds in general have a large array of pollinators including bees, wasps, butterflies, moths, skippers, beetles, and birds. Each seedpod produces numerous seeds with a tuft of tan hairs that facilitate wind dispersal.
Conservation / Management
Habitat loss is a major concern with all prairie species. 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. 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 2-5 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, habitat changes and trends, 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 A. 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.
Great Plains Flora Association. 1986. Flora of the Great Plains. University Press of Kansas, Lawrence, Kansas. 1,402 pp.
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.