Psoralidium tenuiflorum (Pursh) Rydb.
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Psoralea tenuiflora, Psoralea floribunda, Psoralidium tenuiflorum, Psoralidium tenuiflora
Basis for Listing
When the first state endangered species list was created in 1984, Psoralidium tenuiflorum was considered a candidate. But since it had been found only twice in Minnesota, once in 1890 and once in 1915, botanists feared it might be extirpated and declined to list it. Fortunately, after intensive field searches, it was rediscovered at one site in 1993 and two more sites in 1994. Two of the populations are small, numbering less than 10 plants. All three populations occur on bluff prairies, and of those, two are being invaded by Juniperus virginiana (eastern red cedar). These sites will require active fire management if this species is to survive. Psoralidium tenuiflorum was subsequently listed as an endangered species in Minnesota in 1996 given its extreme rarity and need for special management.
Psoralidium tenuiflorum is an erect, much-branched, and fairly large plant up to 1 m (39 in.) tall. It has compound leaves, each with 3-7 leaflets that are 2-6 times longer than wide. The terminal leaflet has a stalk about as long as the stalks on the other leaflets. Flowers are blue to pinkish-purple (rarely white) with a distinct stalk. Pods have an abrupt, short beak (Isley 1962; Great Plains Flora Association 1986).
Psoralidium tenuiflorum occurs on dry to dry-mesic bluff prairies in shallow soil over bedrock. It also grows in dry, rocky to sandy soil on lower portions of slopes.
Biology / Life History
This species produces most of its flowers in spring and, less often, many short, few-flowered stalks throughout summer. Only a few flowers in each inflorescence will develop fruits. Fruits are oval pods. The plant will spread by underground stems (Isley 1962; Larson and Johnson 1999).
Conservation / Management
The protection of prairie remnants within Minnesota is essential for the survival of this and other prairie species. While the bluff prairie habitat that P. tenuiflorum prefers has not been converted as extensively as mesic prairie habitats, the scenic vistas offered by bluff prairies are increasing their attraction for residential development. Protection may be achieved by the outright purchase of a prairie parcel by conservation-minded interests or the purchase of conservation easements. Woodland encroachment in P. tenuiflorum habitat, especially by J. virginiana, is also a major threat and may require mechanical removal of woody species. In addition to mechanical treatments, a regular regimen of prescribed burns is needed to maintain the open conditions required by this species. However, the timing and conditions of prescribed burns are critical. They should be limited to a dormant season when P. tenuiflorum is not actively growing, ideally in spring, but an acceptable alternative is in autumn.
Conservation Efforts in Minnesota
Mechanical removal of J. virginiana is ongoing at the largest population site. One of the small populations occurs on State Forest land, which may offer opportunities for long-term conservation.
Great Plains Flora Association. 1986. Flora of the Great Plains. University Press of Kansas, Lawrence, Kansas. 1,402 pp.
Isely, D. I. 1962. Leguminosae of the north-central states IV. Psoraleae. Iowa State College Journal of Science 37(2):103-162.
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.