Pediomelum tenuiflorum (Pursh) Rydb.
Psoralea tenuiflora, Psoralea floribunda, Psoralidium tenuiflorum, Psoralidium tenuiflora
Basis for Listing
Psoralidium tenuiflorum (slender-leaved scurfpea) is a widespread species found primarily in the Great Plains and prairie regions of the Midwest. In Minnesota, it is restricted to specialized habitats in the southern part of the state. When the first state endangered species list was created in 1984, P. tenuiflorum was considered a candidate. But since it had been found only twice in Minnesota, once in 1890 and once in 1915 (Minnesota River Prairie Subsection), botanists feared it might be extirpated and declined to list it. Fortunately, after intensive field searches, it was rediscovered in 1993. Since then, only a small handful of occurrences have been recorded (The Blufflands Subsection). All currently known populations occur on bluff prairies and at least 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 active management.
Psoralidium tenuiflorum is a fairly large erect and much-branched plant up to 1 m (39 in.) tall. The stem is rather slender and is covered with short stiff hairs. The branches have palmately compound leaves, each with 3-7 small narrow leaflets that are hairy on the lower surface, smooth and gland-dotted on the upper surface. The leaves on the lower portion of the stem have distinct petioles; the leaves on the upper portion are nearly sessile. Flowers are blue to pinkish-purple (rarely white) with a distinct stalk. The flower petals are 5-8 mm (0.2-0.3 in.) in length. Pods are about 8 mm (0.3 in.) long and have an abrupt short beak (Isely 1962; Great Plains Flora Association 1986).
Psoralidium tenuiflorum occurs on dry to dry-mesic south-facing bluff prairies in shallow soil over bedrock (usually sandstone). It also grows in dry, rocky to sandy soil on lower portions of slopes.
Biology / Life History
This species is a long-lived perennial that produces most of its flowers in early summer. The flowers are presumed to be insect-pollinated, but no specific pollinator species have been identified. It is common for only a few flowers in each inflorescence to develop fruits. The fruits are small dark oval pods. Mechanisms for seed dispersal are unknown (Isely 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.
Best Time to Search
The best time to search for Psoralidium tenuiflorum is when the plant is flowering in May and June.
Conservation Efforts in Minnesota
Mechanical removal of J. virginiana is ongoing at the species’ largest population site. One of the small populations occurs on State Forest land, which may offer opportunities for long-term conservation.
Welby R. Smith (MNDNR), 2021
(Note: all content ©MNDNR)
References and Additional Information
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.
The Great Plains Flora Association. 1986. Flora of the Great Plains. University Press of Kansas, Lawrence. 1,402 pp.