Palouse Prairie Foundation plant database (under development)
Genus species:      Common name:     Match: Full Partial
Plant Species: Astragalus arrectus, Palouse milkvetch

Kingdom: Plantae
Phylum: Magnoliophyta -- flowering plants
Class: Magnoliopsida -- dicots
Family: Fabaceae -- pea
Genus: Astragalus
Species: arrectus
Common Name: Palouse milkvetch
Species Code: ASAR7
Origin: endemic to the Palouse region in eastern Whitman County and adjacent parts of Idaho, although disjunct populations occur in central Washington.
Rare: A. arrectus is ranked S2 by both the Natural Heritage Program in Washington and the Conservation Data Center in Idaho.

Form: forb, perennial, erect, 30-50 cm tall, sparsely appressed pubescent overall, with a woody taproot and caudex.
Duration: perennial
Habitat Type: prairie, shrub thickets
Wetland Indicator Status: not listed

Leaves: pinnate compound, 9-22 cm long; leaflets 17-32, but usually 21-31, oblong to elliptical, glabrous above, peduncles longer than the leaves.
Mature height: 12-20 inches
Flowers: borne in dense, erect raceme of 15-35 flowers; corolla whitish, 12-13 mm long.
Flower color: white to pale yellow
Bloom: late May to early June
Bloom starts on: late May
Bloom ends on: early June
Fruit: linear, 12-20 mm long, stipate, the suture nearly dividing the pod; seeds 2 mm, olive-brown, reniform.
Vegetation type:
Astragalus palousensis Piper is synonymous.
A. palousensis in Piper & Beattie 1914, Hitchcock et al 1969.
Reproduces sexually by seed.
n=24 (Hu et al 1999).
Pollinated by insects.
Flowers are perfect.
Fruit is a legume.
Some Astragalus species are toxic to livestock.
Larva of the clouded sulfur (Colias philodice), the silvery blue (Glaucopsyche lygdamus), the western tailed blue (Everes amyntula), and the arrowhead blue (Glaucopsyche piasis) butterflies feed on Astragalus species. While they prefer alfalfa, larva of the orange sulfur butterfly (Colias eurytheme) may also feed on Astragalus species (Pocewicz 2005).
Worldwide, Astragalus is the largest genus of vascular plants, numbering around 2500 species. Around 400 of these occur in North America. Most are narrow endemic species specifically adapted to a particular set of habitat conditions (Hu et al 1999).

Sun requirement: full
Soil moisture: xeric to mesic
Fire: Some Astragalus spp. resist fire by having deep taproots and the ability to form new shoots from the taproot. The new shoots will produce seed the year following a fire (McLean 1969).
Hazards: Some Astragalus species are toxic to livestock.

Sowing time: spring
Transplant time: spring
Stratification: seed should be scarified.
Seed yield: high
Seed harvest: moderate difficulty
Seed first harvest: third season
Seed cleaning: easy
Planting duration: medium
Seed insect problem:
Seed shatter: medium
Seed size: medium
Seed harvest date: early July
Seed comments:

Herbaria: Specimen data and digital resources from The Consortium of Pacific Northwest Herbaria
Key words:
Alternate Genus:
Alternate Species: palousensis
Alternate Variety:

1 protocol in the Native Plant Network
Pullman, Washington Plant Materials Center
Reproduces sexually by seed.

Notes: Astragalus arrectus is a rare species endemic to the Palouse. It should not be collected. Other names include Palouse milkvetch, Astragalus palousensis (Skinner et al 2005).

Hitchcock, C. Leo, Arthur Cronquist, Marion Ownbey, and J.W. Thompson. 1969. Vascular Plants of the Pacific Northwest. University of Washington Press. Seattle, WA. 5 vol.

Hu, Jer-Ming, Michael J. Sanderson, and Martin F. Wojciechowski. 1999. Website for the Largest Genus of Vascular Plants, Astragalus. University of California, Davis. Online at Accessed 10/29/06.

McLean, Alastair. 1969. Fire Resistance of Forest Species as Influenced by Root Systems. Journal of Range Management 22:120-122.

Piper, C.V., and R.K. Beattie. 1914. The Flora of Southeastern Washington and Adjacent Idaho. Lancaster, PA: Press of the New Era Printing Company. 296 pp.

Pocewicz, Amy. 2005. Host Plants of Palouse Butterfly Species. 2 page handout to accompany the April 2005 presentation to the Palouse Prairie Foundation.

Skinner, David M., Paul Warnick, Bill French, and Mary Fauci. 2005. More Palouse Forbs for Landscaping. USDA NRCS Pullman Plant Materials Center and Palouse Prairie Foundation. Online at

Plant Profile from the USDA PLANTS Database
Washington Natural Heritage Program Field Guide to Selected Rare Plants Astragalus arrectus species information