Description and Variation: Swainsonpea is a perennial plant, much branched from the base with a woody taproot and rhizome. The leaves are pinnately compound and alternate on the stem. Each leaf comprised of nine to 25 leaflets which have silvery hairs. The flowers are brick red, drying to purple, and occur on axillary stems, each with four to eight flowers. The fruits of this legume are inflated pods with seeds the size of alfalfa seeds.

Economic Importance: Detrimental: Swainsonpea poses a serious threat to grain growing areas. The seed can be  a serious contaminant in the alfalfa grown for seed. It is known as a troublesome and persistent weed.

Habitat: Swainsonpea is found growing along roadsides, waste areas, fence rows, and in cropland. It prefers wet, irrigated lands.

Geographic Distribution: Swainsonpea is native to Asia from the Caspian Sea to China. It is present in California, Arizona, Nevada, Oregon, Idaho, and Washington. In Washington it is known from the Columbia Basin. No collections are known from western Washington.

History: Swainsonpea was originally imported to the United States as a contaminant of alfalfa seed.

Growth and Development: Swainsonpea is a perennial plant that flowers from May through July.

Reproduction: Swainsonpea reproduces by seeds and rhizomes.

Response to Herbicides: Please refer to the Pacific Northwest Weed Control Handbook for appropriate herbicides for Swainsonpea control.

Response to Cultural Methods: Not known.

Response to Mechanical Methods: Not known.

Biocontrol Potentials: Not known.

Rationale for Listing: Swainsonpea seed is a contaminant to alfalfa, an important crop in Washington. By listing it as a class B noxious weed the goal is to keep it from spreading to alfalfa fields to prevent seed contamination and crop losses.

Notes: Swainsonpea is a noxious weed in Idaho, a Q rated weed in Oregon (newly detected species), and a Class A noxious weed in California.

References:

California Department of Agriculture Detection Manual.

A Guide to the Selected Weeds of Oregon.

Hitchcock, C.L. 1972. Flora of the Pacific Northwest. University of Washington Press.

* Hitchcock, C.L. and A. Cronquist, et al. 1964. Vascular Plants of the Pacific Northwest. University of Washington Press.

Anonymous. 1960. Weeds and Weed Seeds. Seedburo Equipment Co., Chicago, IL.

Whitson, T.D. 1987. Ed. Weeds and Poisonous Plants of Wyoming and Utah. Cooperative Extension, University of Wyoming.

This page was last updated 03/31/07