Swainsonpea or Austrian Peaweed [Sphaerophysa salsula (Pall.) DC.][SWASA][CDFA list: A] Photographs Map of Distribution

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SYNONYMS:Austrian peaweed, Swainsona

GENERAL DESCRIPTION:Herbaceous, long-lived perennial with aggressive creeping roots, to 1.5 m tall. Swainsonpea is potentially noxious wherever alfalfa [Medicago sativa L.] is grown for seed. A close resemblance of the two seeds makes it difficult to separate in seed production systems. Introduced from Asia.

SEEDLINGS:Less vigorous than alfalfa seedlings.

MATURE PLANT:Stems erect to ascending, covered with short white hairs. Leaves compound, odd-pinnate. Leaflets 15-23, oblong to ovate, 0.5-2 cm long. Lower surfaces of leaflets covered with short white hairs; upper surfaces mostly glabrous. Stipules awl-like, mostly 0.5 cm long or less, fused at the base.

ROOTS and UNDERGROUND STRUCTURES:Develops an extensive system of robust, woody, creeping lateral roots that produce new shoots. Roots can associate with nitrogen fixing bacteria.

FLOWERS:May-July. Pea-like, on racemes in the leaf axils near the stem tips. Corollas brick- to orange-red, 12-14 mm long. Stigmas finely hairy. Pedicels 3-7 mm long.

FRUITS and SEEDS:Pods inflated at maturity, ovoid to spherical, grooved on top,1.5-3.5 cm long, with a stalk-like base 4-7 mm long that protrudes beyond the persistent calyx tube, glabrous, membranous to papery, translucent, often mottled. Pods do not release seeds (indehiscent). Seeds several to numerous, flattened, semi-circular to triangular with a slight notch, ~ 2 mm long, smooth, dull dark brown to black.

HABITAT:Disturbed sites, roadsides, irrigation ditches, cultivated crops.

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DISTRIBUTION:Uncommon. Southern San Joaquin Valley (ce Kings, c Kern cos.). To 500 m (1650 ft).

PROPAGATION/PHENOLOGY:Reproduces by seed and vegetatively from creeping roots. Entire pods disperse as units with seeds enclosed. A large proportion of seed is hard-coated and requires scarification to germinate.


SIMILAR SPECIES:Other weedy perennial members of the pea family that can superficially resemble swainsonpea include camelthorn [Alhagi pseudalhagi (M.Bieb.) Desv.] and wild licorice [Glycyrrhiza lepidota Pursh.]. Unlike swainsonpea, camelthorn has thorns, simple leaves, and pods not inflated. Wild licorice has yellowish- to greenish-white flowers, gland-dotted leaves, and bur-like fruits covered with hooked prickles. In addition, many native locoweeds [Astragalus spp.] have leaves and inflated pods that closely resemble those of swainsonpea. However, locoweeds have glabrous stigmas and styles, pods that lack the stalk-like base above the calyx, and most do not have red flowers. Only scarlet milkvetch [Astragalus coccineus Brandegee] has red flowers. Scarlet milkvetch is a desirable native species that occurs only on the western edge of the deserts and east of the Sierra Nevada in sagebrush communities or pinyon woodlands.

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Prevention and control: Swainsonpea is a long-lived perennial legume found throughout the western United States. It may have been introduced to the United States from Asia for forage or soil stabilization. In California, it has been detected in Kern and Kings counties and is limited in its distribution. Swainsonpea is a threat to alfalfa seed producers; its seeds are nearly identical in size, shape, and weight to alfalfa seed, making separation almost impossible. Avoid introducing swainsonpea by using only certified alfalfa seed.
There is little information regarding control of swainsonpea. Tillage may be ineffective due to an extensive creeping root system that sends up numerous shoots, and may spread severed rootstocks to new areas. Tillage equipment should be thoroughly cleaned after working in infested areas.
Mowing or grazing may reduce seed production, but will be ineffective for complete control. Cattle likely prefer the seed pods of swainsonpea, and seed viability probably remains high after passing through animals. Therefore, cattle should be removed from areas after seed production. Like many other legumes, the seeds are extremely hard and may be viable in the soil for many years.
There is no information regarding chemical control of swainsonpea. Herbicides such as glyphosate, clopyralid, triclopyr, dicamba, 2,4-D, and picloram may be effective in noncrop areas. Control in alfalfa may be difficult, but glyphosate applied with a rope-wick applicator may be effective. However, regrowth is likely with any of these treatments and reapplication may be necessary. Optimal time for treatment is unknown, but other perennial weeds may be most susceptible either at early bloom or during the fall when translocation of carbohydrates to the roots is maximized.
Alfalfa is generally more competitive than swainsonpea. Therefore, maintaining a healthy competitive stand may reduce the potential for invasion. Swainsonpea may be favored in older, thinning stands of alfalfa. Serious infestations in alfalfa may require rotating to a grass crop where more effective control options are available.

Rumbaugh, M. D. 1980. Agronomic characteristics of Sphaerophysa salsula DC. Agronomy Abstracts 72:105.
Whitson, T. D., Burrill, L. C., Dewey, S. A., Cudney, D. W., Nelson, B. E., Lee, R. D., and Parker, R. 1992. Weeds of the West. Jackson, WY: Western Society of Weed Science. 630 pp.

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