Alkali Swainsonpea

Perennial, Herbs, Plants with rhizomes or suckers, Nodules present, Stems erect or ascending, Stems or branches arching, spreading or decumbent, Stems prostrate, trailing, or mat forming, Stems less than 1 m tall, Stems solid, Stems or young twigs glabrous or sparsely glabrate, Stems or young twigs sparsely to densely hairy, Leaves alternate, Stipules conspicuous, Stipules green, triangulate to lanceolate or foliaceous, Stipules persistent, Stipules free, Stipules clasping stem at the base, Leaves compound, Leaves odd pinnate, Leaf or leaflet margins entire, Leaflets opposite, Leaflets 10-many, Leaves hairy on one or both surfaces, Inflorescences racemes, Inflorescence axillary, Bracts very small, absent or caducous, Bracteoles present, Flowers zygomorphic, Calyx 5-lobed, Calyx glabrous, Petals separate, Corolla papilionaceous, Petals clawed, Petals red, Petals reddish brown, maroon, Petals orange or yellow, Banner petal ovoid or obovate, Wing petals narrow, oblanceolate to oblong, Wing tips obtuse or rounded, Keel tips obtuse or rounded, not beaked, Stamens 9-10, Stamens diadelphous, 9 united, 1 free, Filaments glabrous, Style terete, Style with distal tuft of hairs, Fruit a legume, Fruit stipitate, Fruit unilocular, Fruit freely dehiscent, Fruit orbicular to subglobose, Fruit rugose wrinkled or reticulate, Fruit exserted from calyx, Fruit inflated or turgid, Fruit glabrous or glabrate, Fruit 11-many seeded, Seeds ovoid to rounded in outline, Seed surface sm ooth, Seeds olive, brown, or black.

Sphaerophysa salsula is a species of flowering plant in the legume family known by the common names alkali swainsonpea, Austrian peaweed, and red bladder-vetch. It is native to Asia but it is known in many other parts of the world as an introduced species and often a noxious weed. It grows in cultivated land and disturbed habitat, easily tolerating alkaline substrates. It is commonly seen in areas where alfalfa is grown, because the seeds of the two species look similar and the weed seed is easily imported with the crop seed.

This is a long-lived perennial herb growing up to 1.5 meters tall. It reproduces via seed as well as by sprouting vigorously from its creeping root system. The stems are coated in short white hairs. The leaves are made up of many oval leaflets each up to two centimeters long. The inflorescence is a raceme of several pea-like flowers each just over a centimeter wide. They may be brick-red to deep pink to brownish or red-orange in color. The fruit is a legume pod up to 3.5 centimeters long. It is inflated and bladderlike, hairless, translucent, shiny, and papery when dry. It is mottled greenish or reddish. It contains several seeds each about two millimeters long.

Like other legumes, this plant contains endophytes. These include the bacterium Paracoccus sphaerophysae, which was recently isolated from the roots of this species, and named for it. Other rhizobia in this species include Shinella kummerowiae and species of the genera Rhizobium, Agrobacterium, and Mesorhizobium.