alfalfa (Medicago sativa), also called lucerne, or purple medic, perennial, clover-like, leguminous plant of the pea family (Fabaceae), known for its tolerance of drought, heat, and cold; for the remarkable productivity and the quality of its herbage; and for its value in soil improvement. It is widely grown primarily for hay, pasturage, and silage. The plant, which grows 30–90 cm (1–3 feet) tall, arises from a much-branched crown that is partially embedded in the surface layer of soil. As the plant develops, numerous stems bearing many trifoliolate leaves arise from the crown buds. Racemes of small flowers arise from the upper axillary buds of the stems. With approaching maturity, corkscrew coiled pods containing from two to eight or more seeds develop abundantly in regions with much sunshine, moderate heat, dry weather, and pollinating insects.
The primary root of alfalfa attains great depths. When 20 or more years of age, this taproot may descend as much as 15 m (50 feet) or more where the subsoil is porous. This accounts for the unusual ability of the plant to tolerate drought. The roots of seedling plants are known to penetrate the soil for 90 cm (3 feet) at two months and for 180 cm (6 feet) with plants five months of age. Not infrequently, newly established fields of alfalfa survive severe summer drought and heat when other leguminous plants with shallower and more branching roots succumb.
Alfalfa has a remarkable capacity for rapid and abundant regeneration of dense growths of new stems and leaves following cutting. This makes possible from 1 to as many as 13 crops of hay in one growing season. The frequency of harvest and the total seasonal yields are dependent largely on the length of the growing season, the adaptability of the soil, the abundance of sunshine, and especially the amount and distribution of rainfall or irrigation during the growing season. Green leafy alfalfa hay is very nutritious and palatable, containing about 16 percent proteins and 8 percent mineral constituents. In addition it is rich in vitamins A, E, D, and K.
Like all crops, alfalfa is beset with hazards of climate, diseases, and insects. Among the more serious of these are winterkill, bacterial wilt disease, alfalfa weevil, lugus bugs, grasshoppers, spotted aphid, and leafhoppers. In humid areas and in irrigated areas, alfalfa stands of three or more years of age have often become badly thinned by infestations of the soil-borne bacterial wilt organism Phytomonas insidiosum.