Melilotus alba is an annual or biennial herb, 0.3-1.2 m tall, from a taproot, root nodules associated with nitrogen-fixing Rhizobium bacteria, contractile roots pull root crown below soil in winter; stems 1-10 from root crown, erect, grooved, usually hairy near top; stipules linear, 0.4-1 cm long, 0.1 cm wide.
Leaves alternate, 3-parted, stalk ca 1 cm long, leaflets 1-4 cm long, 0.8-2 cm wide, oblong, lower leaflets broader than upper, tip blunt, underside finely hairy, margin toothed, stalk of middle leaflet 0.5-0.6 cm long, those of lateral leaflets 0.2 cm long (Yatskievych 2006).
Flowers white, 0.4-0.6 cm long, bilaterally symmetrical, pea-flower-shaped; top petal (standard, banner, flag) 0.4-0.5 cm long, longer than side petals (wings); stamens 10, 9 fused, 1 solitary, style 0.2 cm long, ovary superior, all enclosed within lower petal (keel); calyx tubular, 0.1 cm long, 2-lipped, upper lip 2-lobed, lower lip 3-lobed, tube tapered to base (Radford et al. 1968; Yatskievych 2006); inflorescence spike-like (racemes) 5-20 cm long; flowers self-fertile, also pollinated by bees, also visited by flies, wasps; flowering induced by days over 12 h long; plants dies after flowering; blooms June-Sept. (Turkington et al. 1978; Hough 1983).
Fruit dry, pods 0.3-0.4 cm long, dark brown to black, somewhat net-veined, base of style persisting as pointed tip, 1-2 seeds; fruits July-Oct., usually late summer, seeds dispersed by rainwater and wind, can survive in soil for 20 years; seedlings generally appear in March-April and Sept.-Oct. (Turkington et al. 1978).
Wetland status: FACU-.
Frequency in NYC: Common.
Habitat: Open, disturbed areas, on fill, along roadsides. Drought tolerant, salt tolerant, alkali tolerant, prefers soil of pH 6.5 or higher. Intolerant of shade. More flood tolerant than M. officinalis.
Notes: Escaped, often invasive agricultural plant grown as fodder for livestock. Plants also eaten by deer, rabbits and muskrats, seeds eaten by some birds and small mammals (Martin et al. 1951). Host to a number of insects including a spittlebug (Philaenus leucophthalamus, Cercopidae, Homoptera); a wasp moth, Ctenucha virginica, Archtiidae (Borror and White 1970; Covell 1984; Eckardt 1995; Turkington et al. 1978). Early in the 1920s a hemorrhagic disease of cattle was traced to fermented or moldy sweet clover. Subsequently, the toxic principle was found to be coumarin which has been developed into the medically valuable anti-coagulant drug dicoumarin and the rodenticide warfarin. The effect of coumarin and its derivatives is to lower levels of the coagulant prothrombin in blood (Kingsbury 1964). Differs from Medicago in absence of conspicuously hairy leaf underside, tastes bitter, M. alba blooms about a month later than M. officinalis.