Melilotus officinalis is a biennial herb, 0.4-2 m tall, from a taproot, root nodules associated with nitrogen –fixing Rhizobium bacteria, stems erect or ascending; stipules lance-shaped 0.5-0.8 cm long (Radford et al. 1968).
Leaves alternate, 3-parted (trifoliate), leaflets 1-2.5 cm long, 0.5-2 cm wide (Yatskievych 2006), obovate to oblanceolate (inversely oval or lance-shaped), mature leaflets mostly hairless, margin toothed, the terminal leaflet with stalk to 0.5 cm long, stalks of lateral leaflets ca 0.1 cm long (Radford et al. 1968; Yatskievych 2006), leaves below flower stalks often single, differs from Medicago in absence of hairy leaf underside, tastes bitter.
Flowers yellow, 0.6 cm long, bilaterally symmetrical, pea-flower-shaped; petals 0.5-0.7 cm long, 0.4 cm wide, wings and standard almost equal length; calyx tubular, lobes linear, 0.6-1 cm long, base of tube rounded, often slightly pouched on one side (gibbous), (Radford et al. 1968); inflorescence of racemes 5-15 cm long from upper axils; pollinated by bees and other insects, not self-fertile; blooms May-Oct. (Turkington et al. 1978; Hough 1983).
Fruit dry, pod 0.3-0.5 cm long, pale brown, strongly net-veined, but transverse ridges more prominent than longitudinal ridges, pod notched near base; seeds 1-2; fruits July-Oct. Seeds dispersed by rainwater and wind, can survive for 20 years in soil.
Wetland status: FACU-.
Frequency in NYC: Occasional, apparently not as common as M. alba.
Habitat: Open, disturbed areas, fill, roadsides. An escaped agricultural plant, grown as fodder for livestock. Drought tolerant, more so than M. alba, salt tolerant, alkali tolerant, soil pH 5-8. Intolerant of shade and flooding (USDA, NRCS 2006).
Notes: Generally flowers earlier than M. alba; Host to a number of insects (Eckardt 1995; Turkington et al. 1978). Plants eaten by deer, rabbits and muskrats, seeds eaten by some birds and small mammals (Martin et al. 1951). A source of coumarin. Moderately toxic (see M. alba), also apparently allelopathic but a high nitrogen fixer (USDA, NRCS 2006). Apparently edible if fresh, also useful as a medicinal (Fern 2004).