Melilotus (L.) Mill.

Plants annual or biennial herbs (sometimes short-lived perennials farther south), with relatively stout taproots. Stems erect or strongly ascending, several-angled or ridged, branched, unarmed, glabrous or sparsely pubescent with short, curved hairs toward the tip. Leaves alternate, pinnately trifoliate, mostly relatively long-petiolate, the terminal leaflet short- to relatively long-stalked. Stipules linear above an expanded base (more or less awl-shaped), fused basally to the petiole base, the margins entire, often relatively thin and pale, persistent; stipels absent. Leaflets oblanceolate to obovate, oblong-elliptic, or elliptic, those of the uppermost leaves often narrowly so, the lateral leaflets usually slightly shorter than the terminal one, symmetric, angled or slightly tapered at the base, rounded at the tip, the midvein usually extended as a minute, sharp point at the very tip, the margins sharply and finely toothed, the basal portion often toothless, the upper surface glabrous, the undersurface moderately pubescent with fine, appressed hairs, the venation pinnate, the secondary veins ending in the teeth. Inflorescences axillary, elongate, spikelike racemes with numerous flowers, the stalks mostly longer than the subtending leaves, the bracts 0.8–1.5 mm long, linear to hairlike, inconspicuous, persistent, the flower stalks 0.5–2.0 mm long, downward-curved; bractlets absent. Calyces glabrous or finely hairy, 1.5–2.2 mm long, the tube about as long as to slightly longer than the lobes, bell-shaped, 5-lobed, the lobes more or less similar, triangular to narrowly triangular. Corollas papilionaceous, yellow or white, the banner with the expanded portion obovate to oblong-obovate, narrowed to a short, stout, stalklike basal portion (this sometimes nearly absent), shallowly notched at the tip, curved or bent upward from about the midpoint, the wings in our species slightly shorter than to about as long as the banner, with a stout, stalklike base, the expanded portion asymmetrically oblong to oblong-obovate, with a prominent auricle on 1 side, the keel about as long as the wings and more or less fused with them below the midpoint, boat-shaped, only slightly curved upward, rounded to bluntly pointed at the tip. Stamens 10, all of similar lengths, 9 of the filaments fused and 1 free to the base or nearly so, the fused portion usually much longer than the free portion, often oblique at the tip, the anthers relatively small, attached toward the base, yellow. Ovary short, sessile or very short-stalked, the style slender, usually longer than the ovary, glabrous, the stigma small and terminal. Fruits modified legumes, ovoid to oblong-ovoid, slightly flattened, sessile or tapered to a short, stout stalk at the base, the tip with a slender beak, indehiscent (occasionally dehiscing tardily and irregularly), the surfaces leathery, sometimes appearing wrinkled, with a pattern of raised nerves, light brown to dark brown, gray, or black, 1(2)-seeded. Seeds 1.5–2.5 mm long, ovoid (usually with a shallow, broad, asymmetric notch, somewhat flattened, the surface yellow to reddish brown, smooth, usually shiny. About 20 species, Europe, Asia, Africa, introduced nearly worldwide, mostly in temperate regions.

Melilotus is recognized by the upright habit with ascending branches, pinnately trifoliate leaves with toothed leaflets, elongate spikelike racemes of small, yellow or white flowers, and small, achenelike, 1-seeded legumes. Melilotus is closely related to Medicago and Trigonella L. (which contains fenugreek, T. foenum-graecum L.) in the tribe Trifolieae (Bronn) Endl. In fact, recent molecular phylogenetic research has suggested that the species of Melilotus represent merely a specialized group within Trigonella in which the fruits have become reduced, one-seeded, and indehiscent (Steele et al., 2010). If future studies support this conclusion, then the species of Melilotus will have to be transferred to Trigonella, as the latter is the older of the two generic epithets.

Cultivated sweet clovers are mostly biennials. During the first year’s growth, the plants produce a central, much-branched stem. The root becomes fleshy as the plant stores food for the winter. In the second year, the plant produces vigorous, rapidly growing stems with dense foliage, flowers and fruits. Both M. albus and M. officinalis were introduced into North America by the 1600s (Turkington et al., 1978).

The drying herbage of sweet clovers often exudes a pronounced, sweet aroma. The species are winter hardy, drought tolerant, and highly valued as forage plants. Most species of Melilotus have a preference for alkaline soils. The foliage is bitter, but nutritious, and eaten by cattle and other livestock when they get used to it. The plants contain coumarin, which is responsible for the distinctive vanilla-like smell of the cut foliage. There is no problem with grazing the plants, but under conditions of high heat or spoilage coumarin is converted to dicoumarol, which interferes with vitamin K uptake and prevents blood from coagulating. Cattle fed large amounts of improperly cured or spoiled hay suffer uncontrollable bleeding with any cut or bruise. Low-coumarin cultivars have been developed, however they can interbreed with nearby high coumarin plants. Melilotus also is valuable for soil improvement and as a cover crop for eroded land. It can also become an unwanted weed in wheat fields and is considered an invasive exotic in prairie and glade habitats. The flowers of Melilotus species are attractive to bees, produce nectar over a long period, and are a major source of honey.