Lotus corniculatus L. (bird’s foot trefoil)

Pl. 401 h, i; Map 1778

Plants perennial herbs, from a woody taproot, sometimes with rhizomes. Stems 15–60 cm long, spreading to ascending, branched, often forming loose mounds, angled, unarmed, glabrous or sparsely to moderately pubescent with appressed-ascending hairs, especially toward the tip. Leaves alternate, odd-pinnately compound with 5 leaflets, sessile (sometimes misinterpreted as palmately trifoliate with leaflet-like stipules below a short petiole). Stipules inconspicuous, minute glandlike dots. Leaflets 4–20 mm long, 2.5–5.0(–8.0) mm wide, the basal pair lanceolate or more commonly asymmetrically ovate to broadly ovate (the upper margin angled at the base, the lower margin rounded to somewhat auriculate and clasping the stem), the upper lateral pair and terminal leaflet symmetrically oblong-oblanceolate to obovate, the margins entire, glabrous or sparsely hairy, the surfaces glabrous or sparsely hairy, angled or slightly tapered to a sessile base, angled or tapered (often abruptly short-tapered) to a sharply pointed tip, rarely rounded. Inflorescences axillary, umbellate to loosely headlike clusters of 3–8 flowers, the stalk 2–10 cm long, with fine appressed-ascending hairs or nearly glabrous, with a simple, leaflike bract and a pair of minute glands at the tip, the flower with a stalk 0.7–1.5 mm long above the bract; bractlets absent. Calyces sparsely hairy, the tube 2–3 mm long, conic, the lobes 2–3 mm long, more or less similar, linear to narrowly lanceolate. Corollas papilionaceous, bright yellow, sometimes with the main nerves red toward the base, often turning orange or becoming marked with brownish red with age, the banner 11–16 mm long, 6–8 mm wide, broadly ovate to nearly circular, the wings 10–14 mm long, 3–4 mm wide, oblong, the keel 11–14 mm long, 3–4 mm wide, boat-shaped, fused to above the midpoint, abruptly curved upward near the midpoint, narrowed toward the tip. Stamens 10, of 2 lengths (alternating longer and shorter filaments), 9 of the filaments fused and 1 usually shorter filament free, the fused portion 6–7 mm long, the free portion 2–4 mm long, those of at least the longer stamens flattened and broadened at the tip, the anthers relatively small, attached near the base. Ovary 5–6 mm long, the style 4–6 mm long, slender, curved upward abruptly at the base, hairy at the tip, the stigma small and terminal. Fruits legumes, 1.5–3.0 cm long, 1.5–2.5 mm wide, narrowly oblong to linear, beaked, not circular in cross-section, the sutures not thickened, dehiscing by 2 valves, these green to reddish brown at maturity, twisting spirally after dehiscence, mostly 5–14-seeded. Seeds 1.2–1.5 mm long, 0.9–1.2 mm wide, broadly kidney-shaped in outline, the surface olive green to reddish brown, often mottled with darker brown or black, smooth, somewhat shiny. 2n=24. June–October.

Introduced, scattered to common in the northern half of the state, absent or uncommon farther south (native of Europe, Asia; introduced widely in North America). Upland prairies and banks of streams and rivers; also pastures, old fields, roadsides, and open, disturbed areas.

The bright yellow flowers in umbellate clusters are distinctive for this species. It also is recognized by the peculiar leaves, which sometimes have been interpreted as pseudo 5-foliate leaves (palmately trifoliate but with enlarged leaflet-like stipules) (Allen and Allen, 1981). However, other botanists have interpreted the leaves as pinnately compound with the lowermost pair of leaflets having migrated down the petiole to the stipular position and the true stipules represented by a pair of minute, glandlike processes at the leaf base (Isely, 1981, 1990; Sokoloff, 2000). Support for this latter hypothesis comes (among other sources) from the study of Chrtková-Žertová (1973), who noted that on rhizomes (which she called underground stems) of some forms the leaves are highly reduced, but the glandlike stipules are well-developed.

Steyermark (1963) knew L. corniculatus from only three counties, but it has spread widely, particularly in the northern half of the state, as a result of its use for soil stabilization following highway improvement projects. It is an important forage crop in Missouri, and when planted densely contributes significant amounts of soil nitrogen. Numerous varieties have been described within the native range of the species (Chrtková-Žertová, 1973) and there are also a number of cultivars. Because the material in the United States originated from various cultivars of uncertain origin, the present treatment does not attempt to recognize infraspecific taxa for Missouri materials.