Trifolium dubium Sibth. (little hop-clover)

Pl. 409 a–c; Map 1813

Plants annual, more or less taprooted. Stems 5–40 cm tall, erect or ascending from an often spreading base, sometimes rooting at the lower nodes, unbranched or branched, glabrous or with fine appressed hairs. Leaves short- or moderately petiolate toward the stem base to nearly sessile toward the tip, the longest petioles to 15 mm, mostly shorter than the leaflets. Stipules shorter than the associated petiole, ovate, the fused basal portion less than half the total length, the free portions tapered at the tip, herbaceous but paler toward the base. Leaflets 5–15 mm long, 4–7 mm wide, the terminal leaflet with a stalk 0.8–1.5 mm long, the lateral leaflets sessile or nearly so, obovate, angled at the base, rounded to shallowly notched at the tip, usually with a small tooth at the very tip, the margins shallowly and inconspicuously toothed, the surfaces glabrous or sparsely hairy along the midvein. Inflorescence 5–10 mm long, 6–8 mm wide, more or less ovoid to obovoid dense spikelike to headlike racemes, the stalk 12–30 mm long. Flowers 3–20, short-stalked, the stalk becoming sharply reflexed with age. Calyces 1.5–2.0 mm long, the tube 0.5–0.8 mm, glabrous, the teeth 1.5–2.0 times as long as the tube, unequal (the lower teeth about 2 times as long as the upper ones) slender and moderately (shorter teeth) to long-tapered (longer teeth), lacking a prominent network of nerves and not becoming inflated at fruiting. Corollas 3–4 mm long, longer than the calyx lobes, pale yellow, turning brown with age, the banner straight to more or less incurved, narrowly oblong-ovate, rounded at the tip, finely and relatively faintly nerved. Fruits 1.5–2.0 mm long, ovoid, stalked, the outer wall membranous, 1(2)-seeded. Seeds 1.0–1.5 mm long, ellipsoid, tan to dark brown, shiny. 2n=16, 28. April–September.

Introduced, scattered in the southern half of the state, mostly in the Unglaciated Plains and Mississippi Lowlands Divisions (native of Europe; introduced nearly worldwide, including widely in the U.S., Canada). Banks of streams and rivers, margins of ponds and lakes, openings of bottomland forests, savannas, and glades; also lawns, levees, ditches, cemeteries, railroads, roadsides, and open disturbed areas.

Trifolium dubium is also called least hop-clover, small hop-clover, and shamrock. It is often confused with T. campestre, but can be distinguished from it by the smaller inflorescences with fewer flowers, as well a banner petal that is only finely and faintly nerved (vs. corrugated with strongly impressed nerves in T. campestre). It also commonly has been confused with Medicago lupulina (black medic), but can be distinguished as described in the treatment of T. campestre.

Trifolium dubium is thought by some to be the shamrock of Irish folklore, but others claim that the shamrock may be one of several species of Trifolium, Medicago, or Oxalis (Colgan, 1896; Everett, 1971; E. C. Nelson, 1991). Today, the houseplants most frequently sold in the United States as shamrocks are members of the genus Oxalis (Oxalidaceae). For further discussion, see the treatment of that genus in the present volume.