Trifolium dubium

Herbs annual, 20–40 cm, gla­brous or sparsely hairy. Stems erect to prostrate, branched from base. Leaves pinnate; stip­ules ovate, 0.3–0.5 cm, margins entire, apex acute; petiole to 1.5 cm; lateral leaflet petiolules to 0.5 mm, terminal leaflet stalk 1–1.5 mm; leaflets 3, blades obovate to elliptic-lanceolate, 1–2 × 0.5–1.5 cm, base cuneate, lateral veins prominent, ± parallel, ascend­ing, margins dentate distally, apex rounded or retuse, surfaces glabrous or sparsely hairy. Peduncles 1–1.5 cm. Inflorescences axillary or terminal, 5–20-flowered, ovoid or globose, 0.5–0.9 × 0.6 cm; involucres absent. Pedicels reflexed, 0.2–0.5 mm; bracteoles a fringe of red setae. Flowers 2.8–3.5 mm; calyx campanulate, 1.5–2 mm, glabrous, veins 5, tube 0.8–1 mm, lobes unequal, longer than tube, adaxial 2 shorter, orifice open; corolla pale yellow becoming brown, 2.6–3.2 mm, not or slightly ribbed, banner persistent, spatulate, 2.6–3.2 × 2 mm, apex rounded to broadly acute. Legumes stipitate, ellipsoid, 1.5–2 mm, short beaked. Seeds 1, yellow or pale brown, ellipsoid, 0.9–1 mm, smooth, glossy. 2n = 16, 28, 32.

Phenology: Flowering May–Oct.
Habitat: Disturbed ground, fields.
Elevation: 0–2300 m.

Distribution

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Introduced; St. Pierre and Miquelon, B.C., N.B., N.S., Ont., P.E.I., Que., Ala., Alaska, Ariz., Ark., Calif., Conn., Del., D.C., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., Tenn., Tex., Vt., Va., Wash., W.Va., Wis., Wyo., c, s Europe, w Asia, introduced also in s South America, e Asia (e China), n, s Africa, Pacific Islands (Hawaii, New Zealand), Australia.

Discussion

Trifolium dubium is often confused with Medicago lupulina Linnaeus; the latter may be distinguished by its toothed stipules, deciduous corollas, and shiny, black fruits. Little hop clover may be the co-called shamrock of Irish folklore (E. C. Nelson 1991; P. S. Wyse Jackson 2014); other candidates include other species of Trifolium or species of Medicago or Oxalis.

Selected References

None.

Lower Taxa

None.