Except for the waifs from the West Coast, Trifolium depauperatum and T. fucatum, all our other species are all natives of Europe and the Mediterranean region, and have long been grown as forage crops or for “green manure” in crop rotation, as are other legumes. Trifolium fragiferum L. is known from northern Ohio and Essex Co., Ontario, and will probably be found in Michigan. It is a low creeping species like T. repens, but with pink heads superficially resembling those of T. hybridum.
1. Inflorescence subtended by an involucre consisting of simple bracts (at most 2-toothed at apex) connate at the base or reduced to a narrow ring of tissue; corolla becoming distinctly inflated after anthesis; annual waifs, perhaps not persisting.
2. Flowers ca. 11–22 mm long; involucre with well-developed lobes.
2. Flowers ca. 5–8 mm long; involucre often reduced to a mere ring.
1. Inflorescence without an involucre, although in some species the stipules of the uppermost trifoliolate leaves may resemble one; corolla not inflated; annuals or perennials.
3. Flowers sessile (or pedicels distinctly less than 0.5 mm) in a dense head, never yellow, the middle and upper ones not reflexed in maturity; calyx ± pubescent (rarely glabrous).
4. Head (or a short peduncle) subtended by a pair of opposite leaves with expanded involucre-like stipules; calyx teeth glabrous or (usually) with scattered, irregular, long hairs; short-lived perennials.
4. Head pedunculate with no subtending leaves or bracts and all leaves alternate; calyx teeth densely plumose with straight hairs; annuals.
5. Corolla white or pink, inconspicuous, much exceeded by the calyx teeth; stipules with prolonged setaceous tips; heads ca. 8–14 mm in diameter.
5. Corolla bright or deep red, equaling or exceeding the calyx teeth; stipules without setaceous tip; heads often more than 14 mm in diameter.
3. Flowers on pedicels ca. 0.5 mm or more long, yellow in some species, the middle and upper ones as well as lower becoming reflexed after anthesis; calyx glabrous (at most a few hairs at tips of teeth).
6. Flowers white to pink; petioles of lower leaves all or mostly much longer than the leaflets (usually at least twice as long); heads ca. 1.5–2.5 (–3) cm in diameter.
7. Peduncles arising from erect or ascending stems; stipules gradually tapering to tip; calyx teeth all or mostly exceeding the tube.
7. Peduncles arising from prostrate stems; stipules ± abruptly truncate or obtuse with setaceous mucronate tip up to 2 (–3) mm long; calyx teeth shorter than the tube (or the longest of the unequal teeth equaling the tube).
6. Flowers yellow; petioles of all leaves about equaling the leaflets or shorter; heads ca. 0.3–1.5 cm in diameter.
8. Mature leaves with stipules (except on lower leaves) about equaling or exceeding petioles and the leaflets all sessile or nearly so; heads ca. 12–15 mm in diameter.
8. Mature leaves with stipules much shorter than petioles (except sometimes on the uppermost leaves and in the small-headed T. dubium) and the terminal leaflet usually on petiolule distinctly longer than those of the lateral leaflets (which consist of only the pulvinus); heads ca. 3–12 mm in diameter.
9. Heads mostly ca. 9–12 (–13) mm in diameter, often ca. 20-flowered (or more); corolla 4.5–6 mm long, very strongly veined.
9. Heads 3–7 (–9) mm in diameter, ca. 4–15 (–20)-flowered; corolla 3–4 mm long, only weakly veined.