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Key to Trifolium

Fabaceae

Trifolium

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1 Flowers bright yellow (fading brown); [subgenus Chronosemium].
  2 Leaves palmately trifoliolate (all leaflets essentially sessile); heads 10-13 mm in diameter; flowers 5-7 mm long
  2 Leaves pinnately trifoliolate (the lateral leaflets essentially sessile, the terminal leaflet with a petiolule 0.8-3 mm long); heads 5-13 mm in diameter; flowers 2.5-5 mm long.
    3 Banners with 5 obvious diagonal veins (striations); heads 8-13 mm in diameter, generally with 20-30 flowers; flowers 3.5-5 mm long; petiolule of the terminal leaflet 1-3 mm long
    3 Banners inconspicuously veined; heads 5-8 mm in diameter, generally with 5-15 (-20) flowers; flowers 2.5-3.5 mm long; petiolule of the terminal leaflet ca. 1 mm long
1 Flowers white, pink, purplish, or red; [subgenus Trifolium].
      4 Flowers borne on distinct pedicels, (1-) 2-10 mm long, these often curving or reflexing in age; flowers white, red, or pink; [native and exotic species].
        5 Plants stoloniferous, all or some of the leaves alternate from ground level and long petioled.
          6 Corollas magenta or pink; plants producing solitary cleistogamous flowers near plant base which push into the soil and develop into underground fruits; [very rare, historically in sw. AR, w. LA, and e. TX]
          6 Corollas white to yellowish white, pinkish, or rose-pink; plants not producing underground fruits.
             7 Calyx lobes narrowly triangular, about as long as the calyx tube; peduncles axillary along the stolons; stipules scarious-membranaceous; [plant an abundant introduced weed]
             7 Calyx lobes subulate, distinctly longer than the calyx tube; peduncles terminal, either at tips of the stolons, or at tips of erect flowering branches; stipules green, foliaceous; [plants rare natives].
               8 Peduncle terminal, at the tip of the stolon, lacking leaves; pedicels 2-3 mm long
               8 Peduncle terminal at tip of erect flowering branches, subtended by a pair of opposite or subopposite, short-petioled leaves; pedicels (2-) 4-8 mm long
        5 Plants not stoloniferous, clumped (though sometimes with prostrate or lax stems).
                 9 Calyx lobes broadly oblong, some of them nearly as wide as long, > 1 mm wide, reticulate nerved; calyces mostly glabrous but with ciliate lobes; [uncommon native species in sw. AR, w. LA, and e. TX]
                   10 Calyx lobes narrowly triangular, about as long as the calyx tube (or longer in T. hybridum); stipules scarious-membranaceous; [plants introduced].
                     11 Calyx lobe margins sparsely plumose; [section Trifolium, subsection Alexandrina]
                       12 Perennials; corollas 8-12 mm long, > 2× as long as the calyces
                       12 Annuals; corollas 4-10 mm long, < 2× as long as the calyces.
                          13 Peduncles 0.5-1.5 cm, distal ones shorter; banner apices deeply emarginate or rounded; calyx lobes not scarious-margined, straight; [rare].
                            14 Inflorescences with 1-5 flowers, 5-10 mm long; pedicels straight or slightly reflexed in fruit; flowers 4-5.5 mm; banner apices rounded; [rare introduction in n. portion of our region]
                   10 Calyx lobes subulate to lanceolate, distinctly longer than the calyx tube; stipules green, foliaceous; [natives or rare introductions].
(c) Cressler, Alan M.
                              15 Flowers 4-7 mm long; calyx lobes lanceolate, foliaceous, 3-nerved, 0.4-0.8 mm wide; [common native]
                              15 Flowers 5-12 mm long; calyx lobes subulate, setaceous, 1-nerved, < 0.4 mm wide; [rare natives or rare introductions].
                                16 Leaflets 3-7× as long as wide; stems prostrate; flowers creamy white and purple-veined; plant a perennial; [plants of shale barrens and other rock outcrops, from VA northward]
                                16 Leaflets 1-2.8× as long as wide; stems erect, ascending, or decumbent; flowers white, pink, or purplish; plant an annual or biennial; [plants of a variety of natural woodlands or rare introductions, collectively widespread in our area].
                                  17 Plant decumbent; striking white arrow shaped patterns on leaflets; terminal petiole 5.7-8.2 cm long; corollas white, 9-10 mm; calyx lobes 2-3.7 mm long; [dry limestone woodlands, Bluegrass region of KY (so far as is known)]
                                  17 Plant ascending to erect; faint white lines or no pattern on leaflets; terminal petiole 0.3-3.5 cm long; corollas white, pink, or purple, 5–8 or 9-14 mm; calyx lobes 3-7 mm long.
                                       19 Flowers pink or white (inflorescences often with white banners and pink wings and keels and/or white flowers at tip and pink at base of inflorescences); calyx lobes 2-4 times as long as tube; [cultivated and possibly escaping]
                                       19 Flowers pink, magenta, or rarely white; calyx lobes < 2 times the length of the tube to nearly equal; [various woodlands and barrens, widespread in our region (at least formerly)]
      4 Flowers sessile or on very short pedicels (usually < 1 mm long); flowers pink, purplish, white, or scarlet; [exotic species].
                                           21 All flowers with petals; fruiting heads enlarging, becoming a reddish brown, pubescent ball ca. 2 cm in diameter, remaining aerial; [section Vesicaria]
                                           21 Only 2-5 outer flowers of the head with petals, the others lacking petals and sterile; fruiting heads becoming a subterranean bur, buried by downward curvature and growth of the peduncle; [section Trichocephalum]
                                         20 Plants not stoloniferous, the leaves clustered at or near ground level and/or produced on aerial stems.
                                             22 Inflorescences usually few flowered (3-15); [rare waifs or recent introductions in NC and SC].
                                               23 Stems prostrate and mat-forming; flowers not inflated 4-7 mm long; [rare introduction in Outer Banks, NC]
                                                 24 Perennials; inflorescence globose or ovoid, 1.3-3.8 cm wide; flowers deep pink or reddish purple, 15-18 mm long.
                                                      26 Heads subtended by a pseudo-involucre of 2 (-3) enlarged stipules and/or opposite or subopposite leaves; [section Trifolium].
                                                        27 Flowers purplish red or pink, either 10-14 mm long or 4-6 mm long; [rarely escaping species].
                                                              30 Heads axillary, sessile, in the axils of subtending leaves; calyx tube glabrous (except for a few hairs at apex); [cultivated and rarely escaping]; [section Trifoliastrum]
                                                                   32 Corollas resupinate (inverted 180 degrees, such that the banner is lowermost); calyces 10-veined, markedly asymmetric at base, pubescent; calyx teeth very unequal in fruit; legumes 1-seeded.
                                                                       34 Flowers white, turning pink, banners ca. 14-18 mm; calyces symmetric at base, with tubes 4-5 mm long; leaflets usually elliptical; [common introduction throughout our region]
                                                                           36 Corollas 5-8 mm long; stems, leaves, and calyces conspicuously silky pubescent; calyces usually longer than corollas (the calyx lobes are bristle-like and extend beyond the corollas); [abundant introduction]
                                                                           36 Corollas 9-18 mm long; stems, leaves, and calyces variously pubescent but not silky (T. angustifolium calyces densely plumose); corollas shorter than, equal to, or longer than calyces (often much longer).
                                                                              37 Corollas shorter than or equaling calyces, usually pale pink or purple, rarely white; inflorescences 3–8 cm long; calyces 8–13 mm, densely plumose; [rare introduction but apparently spreading]