Flora of the Southeastern United States
2022 Edition

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Key to Trifolium

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1 Flowers bright yellow (fading brown); [section 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 Standard 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 Standard 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 not white, pink, purplish, or red.
......4 Flowers borne on distinct pedicels, (1-) 2-10 mm long, these often curving or reflexing in age; flowers white, fading pink with age in most species; [native and alien species]; [section Lotoidea].
........5 Plants stoloniferous, all or some of the leaves alternate from ground level and long petioled.
..........6 Calyx lobes narrowly triangular, about as long as the calyx tube; peduncles axillary along the stolons; stipules scarious-membranaceous; [plant an abundant introduced weed]
..........6 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].
............7 Peduncle terminal, at the tip of the stolon, lacking leaves; pedicels 2-3 mm long
............7 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).
..............8 Calyx lobes narrowly triangular, about as long as the calyx tube (or longer in T. hybridum); stipules scarious-membranaceous; [plants introduced].
................9 Calyx lobes not scarious-margined, straight, equal to or longer than the tube
................9 Calyx lobes scarious-margined, becoming divergent and twisted, about equal to the tube
..............8 Calyx lobes subulate to lanceolate, distinctly longer than the calyx tube; stipules green, foliaceous; [plants rare natives].
..................10 Flowers 4-6 mm long; calyx lobes lanceolate, foliaceous, 3-nerved, 0.4-0.8 mm wide
..................10 Flowers 8-12 mm long; calyx lobes subulate, setaceous, 1-nerved, < 0.4 mm wide.
....................11 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]
....................11 Leaflets 1-2.8× as long as wide; stems erect, ascending, or decumbent; flowers white or purplish; plant an annual or biennial; [plants of a variety of natural woodlands, collectively widespread in our area]
......................12 Plant decumbent; terminal petiole 5.7-8.2 cm long; sepal lobes 0.2-0.37 cm long; peduncles 0.23-0.31× as long as the terminal petiole; [dry limestone woodlands, Bluegrass region of KY (so far as is known)]
......................12 Plant ascending to erect; terminal petiole 0.3-3.5 cm long; sepal lobes 0.3-0.7 cm long; peduncles 0.4-4.2× as long as the terminal petiole; [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; [alien species].
........................13 Plants stoloniferous, all or some of the leaves alternate from ground level and long petioled.
..........................14 All flowers with petals; fruiting heads enlarging, becoming a reddish brown, pubescent ball ca. 2 cm in diameter, remaining aerial; [section Vesicaria]
..........................14 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]
........................13 Plants not stoloniferous, the leaves clustered at or near ground level and/or produced on aerial stems.
............................15 Heads subtended by a pseudo-involucre of 2 (-3) enlarged stipules and/or opposite or subopposite leaves; [section Trifolium].
..............................16 Flowers white (fading pink), 7-8 mm long; calyx tube both externally glabrous and 20-nerved
..............................16 Flowers red, pink-purple, or bicolored, either 11-20 mm long or 4-6 mm long; calyx tube not both externally glabrous and 20-nerved (externally pubescent, or 10-nerved, or both).
................................17 Flowers 4-6 mm long
................................17 Flowers 11-20 mm long.
..................................18 Stipules gradually tapering to a long slender tip, longer than the fused part; calyx densely hirsute; stem soft pubescent with deflexed to spreading hairs
..................................18 Stipules abruptly narrowed to a short awn; calyx glabrous to pilose; stem appressed pubescent
............................15 Heads not subtended by a pseudo-involucre of leaves or expanded stipules.
....................................19 Heads axillary, sessile, in the axils of subtending leaves; calyx tube glabrous (except for a few hairs at apex); [section Lotoidea]
....................................19 Heads terminal or axillary; calyx tube pubescent.
......................................20 Calyx bladdery-inflated in fruit; corolla resupinate (inverted 180 degrees, such that the standard is lowermost); [section Vesicaria].
........................................21 Inflorescence with a prominent peduncle; head lobed in fruit
........................................21 Inflorescence subsessile to shortly peduncled; head spherical in fruit
......................................20 Calyx not bladdery-inflated in fruit; corolla orientation normal (standard uppermost).
..........................................22 Corolla 3-6 mm long; [section Trifolium]
..........................................22 Corolla 10-18 mm long.
............................................23 Corolla crimson, 10-13 (-15) mm long; floral bracts absent; heads 1-1.5 (-2) cm in diameter; [section Trifolium]
............................................23 Corolla white, 15-18 mm long; floral bracts present; heads 2.5-3 cm in diameter; [section Mistyllus]