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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].
image of plant
Show caption*© Alan Cressler: Trifolium carolinianum, Heggie's Rock, Heggie's Rock Preserve, The Nature Conservancy, Columbia County, Georgia 1 by Alan Cressler
............ ......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 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]
............ ............ ............ ........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]
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