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Annual, perennial herb, unarmed.
Leaf: generally palmately compound; stipules conspicuous, partly fused to petiole, generally papery or membranous; leaflets generally 3, occasionally 5–9, ± toothed or wavy.
Inflorescence: raceme (generally umbel-like), head, or spike, axillary or terminal, generally many-flowered, generally involucred, generally peduncled; inflorescence bracts 0 or forming vestigial ring or involucre; flower bracts present or not.
Flower: generally spreading to erect, generally becoming reflexed; corolla generally purple to pale lavender, occasionally yellow, persistent after flower; 9 filaments fused, 1 free.
Fruit: generally indehiscent but generally breaking, short, plump, generally enclosed in corolla; base generally stalk-like.
Seed: 1–6.
± 300 species: temps, tropical mountains, n hemisphere, South America, Africa.; foodplant for lepidopterans, cultivated as green manure in crop rotation, fodder. (Latin: 3 leaves) [Ellison et al. 2006 Molec Phylogen Evol 39:688–705; Vincent 2009 Madroño 56:208]
Unabridged references: [Gillett 1980 Canad J Bot 58:1425–1448; Zohary & Heller 1984 Genus Trifolium]

Key to Trifolium

1. Involucre bracts generally fused, generally > 1 mm, generally forming cup or bowl about base of inflorescence; inflorescence not sessile above 1–2 leaves ..... Group 1

1' Involucre bracts 0 or occasionally < 1 mm or forming a vestigial ring; inflorescence occasionally ± sessile above 1–2 ± reduced leaves (stipules of which occasionally involucre-like) ..... Group 2

Group 1:

1. Calyx or banner soon inflated in fruit; involucre bracts generally ± free

2. Calyx hairy, inflated in fruit; inflorescence in fruit ± 2 cm wide, spheric, fuzzy; banner not inflating in fruit; perennial herb, stoloned ..... T. fragiferum (2)

2' Calyx glabrous, not inflated in fruit; inflorescence not as above; banner inflating in fruit; annual, not stoloned

3. Corolla 10–20 mm ..... T. fucatum

3' Corolla 4–9 mm

4. Involucre bracts < 1 mm, fused at base; inflorescence generally 1–1.5 cm wide; corolla 6.5–9 mm; ScV, nw SnJV, CW ..... T. hydrophilum

4' Involucre bracts 2–2.5 mm, ± free to ± 1/2 fused; inflorescence 0.5–1 cm wide; corolla 4.5–7.5 mm; NCoR, CaRF, SNF, Teh, GV, CW, SCo, ChI, PR ..... T. depauperatum

5. Fr 3–4 mm, oblong, stalk-like base 0 or short; involucre bracts ± free or ± 1/2 fused, margins widely scarious, toothed near tip; corolla 5–6 mm; GV, SnFrB, SCoR ..... var. amplectens

5' Fr 2–3 mm, ovate or obovate, stalk-like base 0.5–1 mm; involucre bracts ± free, margins ± membranous; corolla 4.5–7.5 mm; NCoR, CaRF, SNF, Teh, GV, CW, SCo, ChI, PR ..... var. truncatum

1' Calyx, corolla not inflated in fruit (except lower part of banner in Trifolium barbigerum); involucre bracts generally fused into a cup-, bowl-, or wheel-shaped, toothed or cut ring (except generally ± free in Trifolium monanthum)

6. Involucre cup- or bowl-shaped, when pressed generally partly hiding flowers (except in Trifolium grayi)

7. Calyx lobes < 1/2 tube, not bristle-tipped ..... T. microdon

7' Calyx lobes > 1/2 tube, bristle-tipped

8. Calyx lobes, involucre lobes ± entire; plant hairs generally finely wavy ..... T. microcephalum

8' Calyx lobes, involucre lobes, or both toothed, cut, or branched; plant hairs 0 or finely wavy or not

9. Bristle-tips of lower 3 calyx lobes generally forked; involucre margin wavy, finely toothed ..... T. cyathiferum

9' Bristle-tips of lower 3 calyx lobes not divided; involucre margin wavy or not, generally coarsely toothed or cut 1/2 to involucre base

10. Calyx lobes finely toothed, bristle at tip 1–1.5 mm, glabrous; involucre generally glabrous; banner not inflated ..... T. buckwestiorum

10' Calyx lobes entire, bristle at tip 3–4 mm, plumose (or lower lobes divided to near base into 2–3 bristles, each 3–4 mm, generally plumose); involucre generally puberulent; banner inflated below, persistent as twisted beak above

11. Corolla gold- to sulphur-yellow; leaves, stems glabrous; seeds > 3 mm; ScV (North Table Mtn Plateau, Butte Co.) ..... T. jokerstii

11' Corolla purple, pink-purple, white, or 2-colored; leaves, stems generally hairy; seeds < 3 mm; NW, SN, GV, CW

12. Corolla 5–10 mm, ± >= calyx; stalk-like fruit base < 1 mm; inflorescence generally bristly from calyx lobes ..... T. barbigerum

12' Corolla 8–16 mm, > calyx; stalk-like fruit base 1–3 mm; inflorescence not bristly ..... T. grayi

6' Involucre wheel-shaped or bracts ± free, when pressed hiding only bases of flowers

13. Involucre vestigial or bracts 2–5, inconspicouous, generally ± free, 1–3 mm; inflorescence 1–6-flowered ..... T. monanthum

14. Rhizomes thin, white; plant mat-forming, not from central crown or thickened taproot; plant glabrous to ± hairy

15. Fls 1–5, peduncle bent upward at tip; leaflet tips generally acute; SnGb, SnBr, SnJt ..... subsp. grantianum

15' Fls 1–4, peduncle straight; leaflet tips generally rounded or truncate; CaR, SN, SNE ..... subsp. monanthum

14' Rhizomes 0; plant not mat-forming, from central crown or thickened taproot; plant densely hairy

16. Lflet tips generally rounded, truncate, or notched ..... subsp. parvum

16' Lflet tips acute to acuminate ..... subsp. tenerum

13' Involucre generally conspicuous, bracts indefinite in number, generally > 5 mm, fused, if inconspicuous (e.g., 2–3 mm in Trifolium oliganthum) fused at least at base; inflorescence generally 8–many-flowered

17. Perennial; involucre generally 2–3 cm wide; seeds 2–4

18. Thickened fusiform root present; rhizomes 0; KR ..... T. siskiyouense

18' Thickened fusiform root 0; rhizomes present; NW, CaR, SN, SnJV, CW, SCo, PR, SNE ..... T. wormskioldii

17' Annual; involucre < 2 cm wide; seeds generally 1–2

19. Calyx lobes ± <= tube, often toothed or shouldered below tapered tip; calyx tube splitting between upper lobes

20. Fl 5–8 mm; involucre 2–3 mm, generally cut > 1/2 to base; inflorescence 6–10 mm wide, 5–15-flowered ..... T. oliganthum

20' Fl 9–18 mm; involucre > 3 mm, cut < 1/2 to base; inflorescence generally 12–30 mm wide, often > 15-flowered

21. Calyx glandular-hairy or bumpy (occasionally in age glabrous), lobes generally entire; leaflets sharply serrate, longer teeth ± 1 mm ..... T. obtusiflorum

21' Calyx glabrous, not bumpy, lobes entire or 3-toothed; leaflets serrate, teeth < 1 mm ..... T. willdenovii

19' Calyx lobes generally > tube, generally entire (occasionally ± forked in Trifolium trichocalyx), not shouldered below tapered tip; calyx tube not splitting between upper lobes

22. Calyx hairy; involucre small, cut past middle — CCo (Monterey Peninsula) ..... T. trichocalyx

22' Calyx generally ± glabrous; involucre generally well developed, cut to middle

23. Calyx tubular, lobes ± 3-parted — CCo (Monterey Peninsula) ..... T. polyodon

23' Calyx bell-shaped, lobes entire or toothed ..... T. variegatum

24. Infl 1–5-flowered; corolla 3.5–9 mm ..... var. geminiflorum

24' Infl 5–many-flowered; corolla 6–16 mm

25. Infl 1.5–3 cm, 10–many-flowered, corolla 9–17 mm ..... var. major

25' Infl 1–1.5 cm, 5–10-flowered, corolla 6–10 mm ..... var. variegatum

Group 2:

1. Lflets 3–9 (at least some leaves with > 3 leaflets)

2. Lflets generally 7–9; inflorescence 3–5 cm wide ..... T. macrocephalum

2' Lflets 3–7; inflorescence 1–2.5 cm wide

3. Plant silvery or gray, soft-hairy or tomentose; calyx lobe hairs ± 1 mm ..... T. andersonii

4. Longer petiole hairs 1.5–2 mm; peduncles generally << petioles, inflorescence not or ± exceeding leaves ..... subsp. andersonii

4' Longer petiole hairs 0.4–1.2 mm; peduncles generally > petioles; inflorescence much exceeding leaves ..... subsp. beatleyae

3' Plant green, strigose, puberulent, or hairy; calyx lobe hairs < 1 mm

5. Lflet generally elliptic, serrate, teeth 0.1–0.3 mm; inflorescence generally included, 1–1.5 cm wide ..... T. gymnocarpon subsp. plummerae

5' Lflet obovate, coarsely serrate, larger teeth 0.4–0.5 mm; inflorescence generally exserted, 1.5–2.5 cm wide ..... T. lemmonii

1' Lflets 3

6. Fls of inflorescence, except for outer 2–8, sterile, with calyx stalk-like, without petals, forming bur, delivered to ground by growing peduncle; plant prostrate or creeping ..... T. subterraneum

6' Fls of inflorescence fertile, with calyx not stalk-like, with petals, not forming bur, not delivered to ground by peduncle; plant prostrate or creeping or not

7. Calyx inflated in fruit; inflorescence in fruit spheric or star-shaped, red, brown, green, or white, ± woolly, 0.8–2.5 cm

8. Perennial, creeping and rooting or cespitose; bracts of lower flowers forming involucre, ± 2 mm, green; corolla not becoming reflexed ..... T. fragiferum (2)

8' Annual, decumbent to ± erect, not rooting; bracts of lower flowers generally not forming involucre, < 1 mm, white; corolla becoming reflexed

9. Infl star-shaped, 1–2.5 cm; flower bracts reduced ..... [T. resupinatum]

9' Infl spheric, 0.6–1.2 cm; flower bracts cup-shaped ..... T. tomentosum

7' Calyx rarely inflated in fruit (if inflated then << 2 cm wide); inflorescence in fruit not as at 7.

10. Lf pinnately compound; corolla bright yellow

11. Corolla striate; inflorescence 0.8–1.3 cm wide, generally > 20-flowered; petioles of mid-leaves > leaflets ..... T. campestre

11' Corolla weak-striate; inflorescence 0.4–0.8 cm wide, 5–10(20)-flowered; petioles of mid-leaves generally << leaflets ..... T. dubium

10' Lf palmately compound; corolla not bright yellow (except Trifolium aureum)

12. Lflets 4–10 cm, stipules 1–2.5 cm, green; calyx glabrous; corolla green-white to pink ..... T. howellii

12' Lflets or stipules (or both) << above, stipules generally not green; calyx hairy or glabrous; corolla green-white to pink or not

13. Fls generally 1–5 per inflorescence, bracts generally inconspicuous; corolla white to lavender-striate; plant generally small (see Group 1, couplet 13 for vars.) ..... T. monanthum (2)

13' Fls > 5 per inflorescence unless plant stressed; corolla white to lavender-striate or not; plant small or not

14. Pedicels 1–6 mm, flowers generally becoming reflexed

15. Corolla bright yellow, 5–7 mm; calyx 1.5–2 mm; stalk-like fruit base 2–3 mm ..... T. aureum

15' Corolla not bright yellow, generally > 6 mm; calyx generally > 2 mm; stalk-like fruit base 0 or < 2 mm

16. Lflet lanceolate; leaves mostly basal, occasionally 2–3 cauline; plants of s SNH, SNE ..... T. kingii subsp. dedeckerae

16' Lflet wider than lanceolate, if lanceolate then leaves not mostly basal or plants not of s SNH, SNE

17. St creeping, rooting, turf-forming; petioles from ground level, ± equal; corolla white ..... T. repens

17' Plant not creeping, not rooting, not turf-forming; petioles not or some from ground level, lower > upper; corolla white or not

18. Calyx tube puberulent or hairy, occasionally ± so

19. Corolla 5–8 mm; pedicel 2–5 mm ..... T. breweri

19' Corolla 10–18 mm; pedicel 1–2 mm ..... T. longipes (2)

20. Longer petals tapered to beak; 1400–2700 m — MP ..... subsp. multipedunculatum

20' Longer petals acute to acuminate but not beaked; 1200–2700 m

21. Sts generally well developed, not cespitose; petals generally white, short-tapered, acute, or obtuse; 1200–1800 m; KR, NCoR ..... subsp. oreganum

21' Sts often poorly developed, generally cespitose; petals lavender to purple, longer ones acute to acuminate; 1400–2700 m; KR, CaR ..... subsp. shastense

18' Calyx tube glabrous

22. Calyx lobes ciliate with short, flat bristles (or, through breakage, bristle bases); bracts inconspicuous, often in a ring at base of inflorescence ..... T. ciliolatum

22' Calyx lobes not ciliate; bracts subtending individual flowers or 0

23. Lvs basal except 1 pair cauline; corolla 10–17 mm ..... T. beckwithii (2)

23' Lvs cauline; corolla 5–11 mm

24. Infl 1.5–3 cm wide; bracts conspicuous but soon falling; calyx lobes sharply tapered but not bristle-tipped ..... T. hybridum

24' Infl 0.5–1.5 cm wide; bracts 0; calyx lobes bristle-tipped

25. Calyx lobes ± 0.5 mm wide at base, glabrous, tube 1.5–2.5 mm

26. Lflet obovate to obcordate, length 1.5–2.5 × width; CA-FP (except ChI), DMoj ..... T. gracilentum

26' Lflets narrowly elliptic or lanceolate, length 3.5–5 × width; ChI ..... T. palmeri

25' Calyx lobes ± 0.2 mm wide at base, with generally few hairs, tube 0.8–1.4 mm ..... T. bifidum

27. Lflet 2.5–5.5 × longer than wide, notched at tip 0.1–0.4 × length, otherwise generally entire ..... var. bifidum

27' Lflet 1.5–2.5 × longer than wide, truncate or notched at tip < 0.2 × length, otherwise dentate or entire ..... var. decipiens

14' Pedicels < 2 mm, flowers becoming reflexed or not

28. Infl sessile at end of stem or in leaf axils, or in leaf axils on short peduncles 6–10 mm, << leaves; corolla 4–6 mm

29. Infl axillary or at end of stem, peduncles 0 ..... T. glomeratum

29' Infl axillary, peduncles short or short below, 0 above

30. Corolla > calyx lobes; banner tip notched; peduncles short ..... T. cernuum

30' Corolla < calyx lobes; banner tip occasionally ± toothed but not notched; peduncles short below, 0 above ..... T. retusum

28' Infl peduncled (or immediately above 1–2 reduced leaves), peduncle generally > 10 mm, generally > leaves; corolla generally > 5 mm

31. Banner inflated in fruit; involucre vestigial, ring-like; corolla generally 6–9 mm ..... T. depauperatum var. depauperatum

31' Banner not inflated in fruit; involucre 0; corolla 6–9 mm or not

32. Fls generally reflexed soon after opening; inflorescence often turned to side or downward

33. Calyx hairy, lobes twisted or bent, plumose ..... T. eriocephalum subsp. eriocephalum

33' Calyx glabrous, lobes not twisted or bent, not plumose

34. Infl 1–2 cm wide; calyx 3.5–4.5 mm, generally dark or purple-black; corolla 10–12 mm ..... T. bolanderi

34' Infl 1.5–3 cm wide; calyx 3–7 mm, not dark or purple-black; corolla 10–17 mm

35. Calyx 4.5–7 mm; inflorescence erect, axis not exserted ..... T. beckwithii (2)

35' Calyx 3–4 mm; inflorescence generally turned to side or downward, axis exserted, generally forked ..... T. productum

32' Fls generally not reflexed after opening; inflorescence generally ± erect

36. Infl ± sessile above a pair of reduced leaves, stipules, or both (in Trifolium macraei, occasionally 1 of a pair of inflorescences short-peduncled)

37. Infl generally paired, ± sessile or occasionally 1 short-peduncled above (1)2 leaves ..... T. macraei

37' Infl single above subtending leaves

38. Corolla 2-colored (purple with white tips); locally uncommon, s NCoR, n CCo, SnFrB ..... T. amoenum (2)

38' Corolla not 2-colored (red-purple or pink); sporadic or locally common, CA-FP, GB

39. Calyx lobes densely to sparsely plumose, lower one ± 1–1.5 × others; corolla pink

40. Infl spheric, ± 1.5–2 cm; calyx lobes >> tube, densely plumose, 4–6 mm ..... T. hirtum

40' Infl ± short-cylindric, 0.5–1.5 cm; calyx lobes <= tube except lower one, sparsely plumose, 1–2.5 mm ..... T. striatum

39' Calyx lobes sparsely plumose, straight, lower one ± 2 × others, or calyx lobes glabrous, curved, ± equal; corolla red-purple or pink

41. Lflets elliptic to obovate, ± entire; calyx hairy, veins 10, lobes straight; corolla generally red-purple ..... T. pratense

41' Lflets generally lanceolate to elliptic, teeth bristle-like; calyx glabrous, veins 24, lobes straight to curved; corolla pink to purple ..... T. vesiculosum

36' Infl peduncled

42. Corolla <= calyx; calyx lobes > tube, plumose

43. Lflets oblanceolate to obovate, length 1.2–3 × width; corolla 2-colored (purple with white tips)

44. Calyx 4–8 mm, lobes 3–6.5 mm ..... T. albopurpureum

44' Calyx 8–14 mm, lobes 6.5–12 mm ..... T. olivaceum

43' Lflets linear to narrow-lanceolate, -oblong, or -oblanceolate, length 3–8 × width; corolla 1-colored (pale pink to white)

45. Corolla 10–12 mm, ± = calyx ..... T. angustifolium

45' Corolla 3–4 mm, < or << calyx ..... T. arvense

42' Corolla > calyx; calyx lobes > tube or not, plumose or not

46. Perennial; calyx lobes glabrous or hairs few, curved or wavy

47. Infl < 1 cm wide, flowers generally 1–5, erect (see Group 1, couplet 13, for vars.) ..... T. monanthum (2)

47' Infl 1.5–3 cm wide, flowers > 5, ascending-spreading ..... T. longipes (2)

48. Calyx lobes 3–5 × tube; leaflet length generally 6–10 × width; KR, NCoR, CaR ..... subsp. elmeri

48' Calyx lobes 1–3 × tube; leaflet length 2–6 × width in CA-FP, to 10 × in GB; KR, NCoR, CaR, SN, SnBr, SnJt, GB

49. Roots thickened, fusiform; rhizomes ± 0; calyx lobes densely long-hairy ..... subsp. atrorubens

49' Roots generally not thickened, not fusiform; rhizomes present; calyx ± glabrous ..... subsp. hansenii

46' Annual; calyx lobes ± plumose, hairs generally straight

50. Corolla 2-colored (purple with white tips)

51. Corolla 12–16 mm; calyx 10–12 mm ..... T. amoenum (2)

51' Corolla 7–12 mm; calyx 4–8 mm ..... T. dichotomum

50' Corolla 1-colored (purple, crimson, pink, white, or cream)

52. Corolla cream; inflorescence ovate to spheric, 1–2.5 cm; leaflets elliptic to oblanceolate; stipules long-tapered ..... [T. alexandrinum]

52' Corolla crimson or white; inflorescence ± cylindric or ± conic, 2–6 cm; leaflets obovate to obcordate; stipules widely rounded ..... T. incarnatum

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Citation for an individual treatment: [Author of taxon treatment] [year]. [Taxon name] in Jepson Flora Project (eds.) Jepson eFlora, [URL for treatment]. Accessed on [month, day, year].
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