Family: Fabaceae
Arthur Meeks
Key to Wisconsin Trifolium
- 1a.Corollas yellow 2
- 1b.Corollas pink, red, purple, or white 4
- 2a.Stipules of median and upper leaves about equaling or exceeding the petioles; terminal leaflet subsessile to sessile, oblong-ovate T. aureum
- 2b.Stipules much shorter than the petioles; terminal leaflet petiolate, obovate 3
- 3a.Heads 3–7 mm wide, 4–15-flowered; corollas 3–4 mm long, weakly veined T. dubium
- 3b.Heads 9–12 mm wide, 20-flowered or more; corolla 4.5–6 mm long, prominently veined T. campestre
- 4a.Flowers sessile (pedicels 0.5 mm long or less), not reflexed at maturity; calyx pubescent 5
- 4b.Flowers pedicellate (0.5 mm or more long), strongly reflexed at maturity 8
- 5a.Stems creeping and rooting at the nodes or cespitose and not rooting; calyx inflated in fruit T. fragiferum
- 5b.Stems upright, never rooting at the nodes; calyx not inflated 6
- 6a.Heads globose to broadly ovoid, subtended by a stipular involucre (sometimes at the peduncle base); calyx teeth glabrous or with scattered ferruginous hairs; perennials T. pratense
- 6b.Heads ovoid to cylindric, lacking an involucre; calyx teeth densely plumose; annuals 7
- 7a.Corollas bright to deep red, exceeding the calyx teeth T. incaratum
- 7b.Corollas white to pink, inconspicuous, exceeded by the calyx teeth T. arvense
- 8a.Stems erect or ascending; heads borne on axillary peduncles 2–10 cm long; corollas becoming pink with age; calyx with 5 prominent veins, the teeth subequal T. hybridum
- 8b.Stems creeping, rooting at the nodes; heads borne on scapes 6–30 cm long; corollas white; calyx with 10 prominent veins, the teeth unequal T. repens
Many of our species are considered nuisances in manicured lawns despite the beneficial nitrogen fixation they provide. Indeed, many decades ago clover was welcomed in lawns for this purpose, but this fell out of favor with the push for using fertilizers and herbicides to create a “perfect” turf. Trifolium resupinatum L. is known from a handful of old collections, all made from lawns. This species may have been included in grass seed mixes and so the plants weren’t actually weeds. It would key to T. fragiferum but differs in its smaller, resupinate, purple flowers and annual habit. All of our species are introduced from Europe. Native clovers are found in the western United States and to our southeast. Strangely, only T. reflexum L. (buffalo clover) grows within the glaciated portion of the Great Lakes. It comes close to Wisconsin in northeastern Illinois where it occurs in prairies along rivers. It would key to T. hybridum but differs in its longer calyx (6+ mm) and corolla (1+ cm) and acuminate stipules. Gillett & Cochrane (1973) last treated the genus in Wisconsin.
