This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Trifolium agrarium L. Sp. Pl. 772. 1753.
?T. aurenm Poll. Hist. Pl. Palat. 2: 344. 1777.
Glabrous or slightly pubescent, annual, ascending, branched,6'-18' high. Leaves petioled; stipules linear-lanceolate, acuminate, 4"-7" long, adnate to the petiole for about one-half its length; leaflets all from the same point, sessile, obovate or oblong, finely denticulate, narrowed at the base, rounded, truncate or emarginate at the apex, 6"-9" long; peduncles axillary, 1/2'-2' long; head oblong, or oval, 6"-9" long, densely many-flowered; flowers yellow, 2"-3' long, at length reflexed; pedicels shorter than the calyx; standard slightly emarginate, exceeding the pod, conspicuously striate, and brown, especially when dry.
Trifolium procumbens L. Sp. Pl. 772. 1753.
Similar to the preceding species, but lower, more spreading and more pubescent. Leaflets obovate, cuneate at the base, rounded, truncate or emarginate at the apex, finely denticulate, 4"-7" long, the lateral ones nearly sessile, the terminal distinctly stalked; stipules ovate, adnate to the lower part of the petiole, about 2" long; peduncles 3"-12" long; heads 20-40-flowered, globose or short-oval, 4"-6" in greatest diameter; flowers yellow, at length reflexed, about 2" long; standard dilated, not folded, exceeding the pod, striate and brown when dry.
In fields and along roadsides, Nova Scotia to Ontario, Washington, Georgia and Mississippi. Naturalized from Europe. May-Sept.
Straggling or ascending, nearly glabrous, stems 2-20' long, branching. Leaflets obovate, truncate or emarginate at the apex, or even obcordate, denticulate, cuneate at the base, the lateral ones nearly sessile, the terminal stalked; stipules ovate or lanceolate, adnate to the lower part of the petiole; peduncles 3"-12" long; heads nearly globose, 2"-3" in diameter, 3-20-flowered; flowers about 1 1/2" long, turning brown, at length reflexed; standard exceeding and folded over the pod, scarcely striate.
In fields and waste places, Massachusetts to New Jersey, Arkansas, Georgia and Mississippi. Also in Washington, Oregon and British Columbia. Jamaica. Naturalized from Europe. Summer. Said to be the true Shamrock. Called also yellow-suckling. Wild or yellow-trefoil.