This section is from the book "Wayside And Woodland Blossoms", by Edward Step. Also available from Amazon: Wayside And Woodland Blossoms: A Guide To British Wild-Flowers.
Everybody knows a Clover when he sees it; it is therefore unnecessary to take up our space with a general description. Their great value as pasture plants has caused their typical forms of flower and leaf to be well known; but we have so many native species, to say nothing of the introduced kinds, that few besides botanists and agriculturists are acquainted with their specific characters.
All the Clovers or Trefoils are Leguminous plants, and the structure of the individual flower is very similar to that of Lotus and Vicia; but the flowers are much smaller, and are gathered into a conspicuous head. In certain species there are floral bracts, and in some these form an involucre. It is characteristic of most of the clovers that when the seed is set the petals do not fall off, but simply dry up and wrap round the pod. The name of the genus is Latin, and signifies three-leaved. The principal British species are:
I. Subterranean Trefoil (T. subterraneum), so called from its singular habit of burying its pods in the earth when they are ripening. The plant has many creeping stems, covered with soft hairs. The heads of flowers are cream-coloured, and are produced in the axils. The individual flowers are long and slender; only a few in each head are fertile, and in this species the petals fall off" early. The pod is a compressed orb. Dry, gravelly pastures. May and June.
II. Hare's-foot Trefoil (T. arvense). Stems almost erect. Flower-heads numerous, dense, cylindric, softly hairy; flowers pinky-white, minute; teeth of the calyx longer than the corolla. Corn-fields and dry pastures. July to September.
III. Common Purple or Red Clover (T. pratense). (See figure.) This is the clover so commonly grown in meadows as an important ingredient in the hay-crop. Its large oval leaflets are frequently marked with a whitish band that takes more or less of a quarter-moon shape. Its flower-heads are round, afterwards becoming longer than broad, purplish red in colour. Calyx-teeth slender, bristly, not longer than corolla. Top of pod dropping off when ripe. This is the clover Darwin made famous by showing that the cultivated forms must die out but for the humble-bees, whose tongues alone are long enough to fertilize its long flowers. Meadows, pastures and roadsides. May to September.
IV. Zigzag or Meadow Clover (T. medium). Leaflets more pointed than in pratense, and spotless. Stem branched in such a manner as to give it a peculiarly zigzag appearance. Heads larger, and of a deeper purple than pratense. Calyx-teeth half the length of corolla. Pod splitting lengthwise. Pastures, flourishing in lighter soils than pratense. June to September.
V. Soft Knotted Trefoil (T. striatum). Stem more or less reclining, downy or silky. Flower-heads both terminal and axillary, small, rosy-red, broader at the base. Calyx-tube swollen, ribbed, contracted at mouth, teeth not so long as corolla. Dry pastures. June and July.
VI. Rough Rigid Trefoil (T. scabrum). Stems rigid, prostrate. Leaflets rigid, toothed, the veins thickened. Flower-heads broadest in middle. Flowers small, the corolla white, calyx purple; calyx-teeth as long as corolla. Chalky and sandy pastures near sea. May to July.
VII. Dutch Clover (T. repens). Stems smooth, creeping, but not rooting. Leaflets often with a dark spot at the base, below a whitish band. Heads of flowers globose, all produced from the axils, on long stalks. The flowers white or pinkish, attached by short stalks, which are recurved after flowering, so that the pods are all drooping. Meadows and pastures. May to October.
VIII. Strawberry-headed Clover (T.fragiferum). Similar in habit to the last. Flower-head globose, of small purple-red flowers, much larger after flowering, when the calyces swell and take on a red colour, which increases size of head to an inch in diameter, and gives it a strawberry-like aspect. Meadows and pastures. July and August.
IX. Hop Trefoil (T. procumbens.) (See Figure on p. 47.) This must not be confounded with the Hop Trefoil of the farmer (Medicago lupulina), in which the flowers are borne in spikes (see p. 73). The stems are downy, one growing erect, others all round it creeping. The flowers are pale yellow, crowded in the heads, the upper petal (standard) broad, and arched over the straight pod, turning bright brown, which gives the head the appearance of a hop strobile. The pods are always so covered in this species, whereas, in Medicago lupulina they are naked. Dry pastures and roadsides. June to August.
X. Small Yellow Trefoil (T. dubium). Stems slight, creeping, nearly smooth. Heads smaller, on long slender stalk. Flowers yellow, the standard narrow, keeled, turning dark brown after flowering and wrapped round the pod. Similar situations and date to last.