This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
In this genus the leaves are either narrow or, if broad, small. Catkins usually erect, with entire scales. About 160 to 180 species are known, but many of them are so similar in aspect that they are difficult to determine from descriptions. And then the varieties either natural or hybrid are so numerous as to render it impossible to classify them satisfactorily. The ancient Latin name, said to be of Celtic origin.
The following are a few of the more desirable ornamental species, including some of the commoner indigenous ones.
1. S. fragilis. Crack Willow. - A large indigenous tree 60 to 90 feet high. Leaves lanceolate, glandular-serrate, with a long acuminate point. Petiole short, not glandular; catkins on short leafy shoots, rather long and slender, appearing with the leaves. Stamens 2. Capsule distinctly pedicellate. S. Russelliana, the Bedford Willow, differs only in having linear-lanceolate leaves and more spreading branches.
2. S. alba. White Willow. - This is also a large tree and equally common with the last. It has very long linear-lanceolate glandular serrate acuminate leaves clothed with silky hairs on both sides when young. Stamens 2. Capsule glabrous, almost or quite sessile. There are three varieties, distinguished as follows : - alba proper, young twigs olive green, mature leaves silky on both sides; cwrulea, adult leaves glabrous and glaucous beneath; vitelllna, Golden Osier or Willow, young twigs bright yellow.
3. S. Caprea. Common Sallow, Goat Willow, or Palm. - This is an extremely variable species and the commonest of the genus in hedgerows and waste places. It forms a large shrub or small tree, which blooms earlier than any other native species, producing its short thick silky catkins before the leaves. The reticulated leaves are silky, hairy below, and vary in outline from lanceolate to oblong or rotundate, and crenate or entire at the margin. Scales of the female catkins tipped with black. S. cinerea, S. aquatica, and a host of other names belong to this species. S. c. pendula is the Kilmarnock Weeping Willow.
4. S. purpurea. Purple Osier. - A small indigenous shrub with reddish or purple bark. Leaves often opposite, glabrous, lanceolate, serrulate, glaucous beneath. Catkins sessile, narrow, with dark purple scales. Stamens 2; filaments more or less combined, a character peculiar to this amongst British species. This includes a large number of forms, but only one calls for mention, namely, pendula, commonly known as the American Weeping Willow.
5. S. pentandra. Bay Willow. - This is a very distinct and handsome species with broader thicker more shining foliage than any of the foregoing, and five or more stamens. It is, moreover, the latest in bloom of British species, producing its bright yellow catkins after the appearance of the leaves.
6. S. triandra. Almond-leaved or French Willow. - A small tree with glabrous linear-lanceolate glandular-serrate leaves and glandular petioles. Stamens 3. This species is commonly planted in Osier beds.
S. viminalis is another common Osier, distinguished by its entire leaves with a revolute margin and sessile catkins. S. repens is the common creeping or trailing species. S. pendula, syn. 8. Babylonica, is the old Weeping Willow, with very long slender drooping branches and narrow leaves. It is a native of China, according to Professor Koch, not of Western Asia, as formerly supposed, and is sometimes known as Napoleon's Willow. The female plant only is in cultivation. Another handsome Weeping Willow, in cultivation under the 'erroneous names Sieboldii and Japonica, is called elegantissima by Koch.