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.
Technically distinguished from the neighbouring genera by the stalked pod with fleshy or winged sutures and estrophiolate seeds. Three species only are referred to this genus, from the South of Europe and Asia Minor. The ancient Latin name.
1. L. vulgare (fig. 67), syn. Cytisus Laburnum. Common Laburnum. - This showy flowering tree is probably more familiar than some of our native trees, and certainly nothing can exceed the beauty of the pendulous racemes of bright yellow flowers with which it is so profusely clothed during the month of May. There are several varieties in cultivation, which differ from the ordinary type in having larger flowers, in the form of the foliage, etc. Alschingeri, autumnale, crispum and Pdrksii, are amongst the best of the many varieties in cultivation.
The Purple Laburnum, L. Adami, is a hybrid between this and Cytisus purpureas. The flowers are of a dull purplish colour, in long pendulous racemes like the common one, and the leaves have rather shorter petioles; but the most remark-able thing in it is the complete reversion of some parts of the same tree to one or the other of the parents. Thus the three forms may be seen growing on one stem, having the appearance of being grafted upon it. The hybrid is sterile, but the parent reversions fertile.
Fig. 67. Laburnum vulgare. (1/2 Dat. size.)
2. L. alpinum. Scotch Laburnum. - Very similar to the preceding, and some of the varieties in cultivation appear to partake of the characteristics of both. This differs from No. 1 in the pod being quite glabrous, distinctly stalked, and winged along the upper suture. It is also of larger growth, quickly forming a tree from 20 to 30 feet high. A native of South-eastern Europe. L. pendulum and confertum are varieties.
3. L. ramentaceum, syn. L Weldeni. - The racemes in this species or variety are sub-erect, and it is of dwarf shrubby habit, with very hairy leaves. A native of Dalmatia. A variety called serotinum, Autumn-flowering Laburnum, is referred to this species.