The birches are deciduous trees and shrubs with alternate leaves and unisexual flowers produced on catkins, both male and female catkins being borne on the same tree. The male catkins are slender and pendulous, nearly always formed in autumn, but expanding in spring; the flower consists of a perianth and two stamens; they are produced in threes in the axil of a scale. Female catkins shorter, stiffer; the flowers consisting of an ovary with two styles, produced (also in threes) in the axil of a deciduous three-lobed scale. What is here (and commonly) called the seed, is really a tiny nut containing the true seed. It bears a transparent wing at each side, and usually the remains of the two styles at the top. The only other genus of trees with which the birches are likely to be confused are the alders, and they are readily distinguished by the persisting scales of the female catkin, which does not disintegrate like that of the birches, but falls away whole.
Two of the best-known features of the birches are the peculiar bark and frequently white trunks. The bark can often be separated in thin, papery layers, and being impervious to water, is used in other countries for canoe-building and for roofing. The timber, although not as a rule of the best, is put to various minor uses. Some of the Asiatic and American species, however, yield wood of considerable value. An aromatic principle pervades many of the birches, and a fragrant oil is obtained.
As garden trees the birches are chiefly valued for their striking trunks and graceful branches. The silvery-white trunk of the native B. pendula, the creamy-white to pinkish trunks of such species as B. papyrifera, jacquemontii, and ermanii provide some of the most delightful of winter effects. Just as striking, but more rarely seen in gardens, are the rich mahogany or cinnamon hues of the stems of B. utilis and B. albo-sinensis. The rugged trunks of B. nigra and davurica always attract attention, and the darker-coloured ones of B. lutea and maximowiczii are not without their charm. On the whole, no birch exceeds our native B. pendula in beauty, not only for its trunk, but in the singular lightness and delicate grace of its branching also. Most people will agree with the oft-quoted words of Coleridge, that it is
Of forest trees, the Lady of the Woods.’
The young branches and twigs of many birches have a rich red-brown or orange-brown tint, which makes an admirable contrast in winter with such as have white trunks.
So far as I have seen, the birches thrive best on a deep, well-drained loam, and I do not know of any that object to it. But some species, like B. pendula and populifolia, are amongst the best trees for poor, sandy soils. The river birch, B. nigra, thrives well with its roots within reach of water, and is perhaps the handsomest birch to plant where the water-table is high. Others that will grow in such situations are B. pubescens, nana, glandulosa, and pumila.
Whenever possible the birches should be raised from seed, which most of them develop in plenty. It should be sown on the surface of fine soil, and not buried but simply pressed down. An old and good plan when the seed is sown out-of-doors is to cover it until it germinates with a thin layer of brushwood, which gives shade and shelter and protects it from interference by birds, etc.
The common birches are attacked by a gall-producing insect, Phytoptus rudis, which causes an abnormal swelling of the leaf-buds, and distorted, stunted growths.
From the Supplement (Vol. V)
A qualification should have been added to Mr Bean’s statement (page 415) that ‘whenever possible the birches should be raised from seed.’ This is true only of the species, and then only if the seed comes from a wild source. Trees raised from seed gathered in botanical gardens and arboreta often prove to be hybrids, mostly with the native birches predominating in their make-up if the source was a European collection. Mr Kenneth Ashburner has in recent years built up a notable collection of natural-source birches at Chagford in Devon, and another is being formed in the Kew annexe at Wakehurst Place, Sussex, which owes much to Mr Ashburner’s introductions. For his survey of the cultivated species see The Plantsman, Vol. 2, pp. 31-53 (1980); and his note on the bark of birches in Int. Dendr. Soc. Year Book 1979, pp. 47-57.
The following are some recent contributions to the taxonomy of the American species of Betula:
Brayshaw, T. C. – Catkin-bearing Plants of British Columbia. Occasional Paper No. 18 of the British Columbia Provincial Museum, Betula, pp. 136-52 (1976).
Brittain, W. H., and Grant, W. F. – A series of eight papers under the general title ‘Observations on Canadian Birch Collections at the Morgan Arboretum’, published in Canadian Field-Naturalist, Vol. 79, pp. 189-97 and 253-7; Vol. 80, pp. 147-57; Vol. 81, pp. 116-27 and 251-62; Vol. 82, pp. 44-8 and 185-202; Vol. 83, pp. 361-83 (1965-9).
Dugle, Janet R. – ‘A Taxonomic Study of Western Canadian Species in the Genus Betula’, Canad. Journ. Bot., Vol. 44, pp. 929-1007 (1966).
The only comprehensive treatment of the east Asiatic birches is the now out-dated contribution by Camillo Schneider in Plantae Wilsonianae, Vol. II, pp. 455-88 (1916). See also the references under B. platyphylla and B. utilis below.