Some forty-five species of hornbeam are scattered over the temperate regions of the northern hemisphere, scarcely half of which are in cultivation. They are deciduous trees, rarely of the largest size, with zigzag twigs and alternate, conspicuously parallel-ribbed leaves. The flowers are unisexual, both sexes produced on the same tree, but on separate catkins. The pendulous male catkins come on the old wood; the females terminate the young shoots. The male flower consists of numerous stamens produced in the axil of a scale. The female inflorescence is stalked and at first erect, with the flowers in pairs in each deciduous scale; the single flower is subtended by a bract which is unequally lobed at the base and really a compound organ made up of a bract and two bractlets. In the fruiting stage the catkin elongates and becomes pendent, the seed being enclosed in a ribbed nut at the base of the enlarged bract and bractlets.
Hornbeams are hardy trees, and handsome, especially in summer when laden with pendent fruit clusters. As a park tree none is so valuable as our common hornbeam, but for gardens some of the Chinese and Japanese hornbeams are very attractive. They thrive in any good loam, and are at home on chalky soils. All the species should be raised from seed, but the rarer ones can be grafted on common hornbeam, as also must its own varieties be. There are two distinct sections of the genus:
1. Carpinus proper. – Scales of male flowers ovate, scarcely stalked. Bracts of the fruiting catkins loosely overlapping, and so little infolded as to leave the nut exposed – C. betulus, etc. Veins in seven to fifteen pairs.
2. Distegocarpus. – Scales of male flowers narrower, stalked. Bracts of the fruiting catkins closely packed, overlapping, infolded at the base and surrounding the nut. Two of the species described belong to this section – C. cordata and C. japonica. Veins in fifteen to twenty-four pairs. At first glance these species might be confused with Ostrya (hop hornbeam) when in fruit, but in that genus the nut is completely enclosed in a bladder-like organ.
From the Supplement (Vol. V)
An account of the collection of hornbeams at Hergest Croft, Herefordshire, by R. A. Banks, was published in the International Dendrology Society Year Book for 1972, pp. 13-17. The cultivated species are reviewed by Keith Rushforth in The Plantsman, Vol. 7(3), pp. 173-91 and Vol. 7(4), pp. 205-7 (1985-6).
An informal classification of the genus was outlined by Walter Berger in Botanisk Notiser, 1953, pp. 1-47. In this, the members of the section Carpinus are divided into five groups, according to the form of the bracts of the fruiting catkins.