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Up to 41 species of Carpinus are recognised (Govaerts & Frodin 1998), though this may be an overestimate. Hornbeams are deciduous trees or shrubs with a mainly temperate northern hemisphere distribution. The bark is typically smooth or fissured with inconspicuous lenticels, often subtly ridged and furrowed. The trunk is often irregular and ‘lumpy’, the suggestion of bulging muscles under the bark skin giving rise to the American name ‘musclewood’. The trunk tends to fork low down to form a bushy tree with numerous major branches arising from near its base. The branches are often smooth and slightly grooved in the same muscular way as the trunk. Young shoots are frequently somewhat pendulous, but often straighten up in subsequent growth. Winter buds are ovoid and smooth, with multiple scales. The expanding leaves are initially plicate (folded), though they flatten somewhat later, and are arranged in two ranks. They are simple, petiolate and with either serrated or double-serrated margins. The leaves range from ovate to obovate, may be glabrous or tomentose and may be glandular. Carpinus is monoecious. The staminate inflorescences (catkins) are pendulous, solitary or in small racemes, and are formed the previous season, emerging with the leaves in spring. The pistillate inflorescences (also catkins) are solitary, more or less erect, and situated on short, lateral shoots (rarely terminal); unlike on the staminate catkins, the flowers and bracts are not congested. The bracts are rather large and foliose, and each subtends two flowers. The flowers are insignificant. The fruit is a nutlet with distinct, longitudinal ribs and no wings. However, in some species, the papery bracts partially enclose the seed, forming a wing-like structure (Furlow 1997, Li & Skvortsov 1999). A number of accounts have been published in the horticultural literature (Rushforth 1985, 1986b, Muir 1986, Wright 1986), together with a comprehensive key to the species (Rushforth 1987b).
Commencing his survey of Carpinus, Keith Rushforth (1985) commented on its status as ‘a neglected genus’. This is still largely true today, perhaps because the plants’ beauties are subtle compared with the more obvious allure of more popular genera, but the connoisseur of elegance will find much to appreciate. With the exception of the familiar large C. betulus and its close relatives, most species are apparently small trees and suitable for planting as understorey specimens in open woodland, or as larger elements in a border of shrubs, but it may be that over time they too will become quite large. The basal branching means that they can become quite broad quite quickly, so that trees are often broader than they are tall. The lightly shaded space under such trees would be an ideal situation for cultivating summer-dormant bulbs. All Carpinus seem to be tolerant of a range of soil types, and are successful on calcareous and clay soils, but many recently introduced species seem to suffer badly from dry conditions, at least when young. Hardiness is not as well understood as it might be, and several species from apparently suitable provenances remain poorly represented in collections despite repeated introductions. Another danger comes from late frost damage to expanding shoots.
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.
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.