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There are eight species in the genus Castanea, found in North America, eastern Europe, central and eastern Asia. The Sweet Chestnut, C. sativa Mill., is naturalised in many parts of western Europe (Govaerts & Frodin 1998). Chestnuts are deciduous trees or shrubs with vertically furrowed bark. The terminal buds on branchlets are aborted; the axillary bud nearest the stem tip then develops and is enclosed in two outer scales. Chestnut stems can be identified in winter as their ‘terminal’ bud has an adjacent leaf scar. The leaves are spirally arranged, but twisted to appear as if in two ranks. They are thin and leathery, with unbranched secondary veins running in parallel; each vein terminates in a marginal tooth. Stipules are prominent in young growth, but soon fall. Castanea is monoecious; inflorescences may be staminate only or may bear both staminate and pistillate flowers (androgynous). The inflorescences are erect, catkin-like and often produced in profusion in the axils of the leaves on the edge of the canopy. The staminate flowers are small, cream and clustered in groups of one to three (to five). The pistillate flowers are borne on the lower part of androgynous catkins and are subtended by a symmetrical cupule; the cupule is spiny and has two to four valves. The fruit is a nut and there are one to three in each cupule (Nixon 1997, Huang et al. 1999).
The numerous qualities of Castanea sativa give this small genus a particular significance. On another point of interest, the ravages of chestnut blight in the American Chestnut C. dentata are an object lesson in the necessity for rigorous phytosanitary checks on newly imported plant material – and indeed led to the introduction of phytosanitary laws in the United States. Anagnostakis (1992, 2001) has documented that the disease was almost certainly imported into the United States on plants of the Japanese C. crenata, distributed through the horticultural trade. In the United States the Chinese C. mollissima is the usual substitute for the native species, and there is an active programme to breed resistant hybrids with a majority of C. dentata genes (Sternberg 2004).
There appear to be about ten species of chestnut known, but the number varies much in consequence of the varying conception of their specific limits. In any case they constitute a well-marked group of deciduous trees and shrubs, with alternate, parallel-ribbed, conspicuously toothed leaves, always approaching the oblong or narrow oval in shape. The leaves of all the chestnuts have a strong family resemblance; the only leaves anything like them in hardy trees occur in a few oaks. The unisexual flowers are produced in long, slender catkins from the leaf-axils of the young shoots during July. The lower catkins are entirely male; but from the axils of the later leaves there come shorter catkins, at the base of which one to three female flowers are borne. The flowers of all the chestnuts are pale yellow, and have little beauty of colour; but a tree well laden with catkins has a distinct appearance, the enjoyment of which to many people is spoilt by their heavy, unpleasant odour. The nuts are always enclosed in the well-known prickly burs.
The older botanists made C. dentata and C. crenata both forms of C. sativa, which may have led to their not being introduced, and to their present rarity. They are, however, distinct enough, especially as seen in the living state, although it is not easy to make the distinctions very clear on paper. It does not seem likely that any other than C. sativa will be of much value in Britain either for timber or nuts, although the variety ‘Paragon’, sometimes grown, is considered to have the ‘blood’ of C. dentata in it. The ordinary C. sativa varies extraordinarily in the size and quality of its nuts. There are numerous trees in Kew Gardens, some of which bear large, excellent nuts and others that never produce a nut worth eating. The merit of the better forms seems to be due largely to their being able to suppress all but one of the three or four nuts which each bur normally encloses. This enables the survivor to develop into a fine nut.
The chestnuts like a hot summer. Even during the driest and hottest seasons, like that of 1911, one rarely sees any of this genus suffering. They appear to thrive in any well-drained, loamy soil, even of moderate quality, but are said to be averse to calcareous soils. They should always be raised from seeds except in the case of the fine fruiting forms, which are grafted easily in spring on seedlings of the common sort.