There are about 18 species of Carya, distributed in North America, Mexico and eastern Asia. Hickories are monoecious, deciduous trees or rarely shrubs. The bark is grey to brown and smooth with some fissures in young trees, becoming deeply furrowed or breaking into plates or strips in older trees. Bark is often of taxonomic significance in Carya – though usually absent from herbarium specimens. The terminal buds are naked or protected by valvate or imbricate scales. The leaves are imparipinnate with 3–17(–21) leaflets; the leaflet margins are serrate, the lower surfaces covered with glandular scales and non-glandular hairs. The inflorescences are terminal or axillary, on old or new growth; the staminate and pistillate flowers occur on separate spikes; the staminate spikes axillary, pendulous, in clusters of three at the base of new growth or rarely on old growth; the pistillate spikes terminal, erect, on new growth. The staminate flowers are subtended by a bract, with (usually) no sepals and no petals; stamens (2–)3–7(–10). The pistillate flowers are subtended by a bract that is fused to the ovary, with no sepals and no petals; stigmatic disk four-lobed. The fruiting spikes are erect; the fruit is a nut, enclosed in a dehiscent, four-valved husk; the nuts may be smooth or pitted (Elias 1972, Stone & Whittemore 1997, Lu et al. 1999).
Carya is a genus under-represented in European and especially British collections, which at first sight seems odd considering the magnificence of hickory trees in the eastern United States (Flanagan 1987, Andrews 2007). Bean (1976a) blamed it on the difficulty of obtaining good specimens from nurseries, but the problem is at least as likely to be the comparatively cool summers experienced in maritime Europe. Like many other North American trees the hickories need a very hot, humid summer to flourish to their best potential. With the exception of C. tomentosa (in Cornwall), all the UK champion Carya trees are in southeastern England, where summer temperatures are greatest. Even if summer temperatures continue to rise, the difficulties and discouragements presented by transplantation make these poor ‘nurseryman’s’ trees, so they are only likely to be planted by those few enthusiasts prepared to grow their own trees from seed. This is equally true in North America, where not many replacement hickories are being planted, leading both Dirr (1998) and Sternberg (2004) to plead for more planting to be done. Bean’s recommendation to plant out as young as possible is very sound – preferably into a rich soil.
It is curious that none of the Asian species seem to be in cultivation. Carya cathayensis Sarg. was grown at Kew in the 1980s (M. Flanagan, pers. comm. 2007) but has since been lost. The four Chinese species are all from low altitudes with southerly distributions. Of the Mexican taxa, C. ovata var. mexicana (Hemsl.) W.E. Manning and another unidentified species from Nuevo León are in cultivation at the University of California Botanical Garden in Berkeley.
From Bean's Trees & Shrubs
Of the twenty or so species of hickory as yet recognised about half are in cultivation in the British Isles. They are all, save one, natives of eastern N. America. From its two allies, Juglans and Pterocarya, the genus is distinguished by its pith being solid, and not, as in the others, divided into thin transverse plates; and from Juglans in particular by the branched male inflorescences and four-valved fruit. The hickories are large, deciduous trees with pinnate leaves; the leaflets rather wide apart on a common stalk, themselves nearly or quite stalkless. Male flowers mostly in three-branched, slender catkins, produced either at the end of the previous year's shoots or at the base of the young ones of the current year; whilst the few-flowered, female inflorescence terminates the young shoot. Nut surrounded by a husk, which often thickens and becomes hard by the time the seed is ripe.
Considering their great beauty of foliage and stately habit – and there is scarcely any tree more striking than a well-grown young hickory – this genus is strangely uncommon in gardens. The reason appears to be their dislike of disturbance at the root, which makes them unsuited to ordinary nursery conditions. The frequent transplanting which is practised by good nurserymen to ensure success at the final removal of their stock is, in my experience, worse than useless with hickories. It induces a stunted, ultimately diseased condition, from which, at the best, it takes them long to recover. The great secret with hickories is to get them in their permanent places early. To anyone desirous of trying these fine trees I would recommend the following procedure. The best species to experiment with are C. ovata, cordiformis, glabra, and tomentosa. Nuts of these should be obtained in autumn from a reliable American seedsman as early as possible after they are ripe. During the winter they should be kept in a box of moist earth, either inside or out-of-doors. In spring the nuts may be placed singly in 6. in. pots, in a slightly heated frame or greenhouse. After they have germinated, all that is necessary is to protect them from frost until they are planted out about the end of May, if sufficient progress has been made. Caryas need a deep, loamy soil if they are to thrive permanently. Previous to planting the seedlings out, the ground should be well worked, and it is wise to put a couple together to anticipate failures; afterwards the weaker one can be removed. To avoid accidents each plant or plants should be enclosed by small-meshed wire-netting.
The object of all this trouble is to avoid the destruction of the tap-root, which is inevitable if ordinary nursery treatment be adopted. A young tree in deep loam, undisturbed, and with its tap-root preserved, will be a better tree in ten years than another treated in the ordinary way will be in twenty. This method, although the best, may not always be practicable. If perforce the seedlings have to be grown on for several years before planting in their final positions they should be grown in light soil and transplanted every year or two to induce a fibrous root system. Such trees will be more difficult to establish than they would have been if planted out in their first year of life, but at least are preferable to ones which have not received this attention.