A genus of about 8 species of medium-sized deciduous trees. Bark soon becoming rough and scaly. Twigs slender, often zig-zag; buds alternate, ovoid, with many scales. Leaves ovate, serrate, usually doubly and sometimes deeply so; lateral veins often in many parallel pairs; petiole short. Plants monoecious: male flowers in catkins clustered terminally and naked through winter, expanding early in spring, bracts overlapping, each subtending one flower, stamens 3–14; female flowers in short, dense terminal catkins, flowers paired, with a sack-like bract which after pollination becomes sealed to form a bladder which has stinging hairs at the base, ripening whitish then brown. Nutlet ovate, ribbed. (Li & Skvortsov 1999; Bean 1976).
Hop-hornbeams are eye-catching trees in early spring, when the male catkins expand to 10 cm long and turn creamy-yellow with pollen; like those of Corylus and Alnus they are exposed in embryo through winter. The hop-like clusters of fruit are also attractive, and nestle among the dense masses of summer foliage, ripening almost white then finally brown. For the remainder of the year, hop-hornbeams look very like their hornbeam cousins (Carpinus): they have the elegance common to many shade-bearing understorey trees, with masses of fine twigs which sometimes arrange themselves into rather level tiers; their ovate leaves, neatly corrugated with parallel veins, turn yellow in autumn. Their timber is equally dense and tough; the generic name Ostrya derives from the Classical Greek name for a tree with hard wood.
There is in fact one group of hornbeams (Section Distegocarpus) whose fruit-clusters look equally hoplike, although on closer inspection the bracts are not sealed around the nutlets to form the characteristic bladders of Ostrya; one of these ‘false hornbeams’ in particular, C. fangiana, has much longer and even showier fruiting catkins. Recent phylogenetic analyses (Li 2008; Dong et al. 2022) have suggested that Ostrya and Carpinus section Distegocarpus both remain close to the common root from which the larger group, Carpinus section Carpinus, appears to have evolved; it is possible to interpret these results as showing Ostrya nested within the genus Carpinus. The European Hop-hornbeam (O. carpinifolia Scop.) was described by Linnaeus as Carpinus ostrya, but future taxonomic fashions seem unlikely to submerge Ostrya into Carpinus: the way the fruit-bract seals itself to form a bladder around the nutlet is a feature unique to the hop-hornbeams. This bladder, which bears stiff, stinging hairs at its base, has presumably evolved to protect the nutlet against small seed-eating animals; the hairs are an irritation to anyone collecting seed from Ostrya (Dirr 2009) but are unlikely to trouble the casual observer.
The brown bark of Ostrya, which quickly cracks into scaly ridges, offers another feature to distinguish the group from Carpinus, in which the bark tends to remain rather smooth and grey, although in Section Distegocarpus it can become comparably rugged. Ostrya has also evolved to exploit more free-draining or rocky soils than most hornbeams; in the eastern United States in particular there can be a sharp disjunction between wet or flood-plain woodlands where Carpinus caroliniana grows and the adjacent, sandy hillsides where Ostrya virginiana (Mill.) K. Koch is found (Dirr 2009). One consequence is that Ostrya are not so reliable as Carpinus in urban situations, where soils may be compacted and poorly oxygenated; O. virginiana also has a reputation for being intolerant of pollution from road-salt and traffic fumes (Stewart 2010) and of being difficult to transplant (Fehrenbach 1984). This said, Ostrya are tough trees, unpalatable to browsing deer and remarkably free from pests and diseases in general (Dirr 2009). None are known to be calcifuge, and all can tolerate at least some shade. In European gardens, Ostrya carpinifolia is the only species to be at all common – the current fashion of exotic Carpinus among tree-collectors, particularly in the UK, has yet to rub off on its closest allies.
Like the bulk of Carpinus, Ostrya forms a uniform group whose species can be very hard to differentiate. The commoner species are theoretically distinguishable by the number of pairs of side-veins in the leaf, but examples of unknown provenance in cultivation often straddle the suggested parameters and are liable to get renamed. (There are probably too few collections growing multiple Ostrya species for hybridisation to be a likely issue). The presence or absence on the twig and petiole of glandular hairs – shaped like miniature drum-sticks – is also a useful feature for identification, although these are effectively microscopic.
Ostrya could be considered a text-book example of a genus whose distribution has been drastically fragmented by Ice Age climate change: there is one species which has spread widely across the eastern United States and south-eastern Canada from refugia in the southern Appalachian Mountains (Ostrya virginiana), one species which has spread across south-eastern Europe and south-west Asia from refugia in the Caucasus and similar mountain ranges (O. carpinifolia), and one species which has spread across eastern Asia from refugia in the mountains of central China, Korea and southern Japan (O. japonica Sarg.). Further south, in the mountains of China and also extending northwards from central America, the genus survives in greater variety but often in tiny numbers, and these populations may not yet be well understood. From southern China, three endemic species of Ostrya are currently acknowledged: O. chinensis I.M. Turner was first described in 1938 by Alfred Rehder (under the illegitimate name O. multinervis) whose leaves can have up to 25 pairs of side-veins (Li & Skvortsov 1999); O. trichocarpa D. Fang and Y.S. Wang was described from subtropical forests in south-west Guangxi in 1983 (Li & Skvortsov 1999); O. rehderiana was named by J. Chun in 1927 from a population in the Tianmu Shan of north-west Zhejiang, where it remains Critically Endangered (Shaw et al. 2014). (A fourth potential species, O. yunnanensis, was described from the Luquan Xian in north-west Yunnan by W.K. Hu in 1979, but genetic sequencing in 2016 showed it nested within O. chinensis (Lu et al. 2016).) Of these three taxa, only O. rehderiana is known to be in cultivation in the west.
From the canyons of the south-western United States, Ostrya knowltonii Coville was named in 1894, and the Critically Endangered O. chisosensis D.S. Correll in 1965. Arguably, Ostrya at this point provides another text-book example, but one has more to do with politics than natural history: the potentially diverse populations which extend southwards through Mexico into central America are usually treated by American botanists as synonymous with O. virginiana, and at best as a variety or subspecies; for convenience, this account follows Plants of the World Online (Plants of the World Online 2022) in describing them under O. virginiana subsp. guatemalensis (H. Winkl.) A.E. Murray. A comparable situation is seen in Carpinus, where Chinese botanists have described almost a hundred potential species from subtropical parts of their nation but New World botanists – nearly all of them based in the United States – conventionally treat the Mexican populations, which could be just as complex, in synonymy with the USA’s C. caroliniana.
|1a||Twig and leaf-stalk usually with at least some glandular hairs||2|
|1b||Hairs on twig and leaf-stalk never glandular||4|
|2a||Leaf with at least 10 pairs of side-veins||3|
|2b||Leaf small but broad, with 5–8 pairs of side-veins: west North American species||Ostrya knowltonii|
|3a||Leaf with 12–15 pairs of side-veins; east North American species||Ostrya viriginiana|
|3b||Leaf with 10–15 pairs of side-veins; east Asian species||Ostrya japonica|
|4a||Leaf with at least 12 pairs of side-veins; Old World species||5|
|4b||Leaf slender, with fewer than 12 pairs of side-veins: North American species||Ostrya chisosensis|
|5a||Leaf with 12–15 pairs of side-veins, fruit-bladders ovate: European and W Asian species||Ostrya carpinifolia|
|5b||Leaf with 13–16 pairs of side-veins, fruit-bladders with elongated stalk-like bases: Chinese species||Ostrya rehderiana|