Carpinus caroliniana Walt.

Carpinus caroliniana occupies the same role in the lowland forests of eastern North American as C. betulus does across Europe but, in competition with a much wider variety of tall-growing trees, it is more precisely confined as a slow, shade-tolerant understorey plant of heavy, difficult soils and flood-plain woodlands; it never grows as big as C. betulus, and probably does not live so long. It produces a similarly hard and heavy timber, and had numerous medicinal uses among Native American tribes (Furlow 1997). Plants from the northern end of the species’ range are rather hardier than C. betulus, but the species is less pollution tolerant in urban situations (Dirr 2009). Like so many American trees, the autumn colour can be brilliant, orange and scarlet tints mixing with the European hornbeam’s characteristic yellow. The fruit bract is less symmetrical than the European tree’s, and the side-lobes are less developed. The foliage and the general aspect are very similar, but the shorter and less pointed winter buds of the American tree offer a useful differentiating feature (Bean 1976); until late summer however, next winter’s buds are seldom sufficiently developed to be distinguishable. Recent phylogenetic analyses (Dong et al. 2022) have indicated that C. caroliniana is not very closely related to other members of the genus, except for the Mexican C. tropicalis – as might have been expected, given these taxa’s long isolation in the New World. The closest Old World allies seem to be not C. betulus but the east Asian species C. viminea and C. laxiflora, whose fruit-bracts are shaped similarly.

Carpinus caroliniana is stated to have been introduced to Britain in 1812 (Bean 1976), which seems a little late for an abundant tree from its part of the world, but this date was probably derived from a reference within Arboretum et Fruticetum Britannicum (Loudon 1838) to a tree at Trentham Park in Staffordshire which was supposed to be have been about 25 years old when J.C. Loudon sent out his requests for information and measurements to the leading collections of the time. However, as this tree was 11 m tall and 31 cm dbh by this stage, it must have been older (or was really a more vigorous example of C. betulus). Another specimen about ten years younger and only 4 m tall was also recorded by Loudon from Dalhousie Castle in southern Scotland (Loudon 1838). Neither example has been recorded since, and the next evidence for the American Hornbeam’s cultivation in Europe may not be until 1916, when five were planted at the Royal Botanic Gardens, Kew; the single old tree which survived near the Elizabeth Gate until 2010 (when it was 12 m × 36 cm dbh) lacked a confirmed planting date and may or may not have been one of these (Tree Register 2022), but it does seem true that C. caroliniana has a high failure rate in north-west Europe. The species has certainly never been more than a collector’s tree here, and, even in collections, some examples which have long been labelled as the American tree turn out on closer inspection to be ordinary C. betulus: this is true of the fine example planted in 1929 at Clay Island in the Westonbirt National Arboretum (Tree Register 2022). C. caroliniana might prefer the longer warmer summers of southern Europe, but there are too few examples for this hypothesis to be tested. In recent decades, the species has been caught up in the general enthusiasm for planting hornbeams in big gardens, particularly in England, and this slightly more widespread presence may finally allow it to prove its worth in Europe.

Across the larger part of the native range plants with scattered dark glands under their leaves are distinguished by some authorities as subsp. virginiana (Marshall) Furlow – a combination which was not published until 1987. (The variant is also occasionally known as C. virginiana (Marshall) Sudworth, but should not be confused with the closely related hop-hornbeam Ostrya virginiana (Mill.) K. Koch, which shares the vernacular name ‘Ironwood’ and was long known as C. virginiana Mill. The plant first described by the American botanist Humphry Marshall in 1785 as ‘C. betulus virginiana’ was C. caroliniana sensu lato, thought the example in question may have belonged within subsp. virginiana.) Hornbeams from the south-eastern coastal United States and the lower Mississippi valley (subsp. caroliniana in Furlow’s understanding of the species) lack the dark glands and tend to have smaller, longer-tipped leaves with finer secondary teeth, but there is a broad band of introgression from the Carolinas south to northern Georgia and westward to Missouri, Arkansas and southeastern Oklahoma (Furlow 1997). In collections, C. caroliniana has seldom been identified to subspecific level but most examples, particularly in colder climates, are likely to belong to subsp. virginiana.

In North America various clones have been selected for a more upright habit, suited to use as street trees, for general hardiness and for reliable autumn colour. A couple of different cultivars are grown in European collections and are now offered by a few specialist nurseries here.