Nothing is more refreshing in a summer garden than the sight of lush green leaves against a clear blue sky, but the demands of summer's heat, and changeable moisture conditions, make it difficult for trees and shrubs to keep their fresh green character throughout the season. Hornbeams, however, are trees that meet the challenges of the dog days with sprightly green leaves crowning silvery trunks.
There are a number of Carpinus, or Hornbeam, species that are excellent trees for the landscape. They are all deciduous relatives of Birches (Betula spp.) reaching heights of 20' to as much as 60' (with maturity) and spreading to 30' - with specific size depending on the type. Lovely, slender branches arch up and away from their distinctive, smooth-barked trunks. The trunks are silvery or blue-grey, and especially handsome combined with their bright green leaves. The foliage is similar in apprearance to that of Birches but leaves are generally smaller, and a bit more refined, often with better quality throughout the growing season. The fruits of Carpinus are delicately intriguing, as if hundreds of tiny, papery whirligigs had been strung together in dozens, and dangled from the branches. These fruit are a good tool for identifying Carpinus, particularly when separating it from its look-alike cousin Ostrya virginica, the Hophornbeam. Unlike Hornbeam, Hophornbeam's fruit clusters resemble the fruit of the Hops plant.
Carpinus caroliniana is the single North American native, the American Hornbeam. It is often called Musclewood, referring to the undulating trunks, which bear an uncanny resemblance to the limbs of health club devotees. Carpinus caroliniana is also called Ironwood because its wood is very hard and was traditionally used for mallets, golf clubs and tool handles. It is a small understory tree, generally 10 to 30' tall, found in moist places throughout the eastern half of the US, and no less lovely for its common occurrence. Musclewood can have spectacular fall color in shades of canary yellow and scarlet, but this character varies throughout the seedling population, so purchase plants in the fall to preview their fall color. It grows very well in shade and so is a good choice for creating scale in gardens under towering older trees, but it also performs admirably in full sun, as long as there is adequate moisture. It is best to transplant Musclewood balled and burlapped. It will tolerate flooding and many other urban landscape conditions, even those near parking lots and streets, although other Carpinus are probably better choices for these especially stressful locations. It is completely hardy throughout the southeast, from the coast to the mountains, and may be somewhat more heat tolerant than other Carpinus. Of the Carpinus, Musclewood is the most likely to suffer disease and insect problems on the foliage but these are not usually severe. Musclewood is generally propagated from seed collected while green and stored in a moist medium at 40F for 4 months. It is somewhat difficult to propagate, both from seed and vegetatively. There is an uncommon cultivar called 'Pyramidalis' with an inverted pyramidal habit of variable character.
Among the non-native Carpinus, C. betulus, the European Hornbeam, named for its native region, is the most widely grown (although no Hornbeams are especially common in the trade). It is very similar in appearance to American Hornbeam but is somewhat larger, reaching 50-70' with age, with larger leaves and more upright, pyramidal habit. The foliage of this strikingly beautiful tree remains clean and clear green all summer, like emerald velvet against the sky. It is not a rapid grower, making it a good choice for size-restricted areas, especially if some of the upright cultivars described below are used. It is completely hardy throughout the southeast, but may not be at its best in coastal areas. European Hornbeam prefers well-drained sites in full sun but has also performed well in the heavy Piedmont soils at The NCSU Arboretum (now the JC Raulston Arboretum). This elegant tree will remain beautiful in street boxes, parking lot planters, and low-walled condominium patios. It remains dignified 12 months of the year, even through the ravages of today's asphalt-jungle landscape, displaying an elegant tracery of branches in winter above its smooth, handsome bark.
There are a number of cultivars of European Hornbeam which are superior to seedlings for landscape use. Two cut-leaf forms are 'Asplenifolia' with a lacy texture created by deeply lobed foliage (similar to that of a cutleaf Beech) and 'Incisa' with shorter foliage more coarsely lobed. 'Columnaris' and 'Fastigiata' are two upright, narrowly columnar forms which are often mixed in the trade and which are both excellent selections with nearly identical landscape character perfect for street trees, hedges, patio specimens or anywhere a quality small tree is needed. 'Globosa' is a very rounded form - almost a large, circular shrub - which makes a good hedge or screening plant. 'Horizontalis' has a flattened crown, while 'Pendula' has a weeping habit. The new foliage of 'Purpurea' emerges burgundy but quickly changes to green (especially in the warm south). The leaves of 'Quercifolia' are reminiscent of oak foliage while those of 'Variegata' are splotched with light yellow and 'Albo-variegata' leaves are marked with white.
Propagation of European Hornbeam is relatively difficult both from seed and from cuttings. Named selections are usually grafted onto seedling understock and researchers have worked to improve vegetative propagation techniques. In the past, these difficulties have contributed to the relative scarcity of Hornbeams in commercial production but as more growers work with grafting and specialty crops, Hornbeams become more widely available. Their year-round beauty and grace add immediate artfulness to the landscape. In the west half of The NCSU Arboretum (now the JC Raulston Arboretum), a number of different Carpinus display their impeccable character where any gardener can experience the green refreshment of Hornbeam in full summer leaf.