Consider a Hornbeam
Hornbeams are hardwood trees in the family Betulaceae (birch) and the flowering plant genus Carpinus. The 30–40 species of hornbeam occur across much of the temperate regions of the Northern Hemisphere, with the greatest number of species in East Asia, particularly China. Only one species is native to eastern North America, and two species are native to Europe. This article will focus on the native American hornbeam, Carpinus caroliniana, and on the European species, common hornbeam, Carpinus betulus. A third native tree, American hop hornbeam, Ostrya virginiana, will also be described. It too is a member of the Betulaceae family, belonging to the genus Ostrya, a genus of eight to ten small deciduous trees.
The American hop hornbeam is often confused with the American hornbeam. Both trees are commonly called ironwood. Both are understory trees and can grow in shade to partial-shade, share a similar leaf shape, are known for having very hard wood, distinctive bark, showy catkins, and yellow-to-orange-to-red fall leaf color. The American hornbeam can tolerate wet conditions, prefers shade (although it can grow in sun), and is less drought tolerant; the American hop hornbeam is more drought tolerant, cannot tolerate flooding conditions, and can grow in sunnier sites.
The European hornbeam, particularly the cultivar ‘Fastigiata’, is often found in U.S landscapes. It can be used as a tree, or hard-pruned into a hedge. It shares similar characteristics with the American hornbeam and the American hop hornbeam, such as leaf shape, very hard wood, distinctive trunk, and showy catkins. However, the European hornbeam is larger at maturity than either of the native species. Although it can grow in partial shade, it likes full sun and is moderately drought tolerant.
Before I discuss the hornbeams in more detail, I will address the benefits of planting a native tree when site conditions are favorable, and when the native tree satisfies planting objectives.
BENEFITS OF NATIVE TREES
Let’s assume we have decided to add a tree to our landscape, and we feel good about doing something that is beneficial for the environment and for posterity. We’ve heard about the benefits of planting native trees. Don’t all trees offer some benefits and serve different purposes? Perhaps we may simply prefer the ornamental appeal of a nonnative tree.
The landscape can hold many different types of plants, but it’s important to be aware of the particular benefits of native trees so that, if we have an appropriate location and planting objective, we might purposefully select a native tree. Doug Tallamy, author of Bringing Nature Home, describes native trees as an important pillar in the interactive food web, a food web of native plants and native insects that supports wildlife and ultimately, supports us. Because plants and insects in a particular region have evolved together, native insects look to those plants for food. When native plants are replaced with nonnative plants, insects don’t have food and birds can’t feed their young. Wildlife is deprived of native plant food sources, insects, and birds.
There is an impressive chart in the Tallamy book (p. 147) that lists the top 20 native trees and woody plants that support Lepidoptera species (moths and butterflies). Lepidoptera caterpillars are an important food source for birds feeding their young. Hornbeams, the focus of this article, are not included on this top 20 list, but they are nevertheless an important native food source. Carpinus caroliniana is included on Tallamy’s list of Native Plants with Wildlife Value and Desirable Landscaping Attributes for the Mid-Atlantic and Middle States (p. 294).
AMERICAN HORNBEAM (CARPINUS CAROLINIANA)
Native to the eastern half of the United States, the American hornbeam is commonly found in wooded areas as an understory tree in USDA hardiness zones 3-9. It prefers deep, fertile, moist, acidic soil and grows best in partial shade, but will grow in full sun. It does not do well in compacted soil. It is typically found along streams, river banks, flood plains and bottomland, and is planted in landscapes and in naturalized areas. It grows with an attractive open habit in total shade, but becomes dense in full sun.
As noted in TreeBaltimore, Dr. Michael A. Dirr, expert on woody plants and professor of horticulture at the University of Georgia, says this about the American hornbeam:
“I have observed this species in many landscape situations and believe it is much more adaptable than ever given credit. It performs well even in areas inundated with water for several days to a week or two once it is established. Although moderately drought-tolerant, it is probably best to provide even established trees with some irrigation during dry spells in the south.”
Its chief liabilities in cultivation are a relatively slow growth rate and difficulty in transplantation due to its deep, spreading, lateral roots. American hornbeam is more difficult to transplant than European hornbeam.
Its common name, musclewood, comes from its attractive blue-gray bark, fluted with long, sinewy ridges. The extremely hard wood of this tree inspires another common name, ironwood (a name shared with the American hop hornbeam). As the name suggests, the wood will take a horn-like polish and was once used by early Americans to make bowls, tool handles, and ox yokes. Commercial use of hornbeam wood is not practicable, however, due to the limited amount of wood that can be harvested per tree.
The American hornbeam can be grown as a multi-stemmed shrub or as a single-stemmed tree. It is more narrow and upright than the European hornbeam. The American hornbeam grows slowly, about 12” per year, reaching an average height and spread of 20-30′ over a life span of 50-150 years. The largest American hornbeam on record for the Southeast is 75 feet!
The leaves are deciduous, alternate, and simple with a sharp, doubly-serrated margin with larger teeth at the ends of veins. Leaves typically vary from 1-4” in length. Fall color can be beautiful, with yellows, oranges and reds. Although deciduous, hornbeams and hop hornbeams keep their dried leaves in winter. Marcescence is the technical term for plant parts that wither but do not fall off. It can refer to leaves, flowers, or fruit. The marcescent leaves of the American and European hornbeams, and the American hop hornbeam, provide a safe and secure habitat for garden wildlife during the cold season.
The hornbeam is monoecious: A plant or plant species producing male and female reproductive structures on the same plant but on separate flowers. Male and female flowers appear in spring on separate catkins (slim cylindrical flower clusters). The flowers are wind pollinated. The female catkins are about four inches in length, a bit longer than male counterparts, giving way to distinctive clusters of winged nutlets (fruit). Typically, there are 10–30 seeds are on each seed catkin. Maturing in October, the nutlet is held in a bract (a modified or specialized leaf) at the end of a stalk, providing forage for song birds and small mammals. In fall, the bracts change from light green to yellow. Although seeds are dispersed by wind, they are mainly dispersed by birds.
The minimum seed-bearing age of American hornbeam is 15 years. Seed production is greatest at 25 to 50 years and probably ceases at about 75 years.
Seeds, buds, and catkins are eaten by a number of songbirds, ruffed grouse, ring-necked pheasants, bobwhite, turkey, fox, and gray squirrels. Cottontails, beaver, and white-tailed deer eat the leaves, twigs, and larger stems. American hornbeam is heavily used by beaver, because the tree is readily available in typical beaver habitat.
A multitude of insect species utilize the American hornbeam as a larval food source. According to the Native Plant Finder, for the Charlottesville zip code 22901 the American hornbeam is a host plant for 72 species of butterflies and moths, including the Io moth, eastern tiger swallowtail, walnut sphinx, luna moth, and polyphemus moth, to name a few. This database, based on the work of Doug Tallamy and sponsored by the National Wildlife Federation, will provide caterpillar host plant information on specific native plants according to zip codes!
PESTS AND DISEASES
The American hornbeam exhibits no serious insect or disease problems. It shows resistance to verticillium wilt. Leaf spots, cankers, and twig blight are occasional disease problems. It may be susceptible to scales.
EUROPEAN HORNBEAM (CARPINUS BETULUS)
The European hornbeam is native to Western Asia and central, eastern, and southern Europe, including southern England. It grows in USDA hardiness zones 4-8. It requires a warm climate for good growth, and occurs only at elevations up to about 2,000 feet. It can grow in full sun to partial shade, in moist soil, either loamy or clay, and can tolerate highly acidic to neutral soil pH. The European hornbeam is more tolerant of drought than is the American hornbeam.
Like the American hornbeam, trunks have smooth gray bark and distinctive muscle-like fluting. Hornbeam wood is the hardest of any European trees. This deciduous, medium-sized tree matures to 40-60’ tall and 30-40’ wide at a growth rate of about 12-24” per year. When young, its shape is somewhat pyramidal or oval, becoming broader and rounder as the tree matures. It needs little pruning when grown as a tree, but responds well to hard pruning if grown as a hedge. Its average life span is 50-150 years.
Nowadays, the wood is mainly used for furniture, flooring, and wood turning, but traditionally the wood was made into ox yokes. The Romans made chariots from hornbeam because of the strength of the wood. A tonic made from hornbeam was said to relieve tiredness and exhaustion, and its leaves were used to stop bleeding and to heal wounds.
Leaves are simple, alternate, oblong, and doubly serrated, with prominent veins. Leaf length is 2-5”; leaf color is dark green changing to an attractive yellow to orange in fall. Foliage is typically dense and becomes denser with more sun. The bark and buds are ornamental in winter.
The tree is monoecious, as described in the section on the American hornbeam. The European hornbeam also forms male and female catkins in early spring, with female catkins being somewhat longer than the male and greenish in color; male catkins are yellowish. The female catkins form small, brown, winged nutlets (fruit) held in a bract. The nutlets mature in October. Bracts change from light green to yellow in fall.
Common hornbeam is the food plant for caterpillars of many moth species, including the nut tree tussock and the case-bearer moth. Finches, chickadees, and small mammals eat the seeds in autumn.
PESTS AND DISEASES
European hornbeam is largely resistant to pest and diseases, including verticillium wilt (fungal disease). It may be susceptible to armillaria (parasitic fungi), root rot, and scales (insects).
C. BETULUS ‘FASTIGIATA’
The cultivar ‘Fastigiata’ is much more common in commerce than is the species. Sometimes called Upright European hornbeam, it displays a narrow, fastigiate form (branches sloping upward) in youth, but gradually acquires a tear drop or oval-vase shape with age, typically maturing to 40’ tall and 30’ wide. Ovate, toothed, bright medium green leaves (to 4” long) are clean and attractive throughout the growing season with little susceptibility to foliar diseases. Foliage turns yellow-orange in fall. Trunks have smooth gray bark and distinctive muscle-like fluting.
AMERICAN HOP HORNBEAM (OSTRYA VIRGINIANA)
As a member of the birch family, Betulaceae, the American hop hornbeam is related to the alders, birches, hornbeams, and filberts. It is commonly called ironwood, a name shared with a number of other plants, including the American hornbeam.
Native to eastern North America and Mexico, the American hop hornbeam grows in USDA hardiness zones 3-9. Appropriate for shady locations, it also does well in sun where it develops a broader crown. It prefers moist, well-drained soil, and is tolerant of both acidic and alkaline soils. It can grow in clay, loam, or sand. Although it can tolerate drought, it will not tolerate flooding. It can tolerate dry, gravelly soils in partial shade once established. The tree is difficult to transplant and is slow to establish.
The hop hornbeam is a small- to medium-sized understory tree with a generally rounded crown. The tree matures to a height of 25-45’ and a width of 15-40’. It grows about 24” annually, and lives an average 50-150 years. Its dark, yellowish-green leaves (to 5” long) are alternate, elliptical, and doubly serrated, with prominent veins. The lateral leaf veins of Ostrya are forked, while those of Carpinus are parallel and rarely forked. Leaves, which turn yellow to red-orange in autumn, feel like felt. Leaf size is not uniform, with those near the ends of shaded branches reaching up to 6″ in length; leaves higher in the crown or farther back on the branches are much smaller.
Often confused with American hornbeam, the bark offers a point of differentiation. The smooth sinewy blue-gray trunk of American hornbeam easily contrasts with the shreddy, brownish, grooved bark of the American hop hornbeam. As the American hop hornbeam matures, the shreddy bark develops into thin vertical strips, only slightly shredding at the ends. Both young and aged bark are gray-brown in color.
The wood of hop hornbeam is hard and durable. It was once used for runners on sleighs. The bark and inner wood was used to treat toothache, sore muscles, coughs, and many other ailments by Native Americans.
Flowers are monoecious (reddish-brown male flowers and greenish female flowers appear in separate catkins on the same tree). Flowers are not particularly showy, although the male catkins are more prominent and are present throughout winter. Female catkins are followed by drooping clusters of sac-like, seed-bearing pods which, as the common name suggests, resemble the true hops that are used in the production of beer. The large photo at the top of the article shows very showy catkins of the American hop hornbeam.
Several vertebrate animals rely on hop hornbeam as a source of food. These species include the ruffed grouse (buds, catkins), downy woodpecker (seeds), and purple finch (seeds). Among mammals, the fox squirrel, red squirrel, woodland deer mouse, and white-footed mouse eat the seeds and buds, while the white-tailed deer browses sparingly on the twigs and leaves. The caterpillars of several moth species feed on the foliage of hop hornbeam, including ironwood tubemaker moth and ironwood leafminer. Many other moth species are listed in the Moth Table. There were zero results in the Native Plant Finder according to zip codes 22901 and 22903. (Database results are based on native plants that were historically present in a county based on range maps. Specific zip codes may not hold a particular native plant.)
PESTS AND DISEASES
The tree is resistant to many disease and insect problems, although it is susceptible to the gypsy moth. In the woods, it is one of the first trees to be defoliated by that pest. When under stress, the tree is also susceptible to the chestnut borer. It is resistant to wind, ice, and most stresses of urban living, although it is notoriously sensitive to salt.
Returning once again to Bringing Nature Home (in the Afterward), Doug Tallamy calls upon us to make our communities and our own landscapes into spaces he calls, The Last Refuge, a place populated with the plants and animals that evolved there. So although we may not choose to exclusively plant native trees, their inclusion in our landscapes is an important step in strengthening, or even reclaiming, biodiversity. To that end, the American hornbeam and the American hop hornbeam are less familiar native trees that might be great additions to your landscape. If you are interested in a larger tree, or in a tree that can be pruned into a hedge, you might consider the European hornbeam. Evaluate your landscape for sun, moisture, and soil type. Consider your tree options for shape, size, and fall color. Determine your objectives for planting a tree. When you’re ready to choose, perhaps you should try something new–consider a hornbeam!
Bringing Nature Home (Tallamy, Douglas W., 2007)
Native Plant Finder, National Wildlife Federation, https://www.nwf.org/NativePlantFinder/About
American hornbeam, The Morton Arboretum, https://www.mortonarb.org/trees-plants/tree-plant-descriptions/american-hornbeam
“American Hornbeams Show the Upsides to Planting Native Trees, Omaha World Herald, https://www.omaha.com/eedition/sunrise/articles/american-hornbeams-show-the-upsides-to-planting-native-trees/article_b6dbad4f-3daf-5372-a38c-39865a9db99f.html
“Tree of the Season: American Hornbeam,” TreeBaltimore*, http://treebaltimore.org/tree-of-the-season-american-hornbeam/#.Xh0wFoh7nX4 *Baltimore’s program for the coordination of all tree plantings by city agencies, non‑profit organizations, neighborhoods, and community associations.
”The Sinewy American Hornbeam,” In Defense of Plants, http://www.indefenseofplants.com/blog/2019/12/11/the-sinewy-american-hornbeam?rq=hornbeam
Carpinus caroliniana, Missouri Botanical Garden Plant Finder, https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=h540
University of Connecticut Plant Database, http://hort.uconn.edu/detail.php?pid=83
Carpinus caroliniana, USDA FEIS (Fire Effects Information System, Index of Species Information
Plant Database, Lady Bird Johnson Wildflower Center, https://www.wildflower.org/plants/result.php?id_plant=OSVI
American Hornbeam, University of Kentucky, https://www.uky.edu/hort/American-Hornbeam
Musclewood or Ironwood, Ewell A. Stowell Arboretum, Whitehouse Nature Center, Albion College, https://campus.albion.edu/stowell-arboretum/musclewood-or-ironwood/
SelecTree: Tree Detail, European Hornbeam, Calpoly Institute, Urban Forest Ecosystems Institute, https://selectree.calpoly.edu/tree-detail/carpinus-betulus
Hornbeam, Carpinus betulus, Woodland Trust, https://www.woodlandtrust.org.uk/trees-woods-and-wildlife/british-trees/a-z-of-british-trees/hornbeam/
Plant Profiles, Chicago Botanic Garden, https://www.chicagobotanic.org/plantinfo/pp/j
American Hop Hornbeam, Ostrya virginiana, Ohio Department of Natural Resources Division of Forestry, http://forestry.ohiodnr.gov/hophornbeam
Hop Hornbeam, Illinois Wildflowers, https://www.illinoiswildflowers.info/trees/plants/hop_hornbeam.htm
Featured Image, Ostrya virginiana catkins, Katja Schultz from Washington, DC, National Botanic Garden, Washington, DC, Creative Commons