Home » Education » Tree of the Season: American Hornbeam

Whether it’s a visit to the pool, a stop at the snowball stand, or a lazy stroll in the shade of tree canopy, summer is here, and the warm weather has us seeking ways to beat the heat. As the sun peaks high above, a cool, steady breeze is what we’re after. This may be found blowing through a forest near you. Our tree of the season is a species commonly found in just the place you’ll want to be during the next heat wave.

American Hornbeam, or Carpinus carliniana, is an understory tree of the bottomlands found growing very slowly in the shadiest environs of a cool, damp forest. Quite a few are thriving in the Gwynns Falls ravine in West Baltimore. Forest-bound wanderers will notice the slate blue-gray bark and smooth beech-like trunk spiraling upwards with its sinewy musculature often in an arching, irregular reach for filtered sunlight.

Carpinus carliniana in its natural habitat | Photo: Ted Martello

Carpinus is monoecious, meaning it has both male and female flowers on one plant. Its flowers are called “catkins.” The female catkins are about four inches in length, a bit longer than male counterparts. Maturing in October, tiny brown nutlets attached to three-lobed leafy bracts clustered on a hanging stalk provide forage for song birds and small mammals. There’s little nuisance related to fruit and litter drop.

American Hornbeam leaves are arranged in an alternate pattern along the stem; they grow about two-and-a-half to five inches long and about one to two inches wide forming an overall egg shaped outline, minus the tapering tip. Leaf margins are doubly serrate, which means you’ll find tiny teeth within each large tooth. Fall foliage coloration can vary greatly from tree to tree. According to Michael A. Dirr, author and professor of horticulture at the University of Georgia, the species exhibits an “opportunity for selecting superior fall coloring clones,” considering it drops its leaves earlier than its European cousin Carpinus betulus.

The MD Big Tree program nomination is pending, but our intern recently found quite an impressive Carpinus caroliniana: 24” diameter at breast height, canopy height of 30’, and spread of over 50’! These measurements make this the largest known American Hornbeam in Baltimore City.

As explained in Peterson Field Guide to Eastern Trees, there is confusion between two species due to the use of the common name “Ironwood.” This is where you’ll find arborists and plant enthusiasts praising the importance of learning botanical Latin. Ostrya virginiana, or Hop Hornbeam, is often confused with American Hornbeam. Under close study, these two species differ in appearance. Smooth sinewy trunk bark of Carpinus easily contrasts with shreddy, brownish, grooved bark of Ostrya. The lateral leaf veins of Ostrya are forked, while Carpinus are parallel and rarely forked. The Carpinus, known by some as the Blue-beech, is not in the same family as true beech, but it is still commonly called Ironwood because of the strong muscular appearance of its trunk.

Transplanting Carpinus caroliniana can be difficult due to its deep spreading lateral roots. It is easily planted from containers, but field grown “ball and burlap” plants must be planted during winter to early spring. This versatile tree tolerates periodic flooding and a wide range of soil conditions. It grows best in deep, rich, moist, slightly acid soils; however, it will still grow on drier sites. Michael A. Dirr says, “I have observed this species in many landscape situations and believe it is much more adaptable than ever given credit.”

Perhaps most appropriately applied in naturalistic plantings due to its variability in form, those interested in applying it in a more formal setting might want to look into a cultivar known as Palisade®. This cultivar forms an upright oval canopy with ascending branches and improved density. According to a US Forest Service fact sheet on Carpinus caroliniana, its uses vary from Bonsai, hedges, and screens, to specimen, shade, and street tree pits. Tolerating slightly alkaline soils, it may be worth a try in wide grass strips along residential streets, but this is not a salt tolerant tree, so avoid using it along commercial districts where businesses use ice-melting salts in winter. You will find this and its European cousin on the official Baltimore City Street Tree Species recommendations list.

In the recent past, field grown plant availability among local nurseries has been a challenge. Consequently, the City planting contract has not employed American Hornbeam. It is, however, available through our large tree order contract in container form. TreeBaltimore orders small quantities for various planting partners. The plants come off the truck with a densely foliated, uniform branching structure and strong central leader. Maybe you can mix this one into your next planting palate. Fall 2018 orders are already being processed. So in the meantime, make sure you get out to the trailhead and find a hornbeam to hug.

By Ted Martello, Urban and Community Forester, TreeBaltimore

August 2018