American hornbeam's ability to withstand heavy pruning makes this native wetlands tree a good choice for hedges in areas with poor drainage. Since the root system remains small and shallow, hornbeam trees grow near walkways without damaging pavement. American hornbeam could grow to 50 feet, but mature heights from 15 feet to 20 feet are more common. Pruning methods determine whether the tree will be a hedge, a thicket, or a shade tree.
Pruning often begins at the nursery, where most American hornbeam plants are selected for single trunks. Multiple trunks form hornbeam's natural pattern and when planted for hedges that spreading form makes better sense. If hedges are desired, root suckers and strong lower branches should not be completely removed. To fill gaps in hedges select the nearest vertical branch or sucker shoot and use pruning shears to clip it back to the first horizontal growth. Save new branches growing in the desired direction and clip back unwanted growth until the shape is established.
In summer, trimming American hornbeam hedgerows may be a monthly chore. Frequent light shaping with hedge trimmers focuses cuts on small twigs, doing less overall damage to the plant and encouraging denser foliage. Avoid cutting back to last year's wood--this removes the most vigorous growth and creates bare spots in the hedge.
In winter, examine the hedge for fungal canker--marked by dead patches of bark surrounded by whorls of scar tissue. With shears or loppers cut out the diseased limb below the canker. Cankers on the trunk may girdle and kill the tree. With good care an infected hornbeam could outgrow the infection, replacing cut sections with new growth yearly.
Always rake the hedgerow clean of clippings after pruning or trimming. Old clippings in the hedge block light and interfere with new growth. Clippings on the ground or in the tree harbor harmful fungi. Green clippings make good compost.
American hornbeam grown as an individual tree requires different care. Allow new trees to grow naturally, removing lower branches only when these limbs lose vigor. As the tree gains height, the lower space can be opened up by pruning drooping limbs back to the main branch. When the crown is well above head height use a pruning saw to remove any large limbs which interfere with foot traffic. Cut the unwanted branches back to within an inch or two of the main trunk. Undercut by one third the limb diameter and then finish the cut from above. Remove damaged limbs in the crown of the tree with a pruning hook.