One of the most important maintenance tasks in the Arboretum’s horticultural collections is pruning. We strive to showcase the natural growth habits and qualities of our specimens, but cultivated plants need training and maintenance. Our job is to maintain their health—and keep our visitors safe—while enhancing their natural forms.
We prune trees for many reasons. Structural soundness is paramount both for our trees’ well-being and for public safety. Our stormy summer caused major tree damage. In almost every case, the source was the same: tight angles between branches or trunk forks where bark grows into the joint. When this happens, the tree cannot heal over the joint; the bark inclusion persists—and may rot—as the limbs grow. We saw unfortunate damage to our largest Kentucky coffeetree, an otherwise healthy yellowwood, and several other specimens where old bark inclusions snapped.
Fortunately, bark inclusions are preventable. Branches growing at poor angles can be removed, preferably when the tree is young enough to quickly replace the missing limbs. Some trees are very prone to inclusions and must be pruned carefully when young; hybrid Freeman maples, ornamental pears, and yellowwoods are some of the commonest offenders. A significant part of our work involves identifying potential problems and correcting them before trees get large and fragile.
Structural pruning also involves removing competing limbs, low limbs (to make room for pedestrians and lawnmowers), and dead or diseased material. Much of this takes place in winter, and some trees—oaks and elms, subject in the growing season to oak wilt and Dutch elm disease—may only be trimmed while dormant. But limbing-up and dead/diseased limb removal takes place all year. This summer, we focused on cleaning out the crab apple collection. They are prone to pests and diseases, and dying limbs shelter pathogens. Crabs also benefit from enhanced light and airflow, particularly in such a humid summer. Many truckloads of limbs later, we think they look much happier.
Even our shrubs benefit from pruning. Some get hedged into shapes (by a loyal volunteer), some die back to the ground every winter, and some, like our panicle hydrangeas, get specific treatment to enhance flowering new growth. (Check them out—they are having a good summer!) A few shrubs get infestations that we prune out—our Rugosa rose collection saw a major outbreak of rose girdler this year. And most shrubs need periodic “renewal”—removal of the oldest, least vigorous stems.
It is hard work, but we love our pruning. It is one of the best ways to get to know the fantastic variety of plants we care for. All their quirks come into focus—their growth habits, colors, textures, and smells, and problems. I enjoy evaluating each plant and figuring out how to help it (if I can). I keep a collection of wood samples from the trees I work on, as an example of their diversity, and of the individual characters of each species. These images show a small selection of samples from this year’s pruning.
—Stephen Nystrand, Garden Technician
From top, left to right:
Malus baccata, Siberian crabapple
Taxodium distichum, bald cypress
Amelanchier laevis, serviceberry
Cotinus coggygira, smokebush
Tilia cordata, littleleaf linden
Tilia tomentosa, silver linden
Quercus petraea, sessile or durmast oak
Vibrunum lentago, nannyberry
Juglans mandshurica, Manchurian walnut
Carya ovata, shagbark hickory
Pyrus calleryana, Callery pear
Cladrastis kentuckea, yellowwood
Phellodendron amurense, Amur corktree
From top, left to right:
Quercus ellipsoidalis, Hill’s oak
Liriodendron tulipifera, tulip tree
Ulmus x Hollandica, Dutch elm
Pterocarya stenoptera, Chinese wingnut
Gymnocladus dioicus, Kentucky coffeetree
Quercus imbricaria, shingle oak
Crataegus sp., Hawthorn
Quercus muehlenbergii, chinkapin oak
Juniperus virginiana, red cedar
Juniperus chinensis, Chinese juniper
Ulmus wilsoniana, Wilson elm
Quercus prinus, chestnut oak