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Chapter 8: Editing a Mature Woodland: Shaping Change
by Susan E. Dumaine
Change is the essence of a woodland garden. Gardening in the shade requires us to deal with this change and become nature's editor.
For more than 30 years, I've been editing a woodland in Zone 6A of New England, between granite ledges left behind by the last trans-Atlantic collision of tectonic plates. Shallow, acidic soils, usually rock packed but sometimes soft and floury, were deposited between these ledges by the glacial ice pack some ten thousand years ago. Until the early years of this century, this rough terrain served as the woodlot of a large farm in the adjacent glacial lake bed. Throughout New England, the end of small-scale agriculture has brought swift reforestation, and what we found on our two-acre property typifies the pattern of species distribution here. Although the species we encountered are particular to our region, the ongoing process we've employed in editing our woodland can be followed by gardeners across the country.
Editing the Canopy
The first step is observation. We started by taking inventory of the tree species on our site, noting the pattern of distribution as well as the effect of site conditions on growth habit. Back in 1961, we could see five mature, full crowned red oaks from the house. These stood in more open areas. Numerous younger red and white oaks dominated the ravine between the ledges. A few of the red oaks exhibited multiple trunks, evidence that they had once been cut. On the slopes, small groves of hemlock and beech occurred, with a few random examples of young hickory and red maple. Three chestnut sprouts alternately rose and fell to blight; one linden struggled to grow upright. A white pine was then and is now the tallest tree in the valley, progenitor of others that add feathery texture to the skyline. On and near the ledges in front of the house were two large white ash; in the valley, a few ash saplings.
Next comes editing out. In opening up the paths into and through the valley, my philosophy was to be as gentle with the existing canopy as possible, to exploit its virtues and to leave enough well-placed, well-shaped saplings for future canopy. I respected certain features of compelling interest-among them the existing groves and multi-trunked trees, as well as the hickories and oaks. I removed misshapen or redundant maples, beech and ash, which have shallower roots and are more difficult to garden beneath.
Over the years, nature has not entirely respected my philosophy, and remains the senior editor. Several majestic white oaks simply died, perhaps weakened by repeated gypsy moth defoliation. Five mature red oaks have succumbed to heart rot. Surprisingly, these dramatic, emotionally wrenching casualties have caused little obvious damage to other woody material. However, there has been a cost to the fabric of the woodland as a whole, for the meshing of both the canopy and root systems that once provided mutual support in high winds has been compromised.
When needed, we call in an arborist to remove broken branches in the canopy, or to limb up a tree to let in more light. Done in the winter, there is less debris and the cost is usually less. Done in the summer, it is possible to pinpoint what needs to be removed with great accuracy. On our property, as change admitted more light, saplings of ash and red maple grew strongly, and Norway maples, even faster growing, began to invade. I make yearly efforts to eradicate the weedy Norway maples, which are as fecund and undesirable as Norway rats, but the red maples are shapely and well placed. Ash decline, a newly recognized pestilence, is eliminating the ash one by one.
The Understory
In a woodland, understory trees and shrubs serve as visually important links between the canopy and herbaceous layers, helping to define spaces and create passages. Our understory had significant diversity, but was dominated by three distinct groves of arching white dogwood. Other dogwoods—pagoda, gray and blue—were also present. Three witchhazel spread their canopies on the south-facing slope, and in a seasonally wet area, two large spicebush brightened the early woodland with a veil of tiny sulphury flowers. Eight highbush blueberry grew in these moister spots as well, while lowbush blueberry scrambled over dry ledges. Here and there along the valley floor wandered the suckering, warty stems of elderberry. Maple-leaved viburnum grew over the rocky deposits in one end of the valley, pleasant with blue fall fruit and rosy leaves. In moist or dry spots, arrowwood rooted casually from supine branch tips. The blackhaw viburnum, more treelike, suckered into a thick grove, handsome in its fall fruit and foliage. A few young sassafras showed a similar tendency to grove.
Though the valley and its slopes were totally devoid of evergreen understory plants, there was a lot to respect and utilize. Each of the species we found remains somewhere in the garden, either because of its inherent interest or to reflect the habitat conditions. We have removed most of the arrowwood and sassafras, either because of poor form, poor flowering or simply excessive numbers. We also added the evergreens needed to give structure and year-round interest to the island compositions. We looked for variety in size, leaf form and texture in addition to flowers, and the evergreens now include hollies, rhododendrons, cherry laurels, andromeda, boxwood, laurel, yews, dwarf hemlocks, mahonia, leucothoe and viburnum. An unanticipated loss is the white flowering dogwood, which is declining in health and vigor all over the Northeast. In ten years, I have found neither the courage to completely remove it, nor anything to adequately replace it.
Editing by Pruning
Editing by pruning is the key to maintaining the longterm vigor and aesthetic appeal of shrubs. This is true for evergreen woodlanders like carolina, catawba and rosebay rhododendrons as well as for deciduous shrubs like azaleas, viburnums or blueberry. At a minimum, some of the oldest stems need to be cut out regularly as the shrubs reach maturity. This encourages the development of young shoots and allows the shrubs to function as small trees, arching over pathways and framing views. If a truly fresh start is needed, they can usually be cut back to the ground, either just after flowering or in late winter. Either approach will direct the shrub's stored energy into the production of juvenile wood.
In a technique called arborizing, seriously overgrown, lanky rhododendrons and other shrubs can be pruned out so that their stems can be appreciated like tree trunks while their foliage encloses a space overhead. By emphasizing the vertical, many shrubs that have grown too wide for their location can be retained.
Curbing the Aggressors
Finally, you need to study the ground layer and decide what to keep, what to remove and what to add. Compared to the diversity of the woody material, the herbaceous flora in my woodland was depauperate, consisting of species able to tolerate considerable competition from roots and fallen leaves and endure our typically dry summers without supplemental water. Evergreen whorls of marginal shield fern hung from crevices in the ledges. Elsewhere, stoloniferous asters, sarsaparilla, wild oats, hay-scented fern and false Solomon's-seal, aggressors all, covered large sweeps. I still allow them generous space, but stand ready to smother them with mulch-topped strips of old carpet when necessary.
Some of the plants I introduced in making the garden have been overly successful, and I have had to cope with their excess. Plants set out for quick cover—such as golden archangel and even my favorite evergreen groundcover, creeping phlox—do just that, and then don't know how to stop. Grouped for texture, groundcovers such as barren strawberry, foamflower and ginger eventually run into each other, and need to be separated and replanted. Charmers such as celandine poppy and wild bleeding heart, which delight us with their extended flowering period, overwhelm with their numbers in time.
In a woodland garden where water is delivered as needed, an airy mulch maintained and the soil reworked from time to time, almost every plant, from meadow rues to bugbanes, will sooner or later seed about. Before too long, editing becomes central to the garden's maintenance, and the best red pen is a fearlessly used compost pile.
Susan E. Dumaine is a teacher and lecturer for the New England Wild Flower Society, the Arnold Arboretum and the Massachusetts Horticultural Society. She propagates a range of plants and runs Horticultural Thoughts, a garden consultation and design service.