It is the first day of spring as I begin writing this column while three feet of ice-impacted snow cover the garden. From a window I can see the blanketed roof of a neighbor’s house, only 500 feet away, the expanse of white separated by the dark boles of pines and spruce that stand between us, trees with crowns towering a hundred feet above the fallen limbs of winter storms. Two crows sit quietly in the top of the tallest pine, looking down to the Union River and beyond to Union River Bay and on to the Atlantic.
The dark green canopies provide a foil for the trunks and leafless branches of deciduous trees we have planted over the years: a red oak now old enough to bear acorns in abundance, two bird cherries that were mere saplings when I first met them, a Japanese katsura planted where one day it will cast dense shade over a corner of the vegetable garden, two mountain maples, one fringe tree, a multi-trunked enkianthus, a peach tree spreading horizontal branches over an island perennial bed, a serviceberry, a Sargent crabapple, and the ‘Donald Wyman’ crabapple that I planted last spring, its one-inch trunk now half buried in snow.
Downstairs, a kitchen window frames three old yellow birches with catkin-laden branches, their exfoliating honey-colored bark shining in the sunlight. A ruffed grouse hen has recently taken to perching on the high limbs of these trees, nipping at the catkins. Down the drive, birch saplings, both yellow and paperbark, grow in sunlight dappled by an old white pine.
It is hard for me to envision a garden that is not surrounded by tall canopy trees. Wherever I have gardened in this country, from the coastal plain of South Carolina to the shadow of Mt. Rainier in western Washington to the Union River watershed of mid-coast Maine, tall trees have dominated the near landscape.
As a consequence of this relationship with trees, both deciduous and evergreen, my life has been enriched by many different plant and animal species, from the vines, mosses, and lichens that live on those trees to the birds, squirrels, raccoons, spiders, insects, and other creatures that nest and feed in them. In more recent years I have shifted my focus from gardening with these creatures to gardening for them, particularly the insects, planting the garden with trees that support insect pollinators, insect predators, and insect herbivores. Such trees can be called functional garden trees.
Pollinators and predators are often lumped together under the label “beneficial insects”, leaving the herbivores, by default, characterized as “pests”. This distinction is one that Marjorie and I are working hard to eliminate, beginning by eliminating the word “pest” from our vocabularies. I explain to my readers and listeners that all insects, including the herbivores, are beneficial, that all are instrumental in sustaining the complex garden food web characteristic of a healthy garden ecosystem. I explain that many insect species spend part of their life cycle as herbivore and part as pollinator or predator, thus blurring distinctions between the old labels.
A high diversity of functional canopy trees in and around the garden will support a wide variety of insect species, most of which are specialist herbivores that feed only on one or a small group of related tree species. Many of these insect herbivores, including the adult and larval stages of moths, butterflies, and sawflies, will in turn be eaten by spiders and other arachnids; insect predators like the lady bird beetle; parasitoids such as tachinid flies; frogs, toads, and other amphibians; lizards and garter snakes; rodents; insect-eating mammals, including foxes and black bears, both of which obtain 25% of their nutritional requirements from insects; and 90 percent of terrestrial bird species.
Let’s take a closer look at just one species in many garden food webs: the chickadee. This tiny bird, a bubble of air covered with feathers and weighing only one-third of an ounce, depends on caterpillars to rear its young. While an adult chickadee may be seen capturing a variety of insects, between 95 and 100 percent of insects fed to its young are caterpillars. To raise one clutch of chickadees requires between 390 and 570 caterpillars per day for a total of between 6,000 and 10,000 caterpillars per brood, numbers that are truly mind boggling.
Now consider the number of insects consumed by a single red-bellied woodpecker, a bird that weighs three ounces, nine times the mass of a chickadee. Try to imagine the number of insects collectively eaten by titmice, orioles, bluebirds, wood thrushes, robins, and other insect-eating birds in your garden.
Most insect herbivores are diet specialists, consuming only certain parts of specific plant species. The female moth will lay her eggs only on those specific plants and the larvae that hatch from those eggs will in turn be eaten by specific bird species. Thus the greater the diversity of canopy trees and other larval host plants in the garden, the greater the diversity of birds and other animal species that will live there.
When Marjorie and I decide to add a new tree to our garden, the prevailing characteristic is that it must be functional, serving as a larval host plant for moths and butterflies, providing pollen and nectar for pollinators and other beneficial insects, producing fruits for birds and other wildlife, or a combination of these attributes. Most trees that meet these requirements would be regionally native species, trees that have evolved relationships with regional wildlife species, but there are exceptions. With the exception of Malus angustifolia, a species native to the southeastern coastal plain, any crabapple that we might grow would be non-native and yet highly functional, supporting over 300 species of northeastern Lepidoptera, many of which feed only on Malus, as well as producing fruits relished by many species of birds.
Another exception to “natives only” in our garden is the red-vein enkianthus, Enkianthus campanulatus, a small multi-stemmed tree native to Japan and growing to only 10 feet in height. It is a member of the Ericaceae, the family to which blueberries belong, and its flowers are much like those of blueberry, bell-shaped and pendulous. Bumble bees and other native bees are primary pollinators of these flowers.
Of all possible tree species for the garden, which are the most functional? Douglas Tallamy, in his popular book “Bringing Nature Home: How Native Plants Sustain Wildlife in our Gardens” (2007, Timber Press), tells us that native oaks top the list in terms of supporting lepidopteran larvae; 534 species of butterflies and moths depend on the different species of oaks for food. Turn over an oak leaf, find a caterpillar, and you understand what bird food looks like.
In addition to the larvae of moths and butterflies, including hairstreak butterflies and skippers, red oaks (Quercus rubra) support gall wasps, leaf beetles, acorn weevils, aphids, leafhoppers, treehoppers, and plant bugs. This diversity of herbivores attracted to the red oak will in turn bring a diversity of predatory insects and insect-eating birds into the garden to help control herbivores found on vegetable plants, small fruits, and ornamentals.
During nesting season, when songbird requirements for insect protein reach a peak, I have noticed goldfinches and purple finches leave the porch sunflower feeders to feed on insects in the oak. I also see them in the vegetable garden where, along with warblers and hummingbirds, they work to keep herbivore populations at a minimum.
Willows rank second in the list of functional trees, supporting 456 lepidopteran species, followed by cherries (456 species), birches (413 species), poplars and cottonwoods (368 species), and maples (285 species), all of which can be found in or near our garden. The presence of these and other key tree species account for the high diversity of insects, particularly caterpillars, that inhabit our garden and the equally high diversity of bird life.
Gardening for wildness brings a change in how you view the garden. I look at the red oak in Marjorie’s Garden and think of the red squirrels scampering among the high
limbs, dropping acorns to the ground. When I look at the wide-spreading branches of the old yellow birches, I see the plump grouse hen. In late summer, when the leaves of garden trees are tattered and torn by the chewing and rasping of countess caterpillars, I smile with the assurance that all is well.