Gardeners in tune with nature appreciate the importance of native plants in sustaining the garden food web. We know, for example, that caterpillars of numerous butterfly and moth species (members of the insect order Lepidoptera) feed on the leaves of native trees, shrubs, and herbaceous plants and that these caterpillars are a primary source of protein for birds.
In his popular book on how to sustain wildlife in our gardens, Bringing Nature Home (Timber Press, 2007), entomologist Douglas Tallamy ranks our native trees by their ability to support lepidopteran species. Highest ranked tree species include oaks (517 lepidopteran species supported), willows (456 species), cherries (448 species), birches (413 species), poplars (368 species), and crabapples (311 species).
Tallamy’s research, as well as the research of many other scientists, is conclusive: native insect herbivores depend for food on the leaves of trees with which they co-evolved. You can see this for yourself by comparing the appearance of leaves of the native red maple (Acer rubrum) at the end of the growing season with those of the non-native Norway maple (A. platanoides), a widely-planted non-native invasive tree. In late September, the red maple leaves are frayed along the edgers and riddled with holes – literally moth-eaten – while the Norway maple leaves remain relatively undamaged. Knowing this, nurseries and garden centers market Norway maples as “pest-free” trees, assuming this to be an attribute gardeners should appreciate. They are correct, of course, nothing eats the leaves of a Norway maple. But do you really want a tree in your garden that has no place in the garden’s food web? A garden of plants that do not harbor herbivores is a garden devoid of bird life.
For sure, the backbone of a garden in tune with nature should consists largely of native trees and shrubs. Largely, but perhaps not entirely. Equally important in the garden food web are the pollinators, including bumble bees, native solitary bees, and certain species of moths and butterflies, flower beetles, flies, and wasps. Unlike the plant-eating insects which are dependent on native plants, these nectar and pollen feeders will forage for nectar and pollen on the blossoms of some non-native plant species.
Nectar is almost entirely a mixture of sugar and water with only subtle differences between the nectar of native plant species and that of non-native plant species. Pollinators are more interested in the shapes of flowers and the amount of nectar in each blossom. Quoting Tallamy, “Each species of native bee evolved to forage in flowers with particular morphologies. Bumblebees, for example, have long “tongues” that can reach nectar pools at the base of flowers with long corollas, while sweat bees have relatively short tongues. This is why sweat bees spend most of their time foraging for nectar on flowers like composites, where nectar is within easy reach, while bumblebees go for nectar in flowers with long necks where the nectar is protected from the short-tongued bees.”
In Marjorie’s Garden, a good example of an ecologically-functional non-native small tree (or large shrub, depending on how it is pruned) is the red-veined enkianthus (Enkianthus campanulatus), native to Japan. A member of the Ericaceae, the plant family of blueberries, rhododendrons, heathers, and many other garden-worthy trees and shrubs, redvein enkianthus acquired its common name from the red veins in the petals of each flower. In all other respects, the flowers of this non-native plant are similar in shape and size to those of highbush blueberry and are borne in similar pendulous clusters.
This past Monday, I spent a mid-day hour beneath the flowering branches of this tree, monitoring the insects that were seeking pollen and nectar from its blossoms. In the course of that hour, I recorded the following: several bumble bees of at least two species, a single wasp poking its head in and out of the flowers, several honeybees, one flower long-horned beetle harvesting pollen, one unidentified species of solitary bee so small that its entire body fit inside a flower, and one pollinating insect that still remains a mystery. In addition to these pollinators, I also photographed a ladybird beetle in the process of laying a cluster of eggs on the underside of a leaf.
(The ladybird beetle surprised me. Both she and the larvae that will hatch from the yellow-orange eggs prey on soft-bodied herbivores, including aphids and whiteflies. Will the larvae find these herbivores on a non-native tree? Is the beetle also non-native, perhaps a species from the region of the world where enkianthus grows wild? So many questions, and never enough time!)
Another ecologically functional non-native tree in Marjorie’s Garden is the
Sargent crabapple (Malus sargentii), another native of Japan. Ours was in profuse bloom over the past two weeks, attracting a host of pollinators including the same species of flower long-horned beetle found on the enkianthus, two species of hoverfly, and a still unidentified solitary bee.
Without a doubt, native tree species are essential to a garden in tune with nature. In late September, when we chance to look at the still green leaves of our oaks, maples, cherries, and birches, finding many of them riddled with holes and tattered edges, we should take great satisfaction in the diversity of lepidopteran life nourished by these trees, smiling as we imagine the channeling of solar energy from leaf to caterpillar to bird, the trees themselves no worse for the wear.
When it comes to choosing the flowering trees and shrubs for our gardens, what counts is how effectively their flowers will nourish pollinators as well as the beneficial predatory insects, such as hoverflies, that are nectar and pollen feeders as adults. This does not mean that we have to settle for less in terms of ornament, as evidenced by the two examples discussed above. Non-native trees and shrubs, those known to be non-invasive, can be significant contributors in the ecologically functional garden.