Environmental Commentary by Tom Horton
Doug Tallamy, noted champion of native plants, won’t tell homeowners to never plant a crape myrtle or two. But he wants them to grasp that the lovely, low-maintenance Asian import is “biologically inert, a beautiful statue. So ask yourself, how many statues do I need in my yard?”
On the other end of the spectrum, if you had room to plant only one more species, he’d fairly shout, oak!
Tallamy, professor of entomology and wildlife ecology at University of Delaware, has painstakingly been assembling the science for favoring native species over exotics—even over examples of the latter that came here long ago.
He starts by defining the primary duty of a modern yard: to efficiently pass solar energy, the sunlight falling there, upward and onward throughout nature’s food web.
Most any leaf or lawn performs the first act, photosynthesis; but what passes on the energy from there? That’s where grass and exotic ornamentals falter and natives rule.
The natives get chewed by insects, which in turn feed other bugs and spiders—all these ultimately translating sunlight into birds. ‘Pest free,’ an endearing term to most, actually means a plant’s not doing its job of feeding the ecosystem, Tallamy says.
And maybe there was a time it didn’t matter as much; but humans have disrupted natural habitats profoundly and widely. Now, every backyard counts if we’re to preserve more than a glimmer of our original heritage of plants and wildlife, Tallamy argues in “Bringing Nature Home—how you can sustain wildlife with native plants” (Timber Press, 2005)
We’ve got a ways to go. His studies of typical suburban yards across Maryland, Pennsylvania and Delaware find they are about 92 percent lawn and 80 percent alien plants.
These non-natives were often selected for being relatively unattractive to insects in their native habitats. Compounding the problem Tallamy says, leaf chemistry turns out to vary hugely among species of native plants. This means most plant-eating insects have evolved to specialize—only about 10 percent are able to eat a wide variety of plants.
This co-evolution of plant eaters and plants is a bond not easily broken. Tallamy shows that introduced plants like clematis, even after a century, still host about a 40th of the insects as in their native countries. For eucalyptus it’s close to one 50th; for phragmites, about a 50th—after three centuries.
Some gardeners will note that our native birds eat the berries of both native and non-native plants. But berries occur after the critical nesting season for most birds—good fuel for migration; but what if there’s no one to migrate?
It is insects, including the arthropods like spiders, that provide the fat and protein essential to hatching young for more than 90 percent of birds.
Similarly, pollinators like the bees can use non-native plants, but our native bees (honeybees are imports) seem to do better with native plants. Butterflies are more adaptable to alien flowers, although monarchs depend absolutely on native milkweed.
So: Having decided to renounce some lawn, and to downsize your big buy of crape myrtles, what should you plant to maximize your food web, to most productively translate sunlight into birdlife?
For our region, Tallamy’s done some of the best research we have, using moth and butterfly larvae as surrogates to relate native species to insect biomass.
Oaks top the list, with 534 species supported. Willows, cherries, and plums are close behind, with 456 species (not the popular and alien weeping willow, however). Other good ones are birches, crabapples, blueberries, maples, elms, pines, hickories, hawthorns, spruce, walnuts, beech and lindens.
That’s a lot of choice—more than 80 varieties of oak alone. And don’t think you need space for the Wye Oak. There are smaller varieties like dwarf chinquapin oak and turkey oak; and closely planted, oaks like any tree will grow up, not wide. Their roots go deep so they won’t dig up sidewalks or foundations.
Even with native plants, a complex yard is best. Avoid monocultures. Diversity creates a stable food web that includes critters to eat the critters that eat your plants. Birds alone do an immense job of keeping plants from being overeaten. Tallamy says it takes 6,000–9,000 caterpillars to feed one nest of chickadees.
Are the many cultivars of native plants available now kosher? It is his most-asked question Tallamy said. The answer is ‘mostly.’ If it’s a variation that changes green leaves to red, maybe not; but if it’s just to make a species taller or shorter, probably OK.
The old answer to why we’re losing species diversity was ‘loss of habitat.’ It’s also the habitat we’ve been planting all over our yards, the ‘pest-free’ alien ornamentals.
We have to rethink the yard. Pest free is dysfunctional; ornamental is ecologically pretty as well as photogenic.
Tom Horton has written extensively about the Chesapeake Bay and the environment. Distributed by Bay Journal News Service.