Restoring natives to suburbia

Gardeners enjoy their hobby for many reasons: a love of plants and nature, the satisfaction that comes from beautifying home and community, the pleasures of creative effort, the desire to collect rare or unusual species, and the healthful benefits of exercise and outdoor air. For some people, like my wife and me, there is pleasure in just watching plants grow.

But now, for the first time in its history, gardening has taken on a role that transcends the needs of the gardener. Like it or not, gardeners have become important players in the management of our nation’s wildlife. It is now within the power of individual gardeners to do something that we all dream of doing: to make a difference. In this case, the “difference” will be to the future of biodiversity, to the native plants and animals of North America and the ecosystems that sustain them.

For decades, many horticulture writers have been pleading for a fresh appreciation of our American flora, and for almost as long they have been largely (or entirely) ignored. For several reasons, however, the day of the native ornamental is drawing near; the message is finally beginning to be heard. If I were to ask a random group of gardeners to comment on the importance of native plants in their gardens, they would probably recount several arguments that have been made in recent years in favor of natives over alien ornamentals. They might describe the “sense of place” that is created by using plants that “belong” or the dangers of releasing yet another species of invasive alien to outcompete and smother native vegetation. They might recognize the costly wastefulness of lawns populated with alien grasses that demand high-nitrogen fertilizers, broad-leaf herbicides, and pollution-belching mowers. Or they might mention the imperative of rescuing endangered native plants from extinction. These are all well-documented reasons for the increasing popularity of growing native plants.

Owners of native nurseries are also finding it easier and easier to enumerate the benefits of their offerings. Native plants are well adapted to their particular ecological niche and so are often far less difficult to grow than species from other altitudes, latitudes, and habitats. After all, these plants evolved here and were growing just fine long before we laid our heavy hands on the landscape.

Most compelling to me, however, is the use of native species to create simplified vestiges of the ecosystems that once made this land such a rich source of life for its indigenous peoples and, later, for European colonists and their descendants. That most of our ecosystems are no longer rich is beyond debate, and today, most of the surviving remnants of the native flora that formed them have been finished off by development or invaded by alien plant species. Too many Oak Parks, Hickory Hills, and Fox Hollows—developments named, as the environmentalist Bill McKibben has noted, for the bit of nature they have just extirpated—have been built across the country. Although relatively small, strategically placed and connected patches of completely restored habitats might foster the survival of some of our wildlife, I will describe later why such habitat islands can only protect a tiny fraction of the species that once thrived in North America. With 300 million human souls already present in the United States and no national recognition of the limits of our land’s ability to support additional millions, we simply have not left enough intact habitat for most of our species to avoid extinction. All species need space in order to dodge the extinction bullet. So far we have not shared space very well with our fellow earthlings. In the following pages, I hope to convince you that, for our own good and certainly for the good of other species, we must do better. Native plants will play a disproportionately large role in our success.

The larva of Actias luna, the luna moth, is a beautiful member of the family Saturniidae that serves as an important source of food for birds, bats, and other creatures.

The transition from alien ornamentals to native species will require a profound change in our perception of the landscaping value of native ornamentals. Europeans first fell in love with the exotic beauty of plants that evolved on other continents when the great explorers returned home with beautiful species no one had ever seen before. It quickly became fashionable and a signal of wealth and high status to landscape with alien ornamentals that no one else had access to. As the first foreign ornamentals became more common in the landscape, the motivation to seek new alien species increased. Even today, the drive to obtain unique species or cultivars is a primary factor governing how we select plants for our landscapes.

My epiphany
Although I chose entomology as a profession, I understand the thrill of growing an exotic plant for the first time. When I was in graduate school at the University of Maryland, I took a course in woody landscape plants from the noted horticulturist Robert Baker. He introduced me to the world of ornamental horticulture and the many alien species with landscape value. I left that course with an intense desire to plant as many of the species I had just learned about as possible. The only thing that slowed me down a bit was that I had no place to plant them. Still, I gathered seeds from many of the ornamentals on the University of Maryland campus, germinated them in the greenhouse, and planted the seedlings all over the yards of my parents and relatives. Among other things, my parents got a Japanese hardy orange, and I bestowed the gift of Paulownia trees, of all things, on unsuspecting Uncle George. I now find it ironic that, at the same time Robert Baker was turning me on to alien ornamentals, I was taking courses about plant-insect interactions. These were the courses that explained why most insect herbivores can only eat plants with which they share an evolutionary history. All of the information I needed to realize that covering the land with alien plant species might not be such a good idea had been neatly and simultaneously placed in my lap during those months in graduate school, but it was 20 years before I made the connection: our native insects will not be able to survive on alien plant species.

In 2000, my wife and I moved to 10 acres in southeastern Pennsylvania. The area had been farmed for centuries before being subdivided and sold to people like us who wanted a quiet rural setting close to work. We got our rural setting—sort of—but it was anything but the slice of nature we were seeking. Like many “open spaces” in this country, at least 35 percent of the vegetation on our property (yes, I measured it) consisted of aggressive plant species from other continents that were rapidly replacing what native plants we did have. We quickly agreed to make it a family goal to rid the property of alien plants and to replace them with the forest species that had evolved within the eastern deciduous biome over many millions of years. This rather optimistic and, I admit, peculiar use of our spare time has put us in intimate contact with the plants on our property, both alien and native, and with the wildlife that depends upon those plants.
 

Alien plants like Bradford pear (A) and autumn olive (B) are avoided by native insects, while native plant species like black cherry (C) and red maple (D) are good food sources for native insect species.

Early on in my assault on the aliens in our yard, I noticed a rather striking pattern. The alien plants that were taking over the land—the multiflora roses, the autumn olives, the oriental bittersweets, the Japanese honeysuckles, the Bradford pears, the Norway maples, and the mile-a-minute weeds—all had very little or no leaf damage from insects, while the red maples, black and pin oaks, black cherries, black gums, black walnuts, and black willows had obviously supplied many insects with food. This was alarming because it suggested a consequence of the alien invasion occurring all over North America that neither I—nor anyone else, I discovered, after checking the scientific literature—had considered. If our native insect fauna cannot, or will not, use alien plants for food, then insect populations in areas with many alien plants will be smaller than insect populations in areas with all natives. This may sound like a gardener’s dream: a land without insects! But because so many animals depend partially or entirely on insect protein for food, a land without insects is a land without most forms of higher life (Wilson 1987). Even the most incorrigible antienvironmentalist would be hard pressed to make an attractive case for such sterility. Pure anthropocentrists should be alarmed as well, since the terrestrial ecosystems on which we humans all depend for our own continued existence would cease to function without our six-legged friends.

But does the pattern of leaf damage I noticed in my backyard hold true? Does it occur elsewhere? If alien plants do reduce insect populations, by how much do they do so? Do aliens exclude all insect herbivores or just some? Are all alien plant species equally harmful to insects? And is the predicted effect on higher levels of the food web as serious as I’ve suggested? My colleagues and I have started the large, controlled research projects needed to address these important questions, and the data are starting to accumulate. So far, the results provide exciting support for gardeners who have already switched to natives or who are enthusiastic about doing so. If my concern for the fate of our insect herbivores turns out to be justified, these gardeners can and will “change the world” by changing what food is available for their local wildlife.

My argument for using native plant species moves beyond debatable values and ethics into the world of scientific fact. We can no longer hope to coexist with other animals if we continue to wage war on their homes and food supplies. This simple tenet provides an imperative, particularly for the bird and butterfly lovers among us, to fight invasive aliens as if it really matters and to reevaluate our centuries-old love affair with alien ornamentals. Beyond providing a challenge to ecologically minded gardeners, I will also explain how gardening with natives can create plantings that will stay beautiful and in balance without the use of pesticides. Gardening with natives is no longer just a peripheral option favored by vegetarians and erstwhile hippies. It is an important part of a paradigm shift in our shaky relationship with the planet that sustains us—one that mainstream gardeners can no longer afford to ignore.