We gardeners are a hopeful, well-intentioned lot, digging in to spread beauty and fruitful bounty far and wide. For most of us, the landscapes we tend are constrained by the invisible property lines that define our homes. Rarely do we have the opportunity to consider the individual and collective effects of our gardens on the larger landscape. While no gardener or landscape professional would ever intentionally harm the environment, no garden is an island. Now, as our human footprint expands ever outwards, choices made by gardeners are more important than ever.
Experienced gardeners know that they must choose plants that will do well in the soil, moisture, and light available, but beyond that, personal preferences will determine plant selection. Plant origin (indigenous versus nonindigenous) has rarely been a consideration. Gradually, however, an awareness of the myriad benefits provided by indigenous plants is growing. For the first time in the history of gardening, a growing number of scientists, landscape architects, and nursery and other “green-industry” professionals are making the case, based on an accumulation of scientific studies, that plant origin is, in fact, a very important point to consider.
When Frederick Law Olmsted designed New York City’s famous Central Park in 1858, he made extensive use of nonindigenous Norway maples (Acer platanoides), never guessing that this species would aggressively invade and change surrounding wild forests. Recently, however, scientists studying the spread of Norway maple into regional forests have discovered that this tree’s trait of leafing out especially early prevents adequate light from reaching many species of spring wildflowers, making it impossible for them to reproduce. In addition, there is less food available for baby birds in forests overrun by Norway maples. Baby birds depend on a steady diet of insect larvae (caterpillars) that eat forest tree leaves. Because the leaf chemistry of Norway maples is completely different from that of sugar maples and other indigenous maples (Acer spp.), its leaves are unpalatable and cannot be eaten by insect larvae. Unlike indigenous forest trees, Norway maples cannot provide a source of food for either caterpillars or the baby birds that eat them.
The indigenous (or native) plants of the Northeast evolved during the advance and retreat of glaciers and before the arrival of European colonists. The fortunate fact that our mountains range from north to south allowed for many Northeastern plants, along with wildlife, to migrate south to escape the encroaching glaciers and back again as the glaciers receded. It is this shared history of migration and evolution that underlies the key role of indigenous plants as the foundation of the food web. Over time and space, as plants and animals evolved together, they became interdependent in ways we are just beginning to understand. Indigenous plants are always the best, and, in most cases the only, tolerable source of food for indigenous insects. Many of our butterflies, moths, bees, and other pollinators are host specific. In other words, at the larval or caterpillar stage of their life cycle, these insects are utterly dependent on a very few groups of plant species for food. Such plants are referred to as host plants. Some caterpillars are limited to only a single species of host plant, for example, those of the endangered Karner Blue butterfly that can only eat a single species of lupine, Eastern lupine (Lupinus perennis).
Let’s look at how these interdependent plants and wildlife contribute to the food web in a healthy, deciduous forest. While trees are still bare, sunlight wakens the spring ephemerals, flowers that transform and carpet the forest floor. In a burst of activity, they complete their reproductive cycle before trees have even finished leafing out. These first flowers attract and provide nectar for newly aroused flies and bees and other insects. Soon the migratory birds come to nest and reproduce, many of them dependent on the insects that feed on the abundant flowers. As the leaves gradually unfold, caterpillars and other insect larvae are coming out of winter shelter to feast, just in time to provide food for newly hatched baby birds. In autumn, the reproductive cycle comes, literally, to fruition, as migrating birds devour the abundant berries produced by trees, shrubs, and vines of the forest, spreading seeds far and wide. The timing of these events – flowering, leafing out, fruiting, feeding, and nesting – is exquisitely synchronized in a complex dance, worked out over millenia.
These healthy ecological processes are self-perpetuating in the absence of, and sometimes in spite of, human influence. Plant communities are adapted to natural disturbances such as storms, fire, or drought, which may delay or temporarily suspend some processes, but rarely cause long-term damage. Introduction of nonindigenous plants, insects, and blights, on the other hand, disrupts ecological processes on a continuous basis long after the initial disturbance, as these biological organisms have their own self-perpetuating mechanisms, much as cancer behaves in the human body.
In times past, natural systems were the dominant forces on the landscape. Even a large fire or hurricane occurred within the natural matrix; damage was contained and surrounded by the intrinsic elements that would repair the damage. With the rise of human-dominated landscapes, the reverse is true. When bulldozers level 500 acres for a shopping mall or open up swales along a roadside, the area devoid of vegetation is an open wound inviting invasive plant seeds to act like infectious agents.
The most successful nonindigenous invaders exhibit reproductive traits that allow rapid colonization of disturbed areas. For example, many of them produce an overabundance of seeds and can eventually overwhelm indigenous seed sources. The main advantage, however, seems to be that because nonindigenous plants and other organisms did not co-evolve with our flora and fauna, they enjoy immunity from indigenous pests and diseases. They left behind the pests and diseases that would keep them in check in their place of origin.
In light of these facts, what’s a caring gardener to do?
Virtually every American garden already contains plenty of nonindigenous species. Knowing the consequences of their overuse, carefully weigh every decision to introduce more of them. Use nonindigenous plants in the garden as you might use a particularly pungent spice when cooking, that is, sparingly. Consider the functions and ornamental values of the nonindigenous plants you might want to purchase and look for indigenous species that can perform in similar fashion. The ultimate landscape goal should be a stunningly beautiful garden with an abundance of indigenous plants, along with an occasional nonindigenous plant that carries sentimental or historical value to the gardener: the opposite of what we now have.
The goal when using nonindigenous plants is to eliminate uncontrolled reproduction, which, as we now know, can have negative consequences. The motto of the responsible gardener, with regard to nonindigenous plants, should be “What grows in my garden, stays in my garden.” Knowing that gardeners occasionally will find certain nonindigenous plants irresistible, “safe sex” principles are listed below to guide more responsible choices and reduce the chance that any nonindigenous plants selected will cause significant problems.
Principles of “Safe Sex” in the Garden
Avoid nonindigenous trees; most of the trees we plant will outlive us; choose indigenous trees to support hundreds of species of insects over many lifetimes.
Avoid the latest plant introductions from other regions and continents.
Avoid using plants identified as invasive species.
Avoid plants with seeds that are wind-dispersed
Avoid plants that reproduce vegetatively by means of vines or underground runners.
Avoid plants that produce berries you won’t eat.
Use only male dioecious (ex. hollies and gingkos) plants.
Use annuals; avoid those that self-sow.
Use heirloom plants, such as peonies and lilacs, not known for causing problems.
Use nonindigenous plants that are sterile.
Use only a single specimen, as an accent only, if possible.
Use a single cultivar per species; most are clones and without cross-pollination cannot reproduce.
When in doubt, deadhead.