Today gardeners enjoy an unprecedented wealth of plants to choose from in their quest for the perfect landscape. Recently some gardeners have chosen sides and express the opinion that native plants are better plants.
In the sometimes heated discussion about native plants in the landscape, it is important to define terms. The very definition of what is "native" is elusive and is not always agreed upon.
What are the problems and benefits associated with both native and exotic species? What role should the site play in the decision about what to plant?
What is native?
How long must a plant species have inhabited a region in order for us to consider it native to that place -- two hundred years, since colonization, since before agriculture began?
For example, for those who consider two hundred years sufficient, Queen Anne's lace would be native -- yet we know from historical records that this plant has a European origin.
We generally depend on local floras, inventories of the uncultivated plant life of a given region, to tell us which plants are native. However, these inventories are sometimes flawed and are subject to continual debate. Only fossil records can prove that a plant evolved in a certain place, and even these can be misinterpreted.
If we arbitrarily pick a point in time and say "plants in this place before this date are native," we may not be acknowledging that for centuries, indigenous peoples, traders, explorers, and botanists have had an impact on regional floras with their activities.
Geopolitical and ecological boundaries also play a role in defining native plants. To say a plant is "native to North America" or "native to New York State" implies that it is suitable for growth throughout North America or New York State, when in fact it may only occur naturally in limited microclimates or regions and thus only be suitable for growth in equally limited landscape situations.
Ecotypes are plants of the same species that are found in different habitats and have evolved specific adaptations to their differing environments. Red maple, for example, is native from Florida to Canada, but populations have adapted to dry or wet sites, cold or warm climates. Although technically red maple is native to a large section of North America, seed harvested from one ecotype will not necessarily perform successfully in another site because it is not adapted to the new site's conditions.
The greater the geographic range of the species, the more opportunity there is for variation. For example, even though red maple is native to both regions, a red maple ecotype from the southeast US will be ill-prepared for planting in the Northeast because it has adapted to a climate and to site conditions that are utterly different.
What is exotic?
Exotic plants, also known as non-native, introduced, or alien plants, are species that occur in cultivation or in the wild. These plants were transported across boundaries by people and their activities.
According to The Flora of North America, one-fifth to one-third of the plant species encountered north of Mexico have their origins in other continents. Many exotic plant introductions, such as lily-of-the-valley, daylilies, and daffodils, have become naturalized, meaning that they have succeeded in reproducing and spreading to a limited extent on their own.
Unlike invasive plants, however, most naturalized plants are not a severe threat to other species or to an ecosystem. In fact, the ability to naturalize is often considered a desirable characteristic in horticulture. Wildflowers can be true natives or introduced plants that have been naturalized for over two hundred years.
A small percentage of naturalized exotic plants become invasive. Invasive plants are those that reproduce quickly, displace many of the other species in their domain, and are difficult to eradicate. Purple loosestrife in the northern US and kudzu in the southern states are classic examples of invasive plants that profoundly affect the landscape.
Invasiveness can range from the minor nuisance of garden plants, such as Lamb's ears, that tend to edge out their neighbors, to the other end of the spectrum, where the melaleuca tree from Australia is literally drying out Florida's marshes to meet its high water requirements. Melaleuca is an example of an exotic species that is considered a serious invader because it enters disturbed lands, colonizes them and changes the ecology of them in detrimental ways.
Plant Selection: Native or Exotic?
One of the main reasons people promote native plants is to avoid the devastation that invasive plants may bring on the landscape and forest. Exotic introductions that do become invasive, like kudzu, multiflora rose, and honeysuckle are a costly menace nearly impossible to control, much less eradicate.
Why do a small percentage of plants exhibit invasive tendencies, while the majority of plant introductions are benign or beneficial? The answer lies in the combination of two factors: traits that invasive plant species share and traits of the site that make it susceptible to invasion. No plant is inherently invasive under all circumstances!
Although more often it is exotic introductions that invade, native plants can also become invasive pests. Native grape vines like fox grape form suffocating thickets over shrubs and rapidly climb trees, threatening to out-compete their hosts for light. Though native to eastern and central North America, wild grape is an indisputable pest. Other native plants that are often invasive include blackberries, poison ivy, wild onions and cattails. Just as with exotic introductions, it's a small percentage of native species that cause problems.
Native plants have an important role to play in modern landscaping. Arguments that are made in favor of native plants include lower maintenance, regional uniqueness, biological diversity, and wildlife habitat. One theory holds that native plants are easier to care for because they have evolved in a place over many years, developing resistance to climatic extremes, insect feeding, disease pathogens and other stresses of the local environment that a non-native might not be prepared for.
This may be true in some cases; however, it's important to note that native plants placed under stressful conditions fare no better than exotic ones if the plant is not carefully matched to the site. Also, some exotic plants actually perform better and require less maintenance because of the qualities they were selected for, and because their insect predators and disease pathogens are frequently not imported with them.
Another factor to consider is the interaction of native plants with the "built," or non-native environment. In an urban setting, for example, there is no planting site that approximates what would have been there prior to urbanization. The original landscape in both cities and suburbs often has been remade so completely that the microclimate, soil type, soil hydrology, and insect populations no longer are what they were when the native plants of the area evolved.
To put a native tree, for example, on a median strip planting on a downtown street because it is native to the surrounding countryside would be foolhardy unless the tree is known to tolerate the heat of the asphalt and car exhaust, the salt from the snowplows, a limited root zone, intermittent flooding, and periodic drought.
Not an Either/Or Proposition
If native plants are used simply because they are native, without proper regard to site conditions, the results may be unsatisfactory. The most critical issue is not native vs. exotic -- it is appropriate vs. inappropriate plant selection, given the constraints and opportunities of the site in mind. The more closely a plant's characteristics match the site's, the better chance for its survival and vigor. If a native plant meets those requirements, by all means use it!
While we can and should strive to use plants long found in our region, perhaps the term native plants should be used with some humility. Using diverse plantings will create beauty and prevent the susceptibility to disease that can come from large scale single-species plantings, avoiding disasters such as that of the American elm in the mid-twentieth century.
In the quest for a diverse, healthy landscape, which may be a mix of native and exotic species, references are available both to help find the right plant and to avoid the troublemakers. A little research before selecting plants can save time, money, and aggravation.
Reference guides may warn that a plant is invasive under certain circumstances, but they may not, and nursery catalogs frequently won't. Phrases like "a very vigorous grower" can be euphemisms for potential invasiveness. Treat such phrases as red flags!
Be sure to look in more than one plant reference to gather more than one perspective on any species you have in mind, especially if you suspect it may be invasive. After invasive potential is ruled out, the physical limitations and possibilities of the site should be the first and most important consideration in the exciting process of selecting new plants for our landscapes.
Michelle Buckstrup, Graduate Student, and Nina Bassuk, Program Leader, The Urban Horticulture Institute, Department of Floriculture and Ornamental Horticulture, Cornell University