Moving Beyond the Natives/Exotics Debate
by Nina Bassuk1 and Michelle Sutton
1 Department of Horticulture, Cornell University, Plant Science Building, Room 33, Ithaca, NY 14853
Ten years ago, some well-meaning staff at Cornell University in Ithaca, New York, took a look at the blank canvas next to a new library addition and said, "We want native plants!" A four-inch-diameter white oak (Quercus alba) went in, along with flowering dogwood (Cornus florida) and native viburnums such as possumhaw (Viburnum nudum) and mapleleaf viburnum (V. acerifolia).
Sadly, the native trees and shrubs declined or died within the first growing season after they were installed. Their nativeness wasn't to blame, because in the right situations, the plants would have performed beautifully. But this site was heavily disturbed, its topsoil having been denuded and compacted by the construction of the library addition. Planting the white oak in the compacted blue-gray clay here was like planting it in a small clay pot. It died within a year. The viburnums, native to acidic, sandy soils, suffered mightily in the concrete-contaminated clay, where the pH was found to be a whopping 8.0.
Challenges of the Urban Environment
College campuses are quintessentially urban environments, where humans have profoundly altered the growing conditions for plants. In city landscapes, the earth has typically been scraped and graded, often with a total loss of topsoil. Compaction of the soil by heavy machinery and foot traffic is commonplace. Buildings block light, limiting the ability of plants to photosynthesize. Hard, impermeable surfaces channel water into low-lying areas, waterlogging the ground. Lime leaches from aging concrete sidewalks and building foundations, raising the soil pH. Temperatures become abnormally elevated due to the absorption of solar energy by the built landscape (the so-called heat island effect).
These and other factors present challenges for well-intentioned gardeners, not to mention for the plants they wish to grow. In order to create healthy plantings that will thrive without constant attention and intensive maintenance, gardeners must pay attention to their site conditions and select plants that adapt well to them. In heavily urbanized areas, it may not be feasible to cultivate plant communities of natives found in nearby natural areas. A mix of natives and exotics may be the best option.
In the case of the library garden, a redo was in order. This time, the Department of Horticulture at Cornell—where we worked—was brought in to supervise the project. Before we added any plants, we addressed the site conditions. We remediated the compacted clay soil with Cornell-generated compost from a university operation that uses farm animal manures and kitchen scraps. Six inches of compost were added and incorporated by horticulture students using shovels in a terraced environment that machinery couldn't enter. The addition of compost reduced the compaction, brought the soil profile up to give plant roots more volume, and provided veins of organic matter into which the roots could grow.
Our guideline, applicable to anyone dealing with dense clay soils, was to add sufficient organic matter so that the resulting planting medium was at least one third by volume organic matter, and two thirds by volume soil. You really need to add this much to make a meaningful difference in heavy soils. In the heaviest clay soils, you may need to create a 50-50 mix. Instead of tilling, which only addresses the top eight or ten inches of the soil profile, we urge deep incorporation of organic matter down to 18 inches.
After proper soil preparation, we turned our attention to the planting scheme. First, we had to decide what we wanted the garden to achieve in terms of beauty and functionality, including ecosystem services like erosion control, wildlife appeal, and shade for a sitting area (a venue for people to engage in the natural world). We studied and documented the specific microclimates and challenges of the site, which included poor drainage and seasonal standing water, high pH, and varying degrees of shade. Then we drew up a list of trees, shrubs, and herbaceous plants that could handle these conditions. Sustainability of the planting became the highest goal, trumping exclusive use of native plants.
We put in three yellowwoods (Cladrastis kentukea), a native tree known for its ability to withstand seasonally wet, disturbed soils. Yellowwood has attractive elephantine bark and stunning white flowers. The yellowwoods provide shade along with a Freeman maple (Acer × freemanii), a naturally occurring cross between red maple and silver maple that has the ability to withstand wet feet.
As for native shrubs, we chose a dark-leafed ninebark cultivar, Physocarpus opulifolius 'Diablo'. Ninebark can be found growing indigenously in the woodlands of the Cornell campus, and like the straight species, 'Diablo' can handle less-than-ideal soils, seasonally wet feet, and partial shade. We also planted large swaths of partial-shade-tolerant red-twig dogwood (Cornus sericea 'Baileyi'); you can see the straight species in lowlands and ditches all over upstate New York, but 'Baileyi' is a selection that offers exceptionally red stems in winter and doesn't create thickets (such suckering could be desirable on an eroding hillside, however).
We also planted noninvasive nonnative plants to provide shade, year-round interest, and other services to the garden, including magnolia for seasonal beauty (Magnolia 'Ann', a complex hybrid in the Little Girl series); Japanese garden juniper and forsythia for erosion control (Juniperus procumbens 'Nana' and Forsythia 'Courtasol' GOLD TIDE), and firs for bird cover (Abies nordmanniana and Abies koreana, in the more well-drained parts of the garden). A variety of nonnative willows (Salix species) with no invasive reputation were added for their beauty and ability to tolerate poor, wet soils; we also wanted to display them for educational purposes.
In projects like this, we strive to fulfill the vision of the Sustainable Sites Initiative (SITES; sustainablesites.org), a certification program for landscapes analogous to the LEED program for buildings. The initiative promotes the use of "appropriate plants"—those that are adapted to or tolerant of the site conditions, are not invasive (as defined by local groups), and meet the design intent. When these criteria are met, plantings promote a multitude of sustainable ecosystem services, from shade and carbon sequestration to wildlife food and cover.
Appropriate plants may be native to the local region or nonnative. It's important to recognize that any plant, whether native or not, must be able to grow well under existing site conditions. A regionally native plant may be a poor choice if it does not match site requirements or has a weedy habit under the site conditions. (Opportunistic natives can be weedy in highly disturbed sites.) SITES credits can also be received for preserving an existing site with well-adapted native plants or restoring a site to a native-like condition. This would include not just the use of locally native plants but also the restoration of appropriate soil conditions to support those plants.
In keeping with the SITES criteria, in the library garden renovation we were careful to avoid introducing plants that might colonize the nearby woodlands and edge out diverse flora. Interestingly, already at the forest's edge were two "bad actors": Wych elm (Ulmus glabra), a nonnative, and boxelder (Acer negundo), a native (illustrating that not every native is our friend!). Other examples of natives that could cause grief in this highly disturbed site include fox grape (Vitis labrusca), common cattail (Typha latifolia), and eastern cottonwood (Populus deltoides).
Demythologizing Native Plants
No one can dispute that native plants have an important role to play in modern landscaping. We want plantings to tell us where we are, to initialize a place, to ground us in a region. We don't want to see the same garden palette in Phoenix, Arizona, as we do in Ithaca, New York.
But are all the arguments for native plants accurate? 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, insects, pathogens, and other stresses of the local environment. However, native plants fare no better than exotic ones if they are not carefully matched to the site.
Some exotic plants actually perform better in our disturbed urban sites because the environmental conditions resemble those found in their native landscapes, and because the plants' insect predators and diseases are frequently not imported with them. Witness the graceful—and noninvasive—dawn redwood (Metasequoia glyptostroboides), imported from China and virtually indestructible when used in the right spot.
Another perceived benefit of native plants is plant diversity, but that is not the provenance of native plants alone. In Ithaca, for example, more than 250 tree species (natives and exotics) can be found in urban areas, outnumbering the tree diversity in nearby woodlands. (Diversifying the plant palette—whether you're growing natives, exotics, or a mix of the two—not only provides greater aesthetic interest, it also makes a garden more resilient to attack by pests and diseases.)
What about wildlife—encouraging them onto our properties and providing corridors for their movement? It's no secret to the observant gardener that certain wildlife, especially generalist feeders, can be enticed by exotics. Sometimes this happens to our dismay, like when deer find our hostas and hybrid tulips. Nonnative Callery pear tree cultivars (Pyrus calleryana) are proving to be invasive, and for that reason in Ithaca we are no longer planting them, but inarguably, they provide fruit for birds. (Admittedly, if your desire is to attract specialist native animals and insects to your garden, you need to grow the native host plants associated with this fauna.)
Finally, there is the question of availability. In Ithaca we test potential new street trees by observing their tolerance of tough urban conditions. We'd like to use more native hickories like shagbark (Carya ovata) and bitternut (C. cordiformis) for streetscapes, but it is virtually impossible to get them in a large enough size (they currently come as three-foot whips that vandals would have a field day with). There's a production bottleneck that has yet to be overcome by better propagating techniques and consumer demand.
Right Plant, Right Place
Even when production problems are solved, there is still the reality of the site to contend with. At our library garden, even after intensive remediation of the soil, site challenges such as seasonal standing water and a clay soil with a pH of 8.0 remained. We were dealing with a decidedly disturbed, nonindigenous planting environment.
For instance, in a sloped, sunny section of the garden where we wanted low-growing woody plants for erosion control as well as beauty, we chose Juniperus procumbens 'Nana' and Forsythia 'Courtasol'. Junipers and forsythias are noted for their durability and ability to tolerate clay soil and high soil pH. What regional natives might we have used in their place to fulfill the same functions under the same site conditions?
Let's start with the juniper, our evergreen groundcover. The two junipers native to our area, common juniper (Juniperus communis) and eastern red cedar (J. virginiana), were too tall for our purposes. Disease resistance was another deciding factor: Although one shorter variety of common juniper, J. communis var. depressa, is resistant to Phomopsis tip blight (a common disease of junipers in the eastern U.S.), its resistance to Kabatina tip blight (a significant disease in our area) was unknown. Canby's mountain-lover (Paxistima canbyi) is a native evergreen that we have used in other situations, but we felt it would not tolerate the clay soil, even after remediation. Bearberry (Arctostaphylos uva-ursi), a beautiful native evergreen, wouldn't thrive in the less-than-perfect drainage and high pH. Our list of possible natives for evergreen groundcover dwindled, and eventually J. procumbens 'Nana' emerged as the best low-growing, disease-resistant evergreen candidate for the site.
What about a deciduous, bank-stabilizing, low-growing native woody groundcover that would provide beautiful flowers and uncomplaining service akin to the forsythia? Sweet pepperbush (Clethra alnifolia) would bake in the full sun and probably suffer from the high pH. We love the Virginia sweetspire Itea virginica 'Sprich' GOLD SWEET HENRY, but would it have tolerated the full sun, high pH, and imperfect watering regime one finds on a busy college campus? Most likely not. We could have gone with the fragrant sumac cultivar Rhus aromatica 'Gro Low', which provides excellent erosion control and lovely fall foliage, but the plant is already greatly overused on the Cornell campus, and we wanted to increase plant diversity with our garden. (The flowers of 'Gro Low' are also pretty inconspicuous.) We could also have gone with shrubby St. Johns wort (Hypericum prolificum), a native with showy yellow flowers, but we didn't have a ready source for it.
Today, the maturing library addition garden encourages many people to linger in it. As with any garden, it continues to evolve and respond to challenges from college construction and environmental changes such as increased deer predation, weed growth, and variable rainfall. Yet all the plants originally planted have done well with a reasonable amount of maintenance. The garden serves as a shaded seating area, a place for wildlife to find food and shelter, a vegetated slope with no erosion, and a site with little stormwater runoff and good water infiltration. It's also a great teaching tool for sustainable urban horticulture—a resource for students to learn about choosing and using the most appropriate native and nonnative woody plants.