Native or not?

The growing desire for native plants is palpable. Strong consumer interest in natives was evident in surveys in 2010 by the Gardens Writers Association and the Association for Landscape Architects. In response to this trend, nursery growers are planning to produce more native plants in the future, according to Nursery Management magazine’s 2012 State of the Industry report.

Native plants can be used to create attractive, sustainable landscapes that blend naturally with the surrounding flora. Landscapes composed of native plants are considered sustainable because native shrubs do not pose the threat of introducing new species to an area. When established in landscape sites similar to their natural habitat, native shrubs require little maintenance, are well adapted to local soils and climates and attract beneficial wildlife to the garden. As concern over invasive plants grows, garden consumers are seeking to replace mainstay exotic plants with alternative native species.

It can be rather difficult to define what a native plant is and many definitions exist. Perhaps one accurate definition would identify native plants as those that have evolved and adapted to a specific location (naturally occurring) and have remained genetically unaltered by humans. Often though, plants are described as being native to a country, or native to a large geographic region or native to a state. This is where difficulties can arise.

Switchgrass (Panicum virgatum) is found throughout most of the U.S east of the Rocky Mountains and is considered an American native grass. In the Northeastern U.S., switchgrass is typically a 3- to 4-foot tall plant in flower at the end of the summer. Some switchgrasses from Oklahoma and Texas reach sizes of 8-9 feet tall and bear little resemblance to those that grow natively in the Northeast. A similar example is Acer rubrum, the red or swamp maple, which is native to both Florida and Maine. But is it correct to call a Florida genotype of Acer rubrum transported to Maine a Maine native?

Both Taxodium distichum and Oxydendrum arboreum are natives of South Carolina. However, Taxodium is found on the southern coastal plain of South Carolina and is absent from the mountainous portions of the state. Therefore Taxodium would not be considered native to the northwestern half of South Carolina. Oxydendrum, however, is found in the northwestern mountainous parts of the state, but not along the coastal areas.

The use of native plants by horticulturists and by ecologists can be quite different. Ecologists are generally concerned with using plants that are genetically similar to the area they are to be planted in and have been reproduced by sexual methods, rather than vegetative methods. The goal is to utilize varied genotypes that will function well in the target ecosystem. Horticulturists often choose to use selected genotypes (cultivars) that exhibit superior ornamental traits. In most cases, these plants have to be vegetatively propagated to retain the ornamental traits, due to high levels of heterozygosity that typically exist in herbaceous and woody perennials. From an ecological perspective, the use of cloned cultivars of native species is often considered to be less desirable than using seed propagated plants of the same species. This is because cloning for cultivar propagation reduces genetic diversity. It is also likely that the cultivar did not originate from the same ecoregion that it is being used in.

“Improved” selections
Gardeners and landscapers will generally choose to use “improved” cultivars of native plants rather than unimproved “wild” forms, despite the potential ecological benefits of using seed propagated local material. However, one could argue that it may be better to embrace the use of improved native plant cultivars than exotic plants that could potentially become invasive. To increase consumer use of native plants, plant breeders have begun work to develop native plants that possess showy ornamental characteristics that can match the ornamental appeal found in many exotic species.

Native plants can create attractive, sustainable landscapes that blend naturally with the surrounding flora.

In breeding native plants, there are several situations that can arise where it is unclear if the newly bred plants can be considered to be native. For example, if a breeder crosses two native species to develop a new ornamental plant, can the new plant be considered to be native to a location? What if the breeder crosses a Viburnum dentatum from Virginia with a Viburnum cassinoides from Maine? Could the definition of native be stretched to include this interspecific hybrid as a Maine native? Both parent species are native to Maine; however, one of the parents is from a distinctly different state. Also, the hybrid plant is a new genetic combination that likely doesn’t exist anywhere in the wild as a population. A breeder might also cross a New Jersey native plant (Ilex glabra) with an exotic species (Ilex cornuta) from China in an attempt to get larger, shinier foliage in the offspring. Would the offspring from this cross have any status as a New Jersey native plant? A breeder might also employ chemical or physical mutagens to derive dwarf or compact forms of native species that might be unpopular as a landscape plant in their taller, leggier natural habit. For example, a breeder might use a chemical mutagen to try to obtain more compact forms of the leggy red chokeberry (Aronia arbutifolia) that is otherwise an outstanding shrub native to a good portion of the eastern U.S. Should this shrub still be considered native to an area even though its genes are mutated? As greater emphasis is placed on native plants for landscape use, the native status of the plant breeder’s progeny will need to be established.

Production potential
There are many species that have useful landscape characteristics in their unimproved form. Many of these native plants have been widely adopted for landscape use already, such as Ilex verticillata, Cornus florida, Acer rubrum and Clethra alnifolia, but native plant use for landscaping currently just scratches the surface. According to Prides Corner Farms, a large wholesale nursery in Lebanon, Conn., native plants only account for 10 percent of all of their plant sales. Considerable progress can be made to increase use of native plants by taking a closer look at plants that have been overlooked.

There are numerous native species that hold potential to be useful and desirable landscape plants, such as Comptonia peregrina, Corylus cornuta, Myrica gale and Spiraea tomentosa. If a native plant is going to be used to replace an invasive species that performs well in difficult sites, then the alternative native plant must also be adapted to difficult landscape situations. All too often native plants that are intolerant of difficult landscape situations are suggested as replacements for tough invasive species. The poor outcome of such ill-advised substitutions can reduce consumer confidence in using native alternative plants. A commonly made, but unsuitable recommendation, is to replace the invasive winged euonymus (Euonymus alatus) with the native high bush blueberry (Vacinnium corymbosum), because both possess outstanding red fall color. Unfortunately, blueberry requires very acidic soils that are rarely found in the challenging sites where winged euonymus is typically used.

The goal of my research and extension program at the University of Connecticut is to identify novel or under-utilized native shrubs that are adaptable and able to directly replace invasives in landscapes. Native species that have been identified as having good landscape adaptability next need to have production methods developed, which nursery producers can use to profitably produce plants for sale. Once good production methods have been established, the last, but equally important, step is to develop marketing plans and materials to promote consumer use of native plants in place of invasive exotic species.

Jessica Lubell is assistant professor, Ornamental Horticulture, Department of Plant Science and Landscape Architecture, University of Connecticut; jessica.lubell@uconn.edu.