Invasive Plants North American Gardeners Should Avoid and Native Plant Alternatives

The first of my gardening catalogs come in the mail in mid-December, and I, like many gardeners, mull over the many choices available. But as I leafed through it, I noticed to my surprise and horror that several plants listed as troublesome invasives were for sale. Many invasive plants were, as recently as the winter of 2010, still for sale through mail-order and Internet plant sellers.

Invasive plants can appeal to gardeners because they grow quickly, look pretty, and are promoted as "pest-free." But a pest-free plant has no natural enemies or predators to keep it unchecked outside its native ecosystem. Often, these turn into invasive plants, growing and spreading quickly or producing large numbers of seeds and offspring. All of these factors can result in escape of the invasive plant from the garden and invasion of natural areas.

Russian olive was billed as a windbreak or screen. "Gray foliage creates a dense screen 10-20 ft tall," said Gurney's Seed and Nursery Catalog, which also described it as "extremely hardy" and resisting pests and disease. According to the USDA, russian olive was first introduced as an ornamental in the late 1800's. Russian olive has invaded natural areas, crowding out native species. A USDA fact sheet offers alternatives to Russian olive.

The Chinese elm, a fast-growing species popularized partly because of its resistance to Dutch elm disease, was introduced from Asia. Chinese elm's invasive properties have cause problems in Africa, and Chinese elm has had some invasive tendencies in North America and Australia, according to Wikipedia. For American gardens, the Morton Arboretum's 'Accolade' elm, an American elm bred for its resistance to Dutch elm disease, may be a better alternative.

The Cleveland ornamental pear, also called callery pear, or bradford pear, was described by Gurney's as "Gorgeous from spring through fall!" This medium-sized (20-50 ft) tree produces white blooms in the spring, shiny dark foliage in the summer, and red leaves in the fall. This fast-growing tree has become popular as a specimen tree in American gardens, but the Cleveland ornamental pear has been causing problems as an invasive plant in the eastern US. Instead of the Cleaveland ornamental pear, consider the serviceberry (Amelanchier arborea), which produces fruits, or the redbud (Cecis canadensis), which has pink blooms and no fruits. Both the serviceberry and the redbud are plants native to the United States.

Another invasive plant popular in American landscaping is the Norway maple. This plant did not emerge as an invasive plant until many years after its introduction in the United States. Norway maple's ability to tolerate shade has allowed it to grow and penetrate forest canopies. When it grows taller, Norway maple's dense foliage shades out native trees. The red maple (Acer rubrum), sugar maple (Acer saccharum), American basswood (Tilia americana), red oak (Quercus rubra), white ash (Fraxinus americana), and tuliptree (Liriodendron tulipifera) are recommended as alternatives.

Crownvetch (Coronilla varia), a low-growing invasive legume, is sometimes recommended as a groundcover and has been sold by vendors such as Park Seed, which describes it as covering ground "quickly and beautifully." Crownvetch became popular to reduce erosion on sloped banks, hills, or roadsides. It has become a problematic invasive in some states, such as Virginia. Instead, consider a garden filled with native plants or groundcovers, such as prairie alumroot (heuchera), Canadian wild ginger (Asarum canadense), sedges (carex), prairie coneflowers (echinacea), or wild blue indigo (baptisia). These may take a little longer to fill the space in, but you won't have to worry about them invading natural areas.