There was a lot of talk about native plants at a Pennsylvania State University Cooperative Extension Symposium I attended last Saturday in Gettysburg. In their purest form, native plants evolved with the wildlife that inhabits this region. When the native plants disappear, an area's ecology is severely altered.

Nonnative plants, also known as exotics, introduced here from other parts of the world and other regions of the country, deserve our attention, too. Some of the most energetic of these are out-competing native species and changing our ecosystem.

What constitutes a native plant is somewhat flexible and debatable. The definition generally accepted by many plant societies is any plant here before Columbus landed.

If you want to landscape with native plants appropriate to your region, then choosing purple coneflower, for example, which is native to the United States but from the prairie region, would be unacceptable here. You would need to select a plant that occurs naturally in the mid-Atlantic region and plant it in the locations that provide the soil, light and other conditions in which it thrives.

When people look for plants that are best adapted to conditions in their area, natives are generally the first choice for vigor and health. Most have proven themselves to coexist well with other native flora and fauna.

Among some home gardeners, there is a notion that indigenous plants need less care and watering. But if you hope to design a garden using flora exclusively from this region, the plants must be placed where they'll thrive. Some, such as hickories and oaks, can prosper in poor, dry soils. Other varieties, such as buttonbushes (Cephalanthus) and American cranberries, require constant moisture.

Many exotic plants adapt well to our environment. Indeed, some have found it so hospitable that they are changing our ecosystem faster than we can say, "Why don't migratory birds fly through here to feed anymore?" And the answer is, the birds don't stop because "invasive exotics" are overrunning the native plants that provided food and shelter, sending the birds in search of more desirable habitats.

Many environmentally sensitive homeowners among my clients ask me not to include trees, shrubs or perennials that are considered invasive exotics in my designs. But knowing what should and shouldn't be installed has taken time. Sometimes, no one can know for sure whether a plant will become invasive until it has been growing for years.

The following are plants on the Invasive Exotic Plants list offered through the Maryland Department of Natural Resources. These plants are considered natural threats to ecosystems in this area, which includes Virginia, Pennsylvania and other neighboring states in the mid-Atlantic region. You can get a complete list by calling 410-260-8555 or by visiting the department's Web site at www.dnr.state.md.us.

* English ivy (Hedera helix) is considered an invasive plant in our woodlands because it thrives in dense shade as well as in sun, loves moisture, and survives drought. And its evergreen foliage shades out all competitors. It carpets the soil and makes a tangled blanket of vegetation that can grow two feet thick. You can eradicate it to make room for native vegetation by covering it for a year with black plastic or landscape fabric. Pulling it out or cutting it back is a temporary fix.

* Purple loosestrife (Lythrum salicaria), a Eurasian native plant, is a beautiful, long-flowering perennial that has become the classic pest plant in this country. It has naturalized into wet areas from Maine to Minnesota and south into the mid-Atlantic states. In doing so, it crowds out native flora, removing the food and cover that native wildlife depends on.

* Japanese silver grass (Miscanthus sinensis) and fountain grass (Pennisetum alopecuroides) propagate primarily by wind-carried seed, although animals can pick up the grass seeds on their fur or feathers and deposit them elsewhere. In just a few years, fountain grass can cover flower beds and extend into your turfgrass. It will make lawns look shredded when mowed.

* Japanese barberries (Berberis thunbergii) and burning bushes (Euonymus alatus) spread primarily because seeds are picked by birds. These two shrubs can readily be found growing throughout Rock Creek Park and other patches of woodland in this region, such as our own back yard.

My family experienced firsthand an example of the barberry's impact on wildlife. Outside our living-room window this spring, we watched excitedly as the eggs of six finches hatched in a crimson barberry. We showed our neighbors and watched as these tiny chicks grew feathers and the parents diligently looked out for the six nestlings. Then in one night, the nest and all its inhabitants were gone -- perhaps the work of a raccoon or snake. But the real cause of their demise was that the Japanese native shrub was far too low-growing for the protection of our baby finches.

This phenomenon has been documented in a six-year study by Kenneth Schmidt of the University of Memphis's biology department and Christopher Whelan of the Illinois Natural History Survey in Wilmington, Ill. They studied the effects of exotic plants on songbirds and found, as we did, that birds that nest in nonnative plants lose more eggs to raccoons and other predators.

Some native plants can also display invasive tendencies. Among these:

* Virginia creeper (Parthenocissus quinquefolia), a native plant, will run over shrubs as quickly as the invasive Asian exotic porcelainberry (Ampelopsis brevipedunculata).

* Phragmites (I. australis), a perennial grass found all over the world, is so massive, eight to 10 feet tall, that it is capable of taking over a wetland in short order and in any climate.

To become invasive, many plants need a specific habitat, such as wet areas for phragmites. But many other invaders, such as porcelainberry, can adapt to a range of conditions -- wet, dry, sun and shade.

Here are some desirable regional natives:

* Clethra, or summersweet, is a dense, leafy shrub that grows four to six feet tall and bears fragrant summer flowers. It is shade-tolerant and displays russet-red foliage in fall.

* White fringetree (Chionanthus virginicus) is a large shrub with white flowers or a small tree at the fringe of the woods.

* Virginia sweetspire (Itea virginica) is a low-maintenance shrub with long-flowering, sweet-smelling blooms in summer and maroon fall foliage. It thrives in a wide range of habitats, preferring wet sites in sun or shade, with some drought tolerance.

* Common witch hazel (Hamamelis virginiana) is a small tree or shrub at home anywhere in the eastern half of the country, in sun or shade. The plant's yellow, fragrant flowers bloom for three weeks in November, with leaf color of an outstanding yellow.

* Goldenrod (Solidago sp.) has showy golden flowers open now, best used as a meadow plant mixed with wildflowers or on the edge of a woods.

The Alliance for the Chesapeake Bay is a great resource for information about responsible garden design using native plants: Visit it on the Web at www.alliancechesbay.org or call 410-377-6270.

The best way to find sources for native plants is by writing to one of the local native-plant societies:

* Maryland Native Plant Society Inc., P.O. Box 4877, Silver Spring, Md. 20914 (www.mdflora.org).

* Virginia Native Plant Society, 400 Blandy Farm Lane, Unit 1, Boyce, Va. 22620 (www.vnps.org, or call 540-837-1600).

Joel M. Lerner is president of Environmental Design in Capitol View Park, Md. His e-mail address is jml@gardenlerner.com; his Web page is at www.gardenlerner.com.

Spiky summersweet, left, is a dense, leafy, shade-tolerant shrub that blooms in the summer and has fragrant flowers; it is a desirable regional native plant. The invasive porcelainberry, right, cuts off sunlight, weakening trees on the fringe of a forest. It can adapt to a range of conditions -- wet, dry, sun and shade.