Centaurea macrocephala is considered a noxious weed in Washington, an invader of natural areas.

THE PACIFIC Northwest has thankfully not (yet) found its own kudzu vine, the vicious aggressor smothering the southeastern United States. Gardeners play a vital role in keeping it that way - making sure our natural areas remain unthreatened by such an marauding invader, according to Sarah Reichard, assistant research professor at the University of Washington and an international expert on invasive plants.

Reichard took me to task last spring for a column I wrote about the joy of self-seeding plants. Turns out the most successful self-seeders, the plants that appear in cracks in the sidewalk and in spots you never water (the ones I was most proud of), are the most worrisome. Any plant that tenacious and resourceful has the ability to escape your garden, and perhaps to out-compete native flora in parks, prairies and forests.

"Oh come on," you're thinking. "Why does it hurt if I grow a few Verbena bonariensis, or bronze fennel? It is my own garden, after all."

Reichard points out that birds pay no attention to property lines (50 percent of woody species are bird dispersed); they eat the seeds of plants and excrete them elsewhere. Or, you may well have seeds on the sole of your shoe when you walk over to visit a neighbor. No garden is an island when it comes to spreading seed about.

And why should we worry about them escaping our gardens?

Now In Bloom Arum italicum 'Pictum' unfurls its glossy arrow-shaped leaves late in autumn, where they create a tapestry of green and marbled silver all winter. The foliage withers away in late spring to be followed by fat spikes of red-orange berries.

Reichard's answer: Invasive plants create "biological pollution" in wetlands, prairies and forests by destroying the native plants, thus reducing biodiversity. Having fewer types of plants increases the potential for serious fire and can change the whole biology of a region because birds and animals depend on native flora.

Here in the Northwest, the worst pests so far are English ivy, which is sufficiently shade-tolerant to invade forests; Scotch broom, which so increases the nitrogen in the soil that it kills off native species; purple loosestrife (have you gone canoeing in Union Bay recently? It's all you see) and Herb Robert (Geranium robertianum). There is some evidence that this last one, a pungent, lacy little weed, invades not only by out-competing (it tolerates shade), but also by poisoning other plants.

Invasiveness is a two-way street in the global garden.

"Our native plants are bad boys when they go overseas," says Reichard, as if she is talking about a bunch of young sailors. Douglas fir, salal and Oregon grape have all caused problems in different parts of Europe, where they have spread too far and too fast.

So what can a gardener do? Pull out aggressive self-seeders, particularly the ones that pop up in adverse conditions. Avoid Verbena bonariensis, Buddleia davidii, pampas grass, English ivy. Check mail-order catalogs to see if they have a policy about selling invasive plants. Reichard consults with Heronswood Nursery in Kingston, and this year's catalog clearly labels plants that have shown invasive potential. The catalog also lists plants no longer sold for ecological reasons.

"Since mail-order nurseries sell new and sometimes untried plants all over the country," says Reichard, "they can be a recipe for disaster." Talk to nursery owners where you buy plants, and make sure they are knowledgeable and responsible.

Although Reichard is working with the National Academy of Sciences to understand invasiveness, she isn't a purist about native plants. Only about a third of her own garden is planted in natives; she believes there are plenty of non-native, noninvasive ornamentals available to gardeners. Reichard became interested in developing a model for invasiveness when she realized that many species often invade, while others consistently don't. After all, have you ever seen boxwood or Camellia japonica get aggressive?

"It isn't about only growing natives, but about being thoughtful - choosing plants that aren't going to escape your garden and hurt the ecosystem," she says.

Valerie Easton is a horticultural librarian and writes about plants and gardens for Pacific Northwest magazine. Her e-mail address is vjeaston@aol.com