Plant invaders—non-native species that grow out of control—cost the American economy more than $120 billion a year. Most of them don’t get here by accident or mischief, but by invitation: webring them here to beautify our yards.

As Jane Stratton (’72 Fine Arts, Education) strolls along her rows of snapdragons and blanket flowers, she scuffs a fleshy, ground-hugging plant.

“I don’t know what that is, but it’s all over,” she says.

Stratton runs Sunshine Crafts, a business that offers a u-pick garden, subscriptions for a weekly vase of fresh flowers, and arrangements of dried flowers. Her two-acre plot just outside of Pullman brims with gladiolus, daisies, peonies, and more.

Like all avid gardeners, Stratton has encountered her share of aggressive plants that don’t stay where she puts them. About 10 years ago she planted a silvery Artemisia she thought would be lovely in dried arrangements.

“As it got time to harvest it, I noticed it growing everywhere along the roadsides,” she recalls. “And I thought, why did I plant this to begin with? It is pretty, but it just self-seeded. It’s been really hard to grub out.”

Now Stratton is a lot more careful about what she buys, and she keeps a close eye on her living inventory. Any plant that starts spilling out of its assigned space gets a swift correction.

“That’s the key,” she says. “A little elbow grease can go a long way in the beginning, rather than trying to stop this stuff once it’s already established.”

Richard Mack (’71 Ph.D. Botany), an ecologist at Washington State University, visited her garden soon after the Artemisia episode and became a big fan.

“She told me what she did, and I thought, gosh, this is exactly what needs to be done. This is extraordinary. I was really struck by her conscientiousness in taking something out of there that looked like it was going to jump the fence,” he says.

Mack has devoted his career to understanding how plants that are native to a place “jump the fence.” In his view, Stratton is on the front lines of an all-out war against botanical invaders.

While the military metaphor might seem alarmist, the scale of intrusions, and the damage they do, meet any reasonable criterion of “invasion.” A 2000 study by researchers at Cornell University estimated that invasive species—plants, animals, and microbes—cost American businesses and taxpayers at least $122 billion every year in damaged property, lost productivity, and control efforts. More subtle, and perhaps more costly in the long run, is the damage done to natural communities. Invasive species crowd out natives, mangle food chains, increase fire frequency, and speed erosion. They are the main factor in the decline of nearly half of our threatened and endangered species.

The kicker to this tale is that most plant invaders didn’t get to the United States by accident. Some were imported as potential forage or other crops, but the vast majority are ornamentals. We brought them here not to feed ourselves or our livestock, but to beautify our yards.

The same thing has happened all over the world. During the past decade, Mack has been asked to review biosanitation protocols written by the governments of India, China, and Taiwan, all of which are struggling against current invasions and are anticipating more, as expanding trade and improving infrastructure help alien species reach new territory. Everywhere he goes, the pattern is the same: plants imported to beautify gardens or parks jump the fence, run rampant, and disrupt native plant and animal communities in profound and sometimes dramatic ways.

A few years ago, for example, Mack found himself atop an elephant in India’s Corbett National Park. He’d been invited by conservation officials there to consult with them about lantana (Lantana), a sprawling shrub from South America that has escaped cultivation and now clogs hundreds of thousands of acres of open woodlands with impassable tangles of floppy branches. As their elephants waded through the snarl, Mack and his hosts spooked a leopard.

“The poor cat was trying to get out of our way and flee, and he was having all the difficulty in the world,” recalls Mack. “He couldn’t spring. It was the most unnatural movement for a leopard that you can imagine. Normally he could have been out of there lickety-split. Instead, I was on this elephant, looking down, and here was this guy body-surfing over the lantana, trying to get out.”

What dollar value do we put on a leopard’s ability to spring?

Invasive plants may be “weeds,” says Mack, but the two terms don’t mean quite the same thing. A native plant that thrives in roadcuts, burned forests, or vacant lots might qualify as a weed but is not an invader; it prepares the ground for other plants and then recedes to low population levels when its job is done.

“When we talk about invasive species, we’re not concerned with native species which have always played some colonizing role in these ecosystems,” says Mack. He’s also not concerned about non-native plants that become naturalized without harming their new neighbors. Most of our food crops are non-native, for example, as are forsythia, lilac, and other garden standbys.

Mack estimates that about 100 of every 1,000 species that are introduced will become naturalized or self-sustaining beyond the garden gates. Of those, one or more will likely become a serious problem. That sounds like the odds are on our side, but the dizzying pace of global trade means that humans are no longer just another “natural” dispersal mechanism for plants, akin to birds and wind. People in every part of the world bring into their homelands thousands of alien species every year. And as our current struggles show, a single invader can do tremendous damage.

One of the most intriguing aspects of invasive plants is that they rarely tip their hand early on. They muddle along in or near the garden for a few years or decades, before they establish self-sustaining populations. Then, after another lag, they burst out in full assault mode. Most of the species we’re battling in the Pacific Northwest were introduced in the region 100 to 130 years ago.

“We’re dealing with a biological phenomenon, which means that it doesn’t change linearly over time,” says Mack. Their populations grow very slowly at first, then explode in a logarithmic growth curve. He compares this pattern to the rapid spread of an epidemic. “Just as problems with human disease often go undetected, or at least unattended to, until it’s too late, the same thing is true of these organisms. . . And that actually leads to why these problems tend to get out of hand: the public and the policymakers don’t pick up on the danger until it’s virtually too late.”

Whether a species will reach the population explosion stage depends on whether it can reproduce well enough to nudge its numbers up to the point when compound growth kicks in. Small populations just can’t gain traction, even in a favorable habitat. That’s because of stochasticity, a scientific term for chance events. Stochasticity might be demographic—none of the members of a population leave offspring—or environmental—a summer hailstorm wipes out the whole population.

“A small immigrant population has almost no chance of persistence, even if it can tolerate the basic parameters of the environment, simply because of these random events that occur,” says Mack. He contends that we aid and abet potential invaders by protecting them from destruction by stochastic events. We water them during dry spells, shelter them from cold and wind, and nourish them with fertilizers. We even add more individuals if the original population struggles. As we cushion the random blows of nature, we enable a small, vulnerable population to put out more and more offspring, expanding its numbers to the point that it can survive those stochastic events.

Even so, there must be something about invasive plants
that makes them invasive. We coddle all of our garden plants, yet only a few jump the fence. Some of the warning signs are obvious. A plant that readily spreads and puts out a lot of seeds, with little encouragement from the gardener, is probably bad news. Melissa Smith, a graduate student in Mack’s lab, is trying to develop more precise measures of invasive potential. She’s evaluating several species of bamboo for their ability to thrive in inland northwest forests and become invasive in our region. One, golden bamboo (Phillostachys aurea), has already invaded parts of Florida, Texas, and Oregon.

“Some of these species have piqued people’s interest with how vigorously they grow in the introduced habitat,” says Smith. “And then some are completely benign. It would be nice to be able to differentiate between the two.”

She’s measuring traits like drought resistance and how efficiently the plants photosynthesize under a forest canopy. Doing multiple tests is important, says Smith, because she doesn’t want to wrongly condemn a species as a likely troublemaker. If a plant scores as potentially invasive on all five of her tests, “then I could effectively say to some ruling body, ‘Look, this plant does all these things under all these conditions, which other plants do that have proven to be invasive, so we probably shouldn’t let this in, or we should do so judiciously.'”

Over the years, research by other scientists has pointed to predictive features such as the number of seeds produced per year, tolerance of a wide range of conditions, and close kinship with another species that has already become invasive. All of those measures are cause for suspicion, says Mack, but unfortunately, none of them is a sure sign that a species will jump the fence—and lack of such features is no guarantee that it won’t.

“This is one of the perplexing things about biology,” he says. “It’s not mechanics or physics. . . the behavior of one [species] doesn’t necessarily predict the behavior of another.”

Despite the difficulties, he thinks it’s critically important to find some way to evaluate invasive potential. If we don’t, our only option is to wait for an invasion to happen, and then scramble to fight it and remedy the damage it’s done. That’s been our usual approach so far, resulting in what Mack calls a huge “hidden tax” we don’t even realize we’re paying.

“We don’t know how much of a bill we’re actually paying for having shrugged our shoulders over the last hundred years or so,” he says, “and we’re dealing with a lot of pests that, had they been dealt with appropriately early on, wouldn’t be problems now.”


Non-native species arriving in a new territory may die out quickly or survive (top half of graph), depending on the size of their initial population, how well they adapt to the new habitat, and whether we encourage their growth. Eventually some immigrants become naturalized, living and reproducing without human help. After years or decades, a few naturalized species begin to grow out of control (bottom of graph), leading to a full-blown invasion.

One of the most common strategies for guarding against botanical invasions is to let the habitat defend itself. In general, the harsher the climate, and the less it resembles an invading plant’s native range, the less likely it is that the invader will gain a foothold. Kappy Brun, grounds supervisor for WSU, says most of the plants that cause major headaches on Washington’s balmy west side behave just fine in Pullman, because the colder winters and drier summers here keep them in check. She combats stubborn patches of Japanese knotweed, but for the most part, she says, invasive species aren’t a big concern on the Pullman campus.

Smith is reluctant to trust ecological mismatch to protect an area from invasion. Before she came to WSU to study with Mack, she worked as an interpretive ranger with the national parks system. All the parks she worked in, all across the country, had one problem in common.

“The biggest threat that I saw with our parks was just how badly we were losing the fight against invasive species,” she says. “And that’s universal. I even worked in a park in Alaska, and there’s so many problems up there. Which is funny, because—it’s Alaska! You wouldn’t think invasives would be a problem there.”

She recalls one species, white sweet clover (Melilotus alba), that had taken up residence on the silt outwashes from glaciers. Its long, strong roots have stabilized the gravel and sand so much that instead of forming shallow, braided streams that shift with each season, the water now cuts deep channels. That one species of plant has changed the hydrology of a river system.

“It’s mind-blowing,” says Smith. “You wouldn’t think that you’d have such problems in Alaska. It’s got all these parameters that say that very few things should survive at all. But something did manage to get in there.”

Whether eastern Washington is more or less vulnerable to invasions than the west side depends on which species are introduced, says Mack. Summer droughts might make the Palouse less susceptible to invasion by trees but much more susceptible to invasion by annual plants that set seed before the drought begins each year. Even trees and large shrubs can make inroads, if they find pockets of milder habitat along streams or in other sheltered spots, or if the plants themselves adapt to habitats they previously couldn’t abide.

Mack says he’s seen too many cases of unexpected outbreaks to give a thumbs-up to plants that have proven to be invasive elsewhere. He keeps an eye on suspect plants locally, such as a tamarisk tree (Tamarix) growing in a front yard not far from the Pullman campus. It’s easy to see why people plant tamarisk: it’s gorgeous. Its frothy foliage and airy plumes of pink or white flowers make it look like an oversized asparagus plant. But it has a well-earned reputation as a destructive invader. Tamarisk moves salt from deep underground into its leaves (hence its other common name, salt cedar). When the leaves fall at the end of the growing season, all that salt ends up on the soil surface. Few other plants can tolerate it. Throughout the southwestern United States, tamarisk has displaced native riparian plants and lowered the water table enough to affect hydroelectric power generation. In recent years, some strains of tamarisk have evolved enough frost tolerance to allow the plant to invade areas farther north. It has made incursions into the Moses Lake drainage, the Hanford reservation, and elsewhere in eastern Washington.

Yet Washingtonians face no real obstacles to buying and planting tamarisk. That tree near campus wasn’t smuggled in, and it’s not tucked away in a hidden corner of the yard. It was probably bought at a local nursery, at least one of which has carried tamarisk in recent years.

Recognizing the marketplace as a major entry point for dangerous plants, in 2001 a group of horticulture professionals and scientists launched an effort to enlist nurseries, landscapers, and gardeners in the fight to keep invaders out. They developed voluntary codes of conduct regarding species known to be invasive. The St. Louis declaration, as it is known, includes such common-sense guidelines as “Plant only environmentally safe species in your gardens” and “Consider removing invasive species from plant collections.” The Washington State Nursery and Landscape Association has signed on, as have Monrovia, one of the nation’s largest wholesalers of ornamental plants, and the University of Washington. WSU hasn’t considered it yet, although grounds supervisor Brun says it sounds like a good idea.

The pledge has been a tough sell with some nursery owners who foresee a loss of revenue, if they stop carrying these plants. That may be a legitimate concern, although replacing invasive plants with well-mannered ones would seem to present new marketing opportunities. Many plant vendors simply argue that they should be able to sell anything they want, regardless of the
damage it will inflict and what it will cost all of us to deal with it later.

“In my opinion there should be repercussions for that,” says grad student Smith. “We shouldn’t be allowed to plant something or proliferate something that’s costing land, that’s costing money, that’s costing energy and resources. That’s a pretty big deal in my book.”

Of course, nurseries wouldn’t carry these plants, if there weren’t customers who wanted to buy them. Most gardeners tend to think more about a plant’s beauty and ease of cultivation than about its future behavior. We want plants that thrive with little attention, that produce lots of flowers and quickly fill in those empty spots in the yard.

Linda Chalker-Scott, urban horticulturist at WSU’s Puyallup Research and Extension Station, thinks gardeners are the key to cutting off the influx of invasive horticultural species. She recalls that several years ago, few retail outlets carried much in the way of native species. Such plants just weren’t in demand. Now many nurseries carry a large selection of native and locally adapted plants, because the demand is high. Likewise, she thinks gardeners could shift nursery inventories away from invasive plants and toward the many noninvasive alternatives that are available.

“I’m not going to be a plant cop,” she says. “I would never venture to tell someone that they shouldn’t plant something. But usually if they learn of a good alternative, they’re receptive.”

Plenty of information on alternatives is available (See sidebar, “What you can do”); but the continuing use of invasive species in landscaping suggests that many gardeners and plant purveyors either haven’t seen it, or have seen it but don’t care. Mack is frustrated by what he views as widespread indifference to biological invasions, especially compared to the frenzy evoked by other issues that so far have cost much less in terms of economic and environmental damage.

“I’m just struck by which issues the public picks up on and really is willing to follow through on,” he says. “If we had people as worked up about invasive species as they are about GMOs [genetically-modified organisms], then we’d be a lot further down the road. People are much more concerned if you slap a radiation sign on something, than if you simply wrote, ‘This is an invasive species’ and you hung that on the plant in the nursery. Why is that? I’m just struck by the incongruity of it.”

Chalker-Scott thinks it’s not indifference so much as a disconnect between personal actions and larger effects.

“People just assume that whatever happens in their yard isn’t going to affect what happens in the rest of the world,” she says. “They tend to think there’s some sort of magic barrier . . . There is a property line, but plants don’t respect that.”

It’s now mid-January, the season of dreams for snowbound gardeners. The winter plant catalogs have arrived, with their promises of care-free growth and never-ending blooms. Among the offerings are butterfly bush, golden bamboo, English ivy, lantana, and silver lace vine. Mack sighs. “Unfortunately, this isn’t a problem that’s going away anytime soon. As I tell my students, this is a growth industry.”


Rogues Gallery

Turn your yard into a beachhead! Try one of these beauties, all proven invaders in the Pacific Northwest (PDF).


Read more

There are lots of things you can do to help eradicate invasive plants. Here are just a few of them:

Find alternatives to common invasive species at and

Check the Washington state noxious weed list.

Ask your local extension agent about plants of concern in your area and opportunities to help eradicate invaders. In Washington, start here.

Invite an expert to talk with your organization about invasive plants. Check with county extension offices, or contact Richard Mack ( or Linda Chalker-Scott ( of WSU, or Sarah Reichard ( of the University of Washington.

Ask your local nursery to adhere to the principles of the St. Louis declaration.