Brought to the U.S. from all corners of the globe, invasive, non-native plant species are taking over ecosystems across the nation. Can the combined efforts of concerned citizens and government agencies keep this invasion of alien species at bay?
Story and photos by Carrie Madren

Carole Bergmann, forest ecologist for Montgomery County, MD, holds up a clump of Japanese stiltgrass as she teaches future weed warrior volunteers about foreign species.

The mile-a-minute weed’s delicate, triangular leaves look bucolic enough, but I’m snatching handfuls of it off scraggly bushes that cower beneath. Petite thorns curve backward and claw at clothing; where they graze my forearms, itchy red bumps irritate for hours. I feel strangely heroic in thick gloves, ripping out the prickly strands and rescuing the now-sparse, hillside native vegetation from this wicked vine. Sweat dampens my forehead as I ball up the offending foliage like string and cast it near the path below.

As a weed warrior, I joined a local army of volunteers dedicated to defending local forests against non-native invasive plants, which were once purposefully planted for erosion control or landscaping appeal, but have now grown wildly out of control across every U.S. region.

“It’s a big mess,” says Mike Ielmini, National Invasive Species Coordinator for the Forest Service. Estimates reveal that our country’s alien-plant problem exceeds the size of California in acreage, including both public and private lands. And with our global village becoming even more interconnected, the invasive species problem will grow.

“I think it’s going to seriously affect forests in the future,” says Carole Bergmann, a forest ecologist for Montgomery County, Maryland, especially when combined with invasive non-native insects and viruses, such as gypsy moths and sudden oak death. The problem is so massive that it is widely considered to be the biggest threat to biodiversity after habitat destruction. It is a challenge that affects us locally — and often personally, as we witness invasions in our local forests — as well as nationally.

Though the invasive threat seems to have spiraled out of control, ecologists and foresters aren’t giving in. In this article, American Forests investigates our growing American invasive plant problem, and how we’re defending our forests.


Ecological Enemies

It wasn’t until the early 1980s that invasive plants began to spark national concern, says Sarah Reichard, research assistant professor at the University of Washington. That’s when the United Nations appointed a group of scientists to examine invasive weeds in natural areas. “It was eye-opening for people,” she says.

Now, over 1,000 invasive non-native plant species have been identified within the U.S., most sharing common characteristics that give them unfair advantages over natives: They can establish quickly, and reproduce rapidly and widely. A single princess tree, for example, can produce 20 million seeds that are easily transported long distances by wind and water. In addition, invasive species’ seed banks can survive for more than seven years in the soil.

Foreign heritage has excused invasives from the complex array of natural controls present in their native lands, such as herbivores, parasites, pathogens, and competition with other species. Without controls, invasive vines and shrubs create dense stands that crowd out native seedlings and dark shade that blocks succession and understory growth. Invasive vines that lurk at forest edges have the power to strangle and topple tall trees, creating canopy holes that encourage more invasive growth.

Many alien plants also create trouble for animals by crowding out the native plants that wildlife depend on for food. English ivy, for example, hides the leaf litter that robins and towhees dig through for insects. And herbaceous invasives — such as garlic mustard — replace native wildflowers that serve as host plants for butterflies.

Bamboo was prized for its ability to screen for privacy, but left alone, it can take over natural areas.

Left unchecked, these outlaw species drown plant diversity, creating a near monoculture of ecologically worthless vegetation. Take, for example, Japanese stiltgrass, which blankets eastern U.S. forest floors in a lush carpet of green, disrupting succession of other species. The ammunition of these tiny, bamboo-like grasses is their seed abundance: Each plant produces hundreds of tiny seeds, which can embed themselves alongside roads and trails just as easily as in the forest interior.

“We don’t even know how it happens in some forests,” says Luke Flory, a researcher at Indiana University who studies Japanese stiltgrass. All it takes, he says, is a hitchhiker seed buried in a hiking boot, all-terrain vehicle, or mountain bike tread.

In the worst cases, invasives can alter the natural cycle of the forest landscape. Highly flammable cogongrass, a new threat, has the ability to turn forests into savannahs. Currently, cogongrass is the target of the largest effort in history to shut out an invasive plant: $10.6 million from the American Recovery and Reinvestment Act of 2009 is at work fighting the invader in Alabama, Georgia, and Mississippi.

Invasives weren’t always our horticultural enemies. Years ago, we imported foreign species for the same qualities that now make them invasive: tenacity and resistance to pests. Japanese honeysuckle, prized as a shady ornamental plant, was often planted around porches. Bamboo was valued for thick, fast-growth screening; kudzu was imported for erosion control. Others arrived as stowaways that sneaked past border controls, like Japanese stiltgrass, which was once used as packing material.

Now we’re paying the price. Invasive species rob forests of valuable ecosystem services and capabilities — degrading soil quality, water abundance, and diversity. The ecological, economic, and health costs of all types of invasive species exceed $138 billion per year, with plants alone responsible for $34.7 billion in losses, according to a 1999 report led by David Pimentel at Cornell University. In the past decade, as invasives grew out of control, that figure has likely skyrocketed.

Agencies and forest managers alike know the situation is critical, but the alien-species problem is just one of many — including wildfires — pressing for funding and attention. A downturned economy has also set back our defense of forests. But can we afford to lose this ecological battle? “It’s like a hole in the bucket,” says Ielmini of the Forest Service, “We’re losing $138 billion a year just on this problem.”


Assessing The Damage

We know that invasives are everywhere, but exactly how much of the U.S. is under siege by alien plants is anybody’s guess. No U.S. agency has had the resources to do an intensive inventory on public and private land, but according to estimates, invasive plants have affected more than 100 million U.S. acres — including tens of millions of Forest Service acres, and over 2.4 million acres of the National Wildlife Refuge System. In addition, invasives spread daily: The U.S. Department of Agriculture estimated in a 1998 study that invasive nonnative plants spread on public lands at the rate of 4,600 acres per day.

Japanese knotweed crowds out natives in much of the eastern US, some western states, and has even reached Alaska.

Different species plague each region, explains Cynthia Huebner, research botanist with the U.S. Forest Service’s northern research station. Mid-Atlantic forest invasives range from Japanese knotweed to tree of heaven, Oriental bittersweet, and dozens more. Midwestern invaders include autumn olive, privets, stiltgrass, and garlic mustard.

In southern forests, according to James Miller, research ecologist for the Forest Service’s southern research station, some 10,000 native plant species must now compete with nearly 400 non-native invasive plants. The worst invaders include Japanese honeysuckle — which has affected some 12 million southern forest acres — as well as kudzu and privets.

In the Pacific Northwest, less light penetrates the canopy of dark coniferous forests, limiting non-native invasions to some extent. But problem species still include Atlantic ivy, English holly, and butterfly bush. Saltcedar trees have become the bane of western states’ riparian zones as dense saltcedar thickets tap water tables and monopolize riverbanks without providing food to local wildlife.

“There are new species coming in probably every day,” Huebner says. A small number — possibly as small as 1 percent — of non-natives turn out to be villainous. Predicting which ones will be helpful and which will be harmful is a challenge of agency cooperation and research, both at the borders and in the field.

In every region where development touches landscapes, invasives follow. “Anything that’s disturbed is more likely to be invaded,” Huebner says. Natural disturbances — a tornado, a fallen limb, a natural treefall — create opportunity for an invasive seed waiting for a patch of sunlight. Human-caused disturbances, such as new trails, roads, or mowing, bring additional opportunities. In contrast, less fragmented wilderness forest tracts stand the best chance of shutting out invasives.

For these reasons, “urban and suburban areas are much more affected than rural areas,” Bergmann says. That’s because most invasives first take hold in urban regions, where vines and shrubs are introduced as landscaping or erosion control, or tracked in as seeds by hikers or vehicles. The plants then spread to nearby natural areas, where visibly distressed forests are enough to call some to arms.


Weed Warriors To The Rescue

The Capital Crescent Trail near Washington, D.C., cuts through a skinny urban forest tract under constant siege. “I don’t want to live in a neighborhood where vines pulling trees down is normal,” says Lynnwood Andrews, as she joins up with other weed warriors early one August morning. In a gorge below the urban trail, mile-a-minute weed grows over dense clumps of porcelain-berry — invasives conquering invasives — a poignant snapshot of ecological anarchy.

Weed warrior coordinator David Brooks is literally up to his waist in invasives.

For weed warrior volunteers, part of the reward is saving a sapling from a net of porcelain-berry, or seeing plots that used to drown in vines begin to host small native bushes again. They also like the fact that they can do something tangible to defend the local forests they love.

On a small scale, such labor-intensive hand-pulling can successfully keep native plants safe while getting to the root of the problem. But knowing the enemy’s seasonal schedule is key. Because of invasives’ long-lasting seed banks, volunteers must wrangle weeds before they go to seed, or hand pulling is useless until next year, when the problem has multiplied.

Though many local parks rely on volunteers to help control invasives in patches, they’re only a small part of our regional and national defense, which managers are piecing together with limited funds.


Defending Forests

To someone looking out over an acre cloaked in leafy vines or carpeted in alien grasses, ecological ruin seems imminent. But researchers, foresters, and ecologists are figuring out how to halt the enemy advance. Prevention, they agree, is our best defense.

In Wisconsin, Department of Natural Resources Invasive Plant Coordinator Kelly Kearns is on high alert for stiltgrass, which hasn’t yet spread to that state. She’s implored residents to report any sightings of the invader, so that they can be eradicated immediately. So far, she has been successful at keeping stiltgrass out of state lines.

For a preventive strategy to work, education is key. Teaching homeowners and landscapers how to identify local invasive threats and the risks of planting invasive species (such as barberry, winged burning bush, and English ivy) would help slow the invasives’ advance. Keeping problem plants from spreading also requires careful cleaning of forestry and firefighting equipment, especially tire treads, to prevent spreading seeds to other natural areas.

The second line of defense involves early detection and rapid response. Weed warrior volunteers, cooperative weed management groups, hikers, and hunters can be invaluable in bringing attention to new infestations. At that point — when the invasion is small — hand-pulling can be effective.

This mid-Atlantic forest has been spot-treated with herbicide.

When a nuisance plant becomes established in thick stands, however, agencies and land managers must resort to treatment, which can include integrated pest management techniques, annual mowing, or chemical treatment. For infestations of some species that gain a foothold — such as cogongrass — herbicides are the only option. The most common herbicides, like glyphosate (Roundup) or triclopyr (Garlon), kill all that’s green, but new grass-specific herbicides target just the grasses and leave other plants alone.

During treatment, the biggest mistake that forest managers can make is to remove invasives but ignore the next steps: re-treating an area in subsequent years, and replanting the native species. “You may end up in a money pit or make the problem worse,” says Huebner, unless you stick with treatment for years. Over time, infested acres can be returned to their natural state.

Still, we’ve much to learn. How to stop invasive plants from spreading is a topic that the U.S. Forest Service and other agencies are grappling with, Miller notes.

Better laws would help. Some states, such as Washington and Oregon, have state noxious weed laws, which require the removal of certain problem invasive species on both private and public properties. Better cooperation between landowners would strengthen our defenses, too. There’s often a gray area regarding who has responsibility for marginal lands bordering highways and railways, where invasives tend to thrive.

Meanwhile, anyone who cares to defend local forests can join the invasive-fighting ranks by learning what enemy plants look like, gearing up in leather gloves, and squelching local invasive uprisings.

– Carrie Madren writes from Olney, Maryland, and can be reached at

This article was published in the Winter 2011 issue of American Forests magazine.