Foreigners are taking over the Chesapeake Bay region, but not the ones some politicians are talking about this election season.

Largely unnoticed and even abetted by unwitting residents, legions of plants from Asia, Europe and other parts of the world are spreading across the 64,000-square-mile watershed, gradually replacing many of the grasses, wildflowers, shrubs and even trees that have grown here for millennia. Aquatic invaders, such as the northern snakehead and blue catfish, seem to capture all the attention. Less noticed are invasive plants in upland areas, such as the mile-a-minute weed and Japanese stiltgrass.

It’s more than just a worry for garden clubs. Scientists say the rampant spread of nonnative vegetation portends a dramatic transformation of the landscape, with worrisome implications for the diversity of plant, insect, avian and other animal life in the Bay watershed — and possibly even for human health as well.

“Nonnative plants are an ecological disaster,” said Doug Tallamy, a professor of entomology and wildlife ecology at the University of Delaware.

Some exotics — a label sometimes applied to non-native plants — crowd out native species, reducing food sources on which wildlife and people depend. Invasive non-natives attract fewer pollinators, reduce biodiversity and push rare species closer to extinction. They can be a nuisance to humans, too, overgrowing trails and stream banks, impeding hiking, bicycling and fishing. And a pair of exotic bushes may even be aiding in the spread of Lyme disease.

All levels of government and environmental groups throughout the Bay watershed recognize the threat and have taken steps to fight it. Brigades of volunteer “weed warriors” turn out regularly to uproot invasive plants, vines and even trees in parks and other sites.

But the response across the watershed has been halting, uneven and no match for the onslaught. Experts say these outsiders are here to stay.

“Eradicating non-native invasives is impossible,” said Carole Bergmann, a forest ecologist with the Maryland-National Capital Park and Planning Commission.

Bergmann founded Montgomery County’s weed warrior program 17 years ago to preserve native vegetation in areas beyond the local park system’s resources. She figures that she’s recruited and trained 600 volunteers since then. Despite that effort, she said, “Things are definitely much worse today than 20 years ago.”

It’s a national, even global, problem, but one with particular resonance for the Chesapeake Bay, which has been the focus of a three-decade, multibillion-dollar effort to restore its water quality, fisheries, vital habitats and treasured landscapes.

Invasion started in 1600s

Since the 1600s, thousands of species of trees, plants and vines have been imported for nutritional, medicinal and ornamental purposes. Until the mid-20th century, though, few realized that some of those imported plants would go wild, spreading on their own beyond the places where they were planted.

U.S. Forest Service scientist Cassie Kurtz has been tracking their expanding footprint for more than a decade in woodlands in the Northeast and Midwest. Analyzing forest plot surveys from 2005 through 2010 — including sites across all of the Bay watershed except Virginia — she tallied extensive reports of invasive plants such as garlic mustard, Japanese honeysuckle and multiflora rose. The frequency with which they turned up ranged from 10 percent in Maine and New Hampshire to 94 percent in Ohio and Iowa. Each plot is one square mile.

She’s working now on an update of her earlier study, using 2014 and 2015 data. Preliminarily, she said, it “found an increase in all of the invasives.”

Their spread can have a gradual but ultimately dramatic impact on other plants, and even animals, researchers say. Some harbor plant pathogens, while others contain toxins that may be harmful to some animals.

The food web undergirding local ecosystems is being destroyed in many places by invasive plants, contends Tallamy, the University of Delaware ecologist. His own research has found that alien ornamental plants support 29 times fewer animals than do native ornamentals. Insects have evolved over eons to feed on certain plants, he explained, and plants likewise have developed specialized chemicals and traits to attract certain insects.

For instance, a chickadee needs 6,000–9,000 caterpillars to feed her young from the time they hatch until they can fly, he said, and the mother will need thousands more herself over the 30 days before the chicks leave the nest. If there are no plants that caterpillars eat, he suggested, both they and chickadees will disappear from the area.

One insect does seem to thrive in certain invasive bushes. It’s the black-legged tick, better known as the deer tick, the sometimes-carrier of the pathogen that causes Lyme disease.

Infestations of Japanese barberry, a big seller at garden centers, can have almost nine times as many deer ticks as can be found in native bushes, said Marc Imlay, a biologist and ranger with the Maryland-National Capital Park and Planning Commission. Asian honeysuckle harbors 10 times as many, Imlay said, citing peer-reviewed research papers.

Deer, a primary host for black-legged ticks, like to rest in thickets of the invasive bushes, Imlay explained. The ticks, which picked up the Lyme pathogen earlier by feeding on infected rodents, then drop off into the bush to await the next meal.

Deer contribute to the invasive plant problem in other ways as well. Their populations swelled, owing to a lack of natural predators and a decline in hunting. They feed on native plants and tree saplings, but generally not on the exotic counterparts, said Robert Taylor, an ecologist with the city of Alexandria, VA. The paths the animals create through the woods also give invasive seeds bare ground in which to take root.

While there is widespread recognition throughout the Bay watershed that invasive species pose problems, the restoration effort has focused mainly on incursions of nonnative fish and the imported wetland grass, phragmites.

The Chesapeake Bay Watershed Agreement signed in 2014 doesn’t say anything about invasive species, much less plants. They do get mentioned, though, as one of the threats to the health of urban forests in the management plan that’s been developed for increasing and restoring vital habitats.

Money for invasive terrestrial plant management is in short supply, said Sally Claggett, the U.S. Forest Service coordinator at the Bay Program. One reason is that “it’s not exciting work for funders,” who prefer supporting tree planting and other activities where the results are obvious.

Just how much is being spent to battle invasive plants isn’t clear. Federal, state and local funding for such efforts are spread through various budgets, and often lumped with money spent on invasive species of all kinds.

In Maryland, the State Highway Administration is spending $2.8 million a year to replace kudzu and other plants along seven major highways, said Pam Milby, assistant chief of SHA’s landscape operations.

The targeted roads include the entire Maryland portion of the U.S. Capital Beltway and sections of the Baltimore Beltway, U.S. 50, and three other limited access highways, Milby said.

Native species losing ground

The effects of limited funding are obvious, even in relatively well-off jurisdictions.

Cromwell Valley Park is a jewel of Baltimore County’s park system. Its 460 acres of rolling hills, once two farms, are covered with fields and forests riddled with nonnative plants, despite the valiant efforts of some.

Laurie Taylor-Mitchell, who revived the weed warrior program in 2015, is leading Cromwell Valley’s volunteer struggle to manage invasives there. On a recent stroll through the park, she pointed to one infestation after another.

“There’s mile-a-minute, a vine that sticks to everything. There’s multiflora rose, and there’s a ton of Japanese stilt grass. The unholy trinity!” Taylor-Mitchell exclaimed.

She said she and others have pulled up fields of Japanese stilt grass and planted native grasses, only to see the stilt grass roar back. She explained that the imported annual lays down a seed bank every year slightly below ground level.

“The layers of seed build up to the point that the native plant’s seeds cannot make it down to root,” Taylor-Mitchell said. At another spot, mile-a-minute is coiled around delicate native ferns. “They are just hanging on by their fingernails,” she said.

Then, at the edge of a forest, the vine oriental bittersweet has enveloped two medium-size trees.

“These vines are always marauding around, trying to find something. This tree has died; this tree is under serious attack,” she said. Oriental bittersweet and another invasive vine, porcelain berry, block the sunlight that trees need to survive.

Herbicides are the only effective way to manage major infestations of invasive plants but they are a no-go for volunteers in the park system because herbicides can only be applied by trained, certified staff or contractors.

In any case, she asserted, there is nowhere near enough money spent for invasive plant removal in Baltimore County. “Is there anybody out there listening?” Taylor-Mitchell asked. “We have been abandoned by all levels of government.”

Even as weed warriors battle uphill against exotic invasive plants, scores of them are sold in nurseries and planted by landscapers. At Home Depot, for example, consumers can pick up Japanese barberry, burning bush, border privet, English ivy and multiple species of honeysuckle — all notoriously invasive plants.

Home Depot spokeswoman Kathryn Emory said the company’s stores follow all regulations in the states where they operate.

Some retailers have chosen to stop selling some invasive plants. Behnke Nurseries Garden Center in Beltsville, MD, has cut at least nine invasive plants from its offerings over the last 20 years, said Larry Hurley, a former perennial plant buyer at Behnke and now part of the company’s management team.

The agriculture agencies in Maryland, Pennsylvania and Virginia that have sole regulatory power over the nursery industry have long banned the sale of plants deemed a threat to agriculture, such as Canadian thistle. The states even have funds to eradicate patches of those plants and other “noxious pests.”

Regulations target worst plants

Recently, though, Maryland’s Department of Agriculture took its first action against invasive plants sold at nurseries and garden centers.

Under a regulation that took effect in July, retailers are required to post caution signs next to displays of five nonnative plants: burning bush, border privet and three invasive vines that are nonnative members of the wisteria family — Chinese and Japanese wisterias and an Asian wisteria hybrid. The signs urge customers to consider buying native alternatives.

The regulation also bans the sale — next year — of three other exotic plants: shining cranesbill, yellow flag iris and fig buttercup. The prohibition was delayed until April 11, 2017, to give stores time to adjust.

The MDA’s regulatory action is the first of its kind in the three states that encompass most of the Bay watershed. Pennsylvania and Virginia are in the early stages of discussions that could lead to similar restrictions in a year or two.

Virginia recently established a Noxious Weeds Advisory Committee to make recommendations to the state agriculture commissioner. In Pennsylvania, the legislature just formed a Forestry Task Force to draft legislation for action in 2017.

In comparison, another Bay state, New York, has already banned 69 ornamental plants, bushes and trees; it also requires warning signs on six others.

New York’s ban, established in 2014, applies to all three of the plants scheduled to be banned in Maryland, as well as four of the five that now require caution signs.

Bergmann, a member of Maryland’s Invasive Plant Advisory Committee, said she expects the MDA to ban more invasive plants over time. That may slow the spread, but it won’t stop the onslaught, she warned, even if all of them were yanked from store shelves tomorrow.

The natural resources and parks agencies in the Bay states are striving to make people aware of the problems posed by invasive terrestrial plants, offering educational information on websites and through volunteer training.

Daryl Anthony, assistant secretary for land resources in Maryland’s Department of Natural Resources, said the agency is putting an unprecedented effort into improving invasive species management and removal.

In Pennsylvania, the Bureau of State Parks uses a combination of staff, contractors and volunteers to keep invasive vines at bay, said program manager Rachel Reece. But with resources limited, the effort has been prioritized to protect trees near rivers and streams, she said.

‘Global priority’ a losing battle

One conservation group some time ago came to the reluctant conclusion that it can’t turn the tide of exotic greenery taking over the landscape.

Eradication of invasives was once a “global priority” for The Nature Conservancy. After spending “much treasure and effort” worldwide, TNC “realized it was not possible,” said Sam Truslow, TNC’s land steward for Virginia. Around 600 TNC staff and many thousands of volunteers threw themselves into the effort, employing flame throwers, chainsaws and herbicides in addition to research and muscle.

The conservancy’s direct invasive plant work is now focused on saving rare landscapes, Truslow said, such as an unusual type of wetland in the Shenandoah Valley. The conservation group also collaborates with state and local stakeholders on managing invasive plants, but on a far smaller scale than it once did.

TNC and partners have managed to keep the sprawling Adirondack Park in upstate New York relatively free of nonnative invasives. But the effort there has benefited from more than a century of public and private conservation work. And the 3,213-square-mile forest remains largely intact, with relatively little development. Invasive plants tend to attack from forest edges.

The once-intact forests of the Bay watershed have been logged and carved up into farms, roads, towns and suburbs with myriad openings for invasive plants.

Saving small areas is still feasible, according to some experts. But even that hope is tempered by the limited federal, state and local funding for the work.

“For years now, I have prioritized invasive management work so that we spend most of our available funds on the best natural areas, the best biodiversity areas and most environmentally sensitive areas countywide,” Bergmann said.

Volunteers can make a difference, experts say, if their labor is concentrated in one place. Meo Curtis, director of the Maryland chapter of the Izaak Walton League, said the group has been able to remove invasives from a 250-acre tract it owns, but keeping them out requires repeated effort.

“It’s just like a black hole. It needs constant maintenance,” said Claggett of the Forest Service.