Steep hills, herbicide bans and human disturbance create ideal conditions for invasive species to take over. Conservationists cannot keep pace.
Story by Eric Jankiewicz. Art by Tim Robbibaro. NewsWorks Lab contributors.
Visitors to Pittsburgh who stand on the overlook beneath Mount Washington’s flagpole this fall are more likely to see a vista of surrounding treetops and overgrown bushes than of the iconic Downtown skyline.
That’s because, in part, plants from distant shores like Japanese knotweed (Reynoutria japonica) are gaining an advantage due to a combination of factors that include climate change, a lack of natural predators and manmade land disturbance, biologists and plant experts told NewsWorks Lab.
As invasive species take root here in larger numbers, they threaten not only the view and the native plants – but also the entire ecosystem, said Brandon McCracken, an ecological restoration manager with the Pittsburgh Parks Conservancy. Each plant can support thousands of organisms.
“When those native plants disappear, and we allow invasive plants to reduce biodiversity and take over wildlands, we have to expect a decrease in biodiversity and what that land can support,” he said. “It’s critical to have healthy ecosystems, and native plants is where that starts.”
Across western Pennsylvania, invasive species are taking root in unexpected and hard-to-manage places, experts said. That includes Mount Washington, but also Slippery Rock Creek north of Pittsburgh, where knotweed crowds the stream banks, and the Laurel Highlands, where Japanese stiltgrass (Microstegium vimineum) chokes out native ferns in the undergrowth.
Awareness and education are helpful in conservation efforts, but restoration remains key, said Mandy Smith, an educator and master gardener coordinator for Penn State Extension in Westmoreland County. The service provides guidance to farmers, agrobusinesses and other growers. It offers webinars and in-person presentations about invasive species.
“Ensure restoration plans are in place,” she said. “Since knotweed is aggressive and is found along waterways or in vulnerable areas, plants need to be put in after removal to reduce erosion or the continued occurrence of invasive species and/or undesirable weeds.”
At the Pittsburgh Parks Conservancy, McCracken and five other people work full time to maintain various parks and green spaces across the city. This includes Emerald View Park in Mount Washington, known for its stunning views, steep streets and hillsides.
“We do as much as we can and get seasonal help and volunteers, but at the end of the day it’s a small group responsible for maintaining a lot of acres and high demand landscape spaces,” McCracken said.
Coal Hill
The spread of invasive plants in Mount Washington presents unique challenges: steep hillsides that are prone to landslides and hard to reach, a city-wide moratorium on herbicide use in parks, and disturbed soil perfect for invasive expansion, city officials and other experts said.
The challenges are not limited to high-profile areas either. Trellises of stretching knotweed leaves cover hillsides along William Street, which cuts across the eastern face of Mount Washington and ends directly above the Liberty Tunnel. The vines create deeper blind spots and bottlenecks against opposing traffic, said Paul Martinez, a Mount Washington resident for more than a decade.
“The city doesn’t seem to know the difference between unsightly and dangerous, and there are areas that are so overgrown on city steps, trails, and roads that you can’t even get through,” Martinez said.
Pittsburgh’s Department of Public Works has “some backlog in our overgrowth service requests due to some equipment being down – in this case, a specific mower that’s needed to address the invasive knotweed and other overgrowth,” a spokesperson said by email. “DPW is looking into this location to see how they can best handle clearing it.”
Marya Pittaway, a Pittsburgh Park Ranger responsible for Emerald View Park and other areas, said the city simply cannot keep up with the invasive plants.
“We’re very understaffed,” Pittaway said. “Ideally, we would put more resources toward invasive mitigation.”
Roots of the problem run deep, she said. Human disturbance creates pathways for invasive species, and Mount Washington – an area once known as Coal Hill – has a history based on disturbing the land.
“This whole hillside was denuded up here in the 1800s,” Pittaway said. “The response was to plant Norway maple, kudzu and other invasives.”
At the time, Pittaway said, people didn’t have an understanding of native versus non-native plants and the dangers around planting non-native species. Even many botanists valued these plants for their ability to grow quickly and prevent soil erosion, with little consideration for how they might overtake native environments, according to the Alabama Cooperative Extension System.
“Where you have established invasive plants are places where humans have had a huge impact, messed up the soil, done construction,” McCracken said. “Native ecosystems that are really healthy and contiguous and have good soil quality … are generally hard to invade for other plants.”
Misconceptions still contribute to the proliferation of exotic species, McCracken added. He cited bamboo, of which some species can be difficult to contain or remove once introduced, according to the Brandywine Conservancy in Chadds Ford, Pa.
“A lot of times people just don’t know,” he said. “They see bamboo and it doesn’t register as a problem.”
No herbicides
The invasive plants that grow in the city’s parks pose a unique challenge, McCracken and Pittaway said, because the city bans the use of herbicides in its green spaces.
The use of these chemicals has been identified by the Pennsylvania Department of Conservation and Natural Resources as the only guaranteed method of controlling certain plants like knotweed once they have become established.
“Mechanical methods alone are largely ineffective” because of the rhizome-root network of the plant, the department notes.
“A lot of the woody species, you can make a lot of progress with hand-pulling,” McCracken said. “With knotweed, that doesn’t work. You need some amount of herbicide application.”
Because of the restriction in herbicide use, Pittaway said she is experimenting with alternative ways to beat back many of these invasive plants. A master gardener since 2019, Pittaway is introducing native plants like Staghorn Sumac (Rhus typhina) in some areas to have those plants outcompete invasives.
Sisyphean task
For Dani Watson, who lives in the Southside Slopes, the annual removal of knotweed from her backyard seems to be a Sisyphean act, with the plant growing back each time she removes the plants.
“I cleared out almost half of my yard, and I read that if you pick the stalks when they’re little, the whole thing will eventually die, but I was not able to keep up with that even though it was working a little,” Watson said. “It’s all grown back now, 10-12 feet tall.”
As the knotweed continues to take over Watson’s backyard, she worries the plant’s strong root system will threaten the foundation of her house. Knotweed poses threats to property because it can puncture pavement and crack concrete, according to the National Park Service.
“It’s growing up through my walkways, everywhere,” Watson said.
Even if she removes the plant from her yard, her neighbors on both sides still have it growing in theirs.
Now, it’s coming for more.
“I can’t keep up with it,” Watson said. “It has been spreading still, and I’m seeing it move up the side towards the front of the place.”
Know your invasives
Penn State University’s College of Agricultural Studies provides extensive information on identifying and managing invasive species. Some of the more common plants in western Pennsylvania are:
- Japanese knotweed (Fallopia japonica) grows in tall, dense stands up to 10 feet tall that exclude other vegetation. Native to East Asia, it was imported to North America as an ornamental plant in the late-1800’s. It grows almost anywhere, from acidic soil in full sun to shady alluvial soils along rivers and streams.
- Japanese stiltgrass (Microstegium vimineum) has a sprawling, almost vine-like appearance, with a canopy height of between 12 and 36 inches. Its seedheads emerge late-August to early-September with infestations that commonly start along road or trail edges, and then spread outward.
- Oriental Bittersweet (Celastrus orbiculatus) and Garlic Mustard (Alliaria petiolata) grow lower to the ground and are sometimes harder to spot. Despite their smaller sizes, these plants can pose an even greater risk to an area’s biodiversity.
- Tree of Heaven (Ailanthus altissima), which serves as an early food source to the equally invasive and destructive spotted lantern flies. Like Knotweed, it poses a risk to native plants because of how quickly it can spread via its root system.
Eric Jankiewicz and Tim Robbibaro are contributors to NewsWorks Lab.
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