Sprawling patches of Japanese knotweed are choking creekbanks and roadsides all over Northeastern Pennsylvania this summer, highlighting the persistence of the invasive species.
What makes knotweed especially challenging is not just how fast it grows, but how easily it spreads and how resistant it is to removal. Those traits have made it one of the region’s most challenging invasive plants, a problem rooted in the biology of the species itself.
Ecological impacts
Originating from east Asia, Japanese knotweed grows rapidly and develops an extensive underground root system. Introduced to the United States in the late 1800s, the plant now appears throughout Pennsylvania and much of the Northeast, often along waterways, roadsides and disturbed soil.
AISHWARYA BEHL
Early flower clusters emerge on Japanese knotweed along York Farm Road in Pottsville, marking the plant’s peak summer growth. (AISHWARYA BEHL/STAFF PHOTO)
Knotweed lines a shaded stretch of York Farm Road in Pottsville, where the invasive species has taken hold along the roadside. (AISHWARYA BEHL/STAFF PHOTO)
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AISHWARYA BEHL
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Early flower clusters emerge on Japanese knotweed along York Farm Road in Pottsville, marking the plant’s peak summer growth. (AISHWARYA BEHL/STAFF PHOTO)
The plant can reach up to 10 feet tall, but its real strength lies below the surface. Knotweed spreads through an interconnected network of rhizomes, underground stems that store energy and regenerate each spring. Even small fragments of these rhizomes can form new plants, allowing knotweed to establish quickly and persist for years.
“It really likes streambanks and ditches close to water. Stormwater breaks up those underground rhizomes and carries bits downstream. They get jammed someplace, and then they’ll start a new plant,” said Tim Abbey, a horticulture educator with Penn State Extension’s Green Industry Team.
Knotweed often appears in concentrated pockets, particularly along steep slopes and creek corridors.
“It gets so thick, it’s like being in the jungles of Japan. It grows tall and dense, and once it takes over a hillside, you can barely walk through it,” said Ross Mullin, owner of Mullin Lawn & Garden Care in Pottsville.
Knotweed’s dense canopy creates a monoculture, crowding out nearly all native vegetation. Its thick stems and broad leaves block sunlight, preventing other plants from establishing.
“If I can grow eight feet tall, I can shade out anything below me,” Abbey said. “And I’m not aware of any animal that actually eats the foliage, so there’s no nutritional value for native wildlife.”
Along streambanks, knotweed provides poor erosion control compared to native vegetation. Over time, its presence can degrade aquatic habitat and water quality. Penn State Extension notes that knotweed infestations can reduce biodiversity, impair water quality and damage infrastructure.
Effective strategies for managing knotweed
Removing Japanese knotweed is far more difficult than most homeowners expect. Disturbing the soil, especially by digging, often makes the problem worse.
“You can’t really dig it out because digging just fractures the rhizomes. If you take sections of the rhizome and dump them somewhere else, that’ll spread it too,” Abbey said.
Cutting stems also fails to kill the plant. Both the rooted stem and the trimmed portion can regrow if they contact moist soil.
“You can treat it, you can cut it, but if even a little bit of that root system is left behind, it’s going to come right back,” Mullin said.
Because knotweed is a perennial, control requires multiple years of consistent management.
Abbey said the most reliable approach combines repeated mowing to weaken the plant with targeted herbicide treatments later in the season, when knotweed is sending energy downward into its rhizomes for the next year. Treatments applied during this window move into the underground system and help kill the source of regrowth.
“It takes a lot of effort, time and money to actually get rid of it,” he said.
For residents looking to learn more about identification and long‑term control, Penn State Extension is hosting a virtual webinar on Japanese knotweed on Friday, Aug. 7 at 1 p.m. Details and registration are available through Penn State Extension.
A growing challenge for Pennsylvania
Knotweed’s persistence mirrors a broader challenge facing Pennsylvania’s landscapes.
As invasive plants continue to spread, they displace native vegetation, weaken streambanks and strain local ecosystems.
Knotweed is only one example of how quickly non‑native species can take hold, and how important early action is.
“If you notice something on your property that’s manageable — knotweed, stiltgrass, lanternflies — you should try to control it so it doesn’t continue to spread,” Abbey said. “It’s a very serious issue.”