Illustration picture shows Japanese knotweed, in Lierde, Tuesday 15 August 2023. Japanese knotweed grows fast and pushes other species away, which means that the birds and insects that those plants need to survive are also in danger of disappearing. …
PHILADELPHIA - Japanese knotweed can grow rapidly along roads, waterways and disturbed areas, but removing the bamboo-like invasive plant is much more difficult than cutting down its visible stems.
What is Japanese knotweed?
Japanese knotweed is an invasive perennial plant originally introduced to North America in the late 1800s for ornamental use, forage and erosion control.
It can now be found throughout much of the United States and Canada, including Pennsylvania.
The plant commonly grows in areas with moist soil, sunlight and human disturbance. That includes stream and riverbanks, roadsides, railroad corridors, utility rights-of-way, wet meadows, vacant lots and other disturbed areas.
What Japanese knotweed looks like
Japanese knotweed has erect, hollow stems that are light green, smooth and jointed. The stems are swollen at the nodes, giving the plant a bamboo-like appearance.
It generally grows between 3 and 12 feet tall. Early in the growing season, new shoots can grow 3 to 4 inches per day.
Japanese knotweed leaves have pointed tips and squared-off bases. The plant produces greenish-white flowers about 4 inches long from August through October.
The plant may also be visible during winter and early spring as clusters of dry, reddish-brown stems left behind from the previous growing season.
Why it spreads so easily
Japanese knotweed spreads primarily through underground stems known as rhizomes.
Those rhizomes can be moved by flooding, erosion or soil disturbance. Even small pieces left in the ground can produce new plants.
Cut or broken stems can also take root when left on moist soil or placed in water. The plant produces some viable seeds, but its underground system is the primary way it spreads.
Japanese knotweed in flower (Photo by Kelly Sitch, DCNR - BOF).
Why Japanese knotweed is difficult to remove
Controlling the rhizomes is the key to managing Japanese knotweed.
Pulling or digging may work for a small number of plants when the full underground system can be removed. However, any pieces left behind can resprout.
Cutting or mowing the visible stems alone will not control an established population. The Pennsylvania Department of Conservation and Natural Resources says cutting later in the growing season can reduce the height of regrowth, but additional treatment may still be necessary.
Property owners should confirm the plant’s identity and consult local or state guidance before attempting treatment, particularly when it is growing near water.
Why it matters
Japanese knotweed can quickly form dense stands that crowd out native vegetation.
Along streams and rivers, those thickets can displace streamside plants, increase bank erosion and reduce habitat quality for fish and wildlife.
Once a large stand becomes established, it can be extremely difficult to eradicate.
What one Philadelphia group is trying
Friends of the Wissahickon and Philadelphia Parks & Recreation began piloting a mechanical control method near the Upper Valley Green Lot in spring 2026.
The project uses metal mesh that is intended to girdle, or choke, the knotweed stems as they become thicker. The groups plan to monitor the area to determine whether the method could eventually be used at other locations in the park.
A Penn Charter student designed and produced a specialized stake with a 3D printer to help hold the mesh in place.
Friends of the Wissahickon said the experiment could take years or even decades to fully control the knotweed.
The Source: This article was written using information from the Pennsylvania Department of Conservation and Natural Resources and Friends of the Wissahickon.