What is Japanese knotweed?

Japanese knotweed is an aggressive perennial from eastern Asia that was brought to North America as an ornamental in the late 1800s. It’s now one of the most widespread invasive plants along Pennsylvania’s streams and roadsides, and it’s on the state’s noxious weed list as a Class B weed. Getting your Japanese knotweed identification right early matters, because a small patch is a project and a big one is a multi-year commitment.

It grows from a huge underground network of rhizomes. Above ground it forms dense stands of tall, hollow canes that shade out nearly everything else, and along streams it can leave banks bare and prone to erosion once the canes die back in winter.

Japanese knotweed identification: key features

Knotweed is easiest to recognize from June on, when it’s head-high or taller. But it has a distinct look at every stage.

• Spring shoots: In April, bright red to purple shoots push up, looking a bit like fat asparagus, tipped with rolled-up leaves.
• Canes: Smooth, hollow, jointed stems that look like bamboo, green with red or purple speckles. They zigzag slightly between leaves and often reach 6 to 10 feet.
• Leaves: Broad and spade-shaped, up to about 6 inches long, with a flat, squared-off base and a pointed tip, alternating along the stem.
• Flowers: In late summer, creamy-white flowers hang in finger-like sprays from the leaf joints. Bees work them heavily.
• Winter: The canes die back to tan or reddish-brown and stand all winter, which makes stands easy to map.

Where it grows in Pennsylvania

Knotweed thrives in sunny, at least seasonally wet ground: stream banks, river floodplains, ditches and the edges of wetlands. Along central PA waterways like the Susquehanna and its tributaries it’s everywhere. It also handles tough spots like mine spoil, salty road shoulders and old fill.

Floods break off pieces of rhizome and stem and carry them downstream, where they root. People spread it too, by moving soil that contains root fragments, mowing it, or tossing cuttings over the bank.

Japanese knotweed look-alikes

Giant knotweed and the hybrid Bohemian knotweed also grow in Pennsylvania. Giant knotweed has much bigger leaves with a heart-shaped base; the hybrid falls in between. They’re all invasive and handled the same way, so you don’t need to sort them out exactly.

Bamboo is the most common mix-up. Bamboo leaves are narrow and grass-like and stay green through winter, and its canes are hard and woody. Knotweed leaves are broad, and the plant dies back every fall.

How to get rid of Japanese knotweed

The goal is to kill the rhizomes, not just the canes. Cutting, pulling or mowing alone won’t do it, and they often help it spread. Plan on at least two growing seasons of treatment, then a few years of watching for new shoots.

• Small patches by hand: For a young, small stand, cutting canes to the ground every two to three weeks all season, year after year, can eventually exhaust it. It takes persistence, and you must keep every scrap of stem away from soil and water.
• Don’t dig casually: Digging leaves root pieces behind that sprout, and moving that soil spreads knotweed to new places. Excavating a stand is a specialized job.
• Cut-then-treat: Penn State recommends cutting in June, then treating the shorter regrowth with herbicide at least eight weeks later. This works well near water where you need to control spray.
• Herbicide: Glyphosate applied from July until frost, and especially in late summer and early fall when the plant is sending sugars down to its roots, is the main extension recommendation. It has no soil activity, and aquatic-labeled formulations are available for stream banks. Triclopyr and 2,4-D burn the leaves but do little to the rhizomes. Imazapyr works but can injure nearby trees through the soil.

Read and follow the product label; the label is the law. Near streams or ponds, you must use a product labeled for aquatic sites. Applying herbicide for hire in Pennsylvania requires a PA Department of Agriculture pesticide applicator certification, and large or streamside stands are best handled by a licensed vegetation-management company.

Bag or dry cuttings on a tarp until they’re completely dead. Don’t compost them or toss them over the bank.

Keeping Japanese knotweed from coming back

Expect small shoots for a year or two after a stand seems dead. Treat or cut them right away. Once it’s under control, plant fast-growing grasses or native shrubs to hold the soil, especially on stream banks.

Be careful with fill. Don’t accept topsoil or fill from a site with knotweed, and if you have knotweed near an area you plan to dig, deal with it before the work starts.

Questions we get about japanese knotweed

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