Japanese knotweed

About

A tall, bamboo-like perennial from Asia that forms dense stands up to about 11 feet tall. Its deep, far-reaching rhizomes make it one of the hardest weeds to get rid of, and tiny root pieces can start new plants.

Also called Mexican bamboo, Japanese fleeceflower, crimson beauty, fleeceflower

How to identify it

Look for

Stems are hollow and jointed like bamboo, with red or purple nodes and often dark speckles; new spring shoots are bright red to purple. Leaves are alternate, spade- or heart-shaped with a flat, squared-off base, about 4 to 6 inches long. Sprays of small greenish-white flowers come from the leaf joints in late summer.

Look-alikes

True bamboo has slender, papery leaves and woody stems that stay up all winter. Giant knotweed and the hybrid Bohemian knotweed look very similar but often have larger leaves (up to a foot long) with rounded bases; manage them the same way.

Where it grows

Stream banks, damp roadsides, yard edges, forest edges and disturbed lots; it also tolerates poor, acidic or salty soils.

When

Red shoots come up in spring, stems reach full height by early summer, and it flowers in late summer before frost kills the tops.

How it spreads

Mostly by rhizomes that can run 23 to 65 feet sideways and at least 7 feet deep; pieces about an inch long can regrow, and some stands also make seed.

Most common in

Where it is regulated

Minnesota lists it as a noxious weed on its Control list.

What to do, gentlest first

Start at the top. Weed killers are a last step; always read and follow the label.

1. Never move soil or stems from a knotweed patch Prevent

   Rhizome fragments ride along in fill dirt, on equipment and in flood water. Act as soon as you see new shoots, before a colony builds a big root system.

   Clean tools and boots before leaving the patch.

2. Cut canes and dig roots, again and again By hand

   Cut the canes and dig out as much rhizome as you can, repeating whenever it regrows. Expect it to take years; small, young patches are the only ones digging can clear.

   Let cut stems dry in the sun, then bag them. Never compost knotweed stems or roots, since pieces can regrow.

3. Knotweed psyllid (not available to gardeners) Natural enemies

   A tiny sap-feeding insect, Aphalara itadori, was approved for release in the US in 2020 and has been released in several states since. It may weaken stands over time but is not sold and is not expected to kill plants.

4. Glyphosate after flowering Conventional spray

   Spray regrowth with glyphosate from after flowering until the first frost, when the plant is moving sugars to its roots. Cutting in June and spraying the shorter regrowth at least 8 weeks later makes coverage easier. It often takes 3 to 5 years.

   Read and follow the label. Many lawn and brush killers also harm garden plants, so keep spray and drift off anything you want to keep. Use a product labeled for use near water if the patch is by a stream. Imazapyr can harm nearby trees through their roots.

Uses, wildlife and pets

For wildlife

Bees, ants, butterflies and beetles visit the late-summer flowers.

Pets

Not listed as toxic by the ASPCA