Japanese knotweed

Identification

Look for the combination of characters below. A single feature is rarely enough, especially when a native look-alike is present.

Height and habit

Grows up to 11 feet tall and forms dense thickets. The stems are jointed and hollow, superficially resembling bamboo.

Stems

Bright green and smooth when mature, with red or purple nodes and darker spots or streaks. Leaf-bearing sections zigzag from node to node.

Leaves

Alternate, spade- or heart-shaped and up to 6 inches long. The leaf base is squared off, which helps separate it from giant and hybrid knotweed.

New shoots

Emerging in early spring, bright red or purple and tipped with furled, distinctly triangular leaves.

Flowers

In late summer, white or pale green finger-like clusters 3 to 4 inches long, made up of many small five-petaled flowers.

Roots and rhizomes

An extensive network of horizontal underground stems. Small fragments can produce new plants, and cut stems can regrow if they remain in moist soil.

Look-alikes

• Bamboo

  Bamboo has slender, papery leaves that persist year-round and woody, jointed stems. Living knotweed stems are herbaceous and visibly wet when cut.

• Giant knotweed and Bohemian knotweed

  Reynoutria sachalinensis and Reynoutria × bohemica

  Giant knotweed leaves can reach 1 foot and have a rounded base; Japanese knotweed leaves are smaller with a squared base. The two species form the hybrid Bohemian knotweed.

• Broad-leaved dock

  Rumex obtusifolius

  Broad-leaved dock grows as a rosette of basal leaves from a central taproot, while Japanese knotweed sends up many single, rapidly elongating stems.

Range and status

Native range: East Asia

Regulatory status by state

• Pennsylvania Noxious weed

  Japanese knotweed is a Class B noxious weed on the Pennsylvania Department of Agriculture list. Noxious weeds cannot be sold, transported, planted or otherwise propagated in the state. Class B means the plant is widely established and cannot feasibly be eradicated.

  Pennsylvania state profile Regulatory source

• New York Prohibited

  Japanese knotweed is prohibited under 6 NYCRR Part 575. Prohibited plants cannot be sold, imported, purchased, transported, introduced or propagated in New York.

  New York state profile Regulatory source

• Virginia Noxious weed

  Japanese knotweed is a Tier 3 noxious weed in Virginia under 2VAC5-317-20. Moving a listed noxious weed requires a valid certificate or permit.

  Virginia state profile Regulatory source

• ohio Noxious weed

  Japanese knotweed is a prohibited noxious weed under OAC 901:5-37-01, which requires management on public and private property. It is also on Ohio's invasive plant list under OAC 901:5-30-01.

  Regulatory source

• wisconsin Restricted

  Wisconsin classifies Japanese knotweed as Restricted under NR 40. Restricted species cannot be transported, transferred or introduced without a permit; control is encouraged but not required.

  Regulatory source

Why it matters

Dense knotweed thickets shade out other vegetation and form monocultures. Along streams, the plant provides poorer erosion control than native vegetation and can degrade aquatic habitat and water quality. Rhizomes can also damage road and bridge foundations and other infrastructure.

Management