Japanese Knotweed
Fallopia japonicaPolygonaceae
Whole Plant
Flower
Leaf
Stem
Whole PlantFlowerLeafStem
Photos (c) Cheryl Comeau Beaton unless otherwise noted

Synonyms:

  • Japanese knotweed, Mexican bamboo, Fleece flower
  • Polygonum cuspidatum, P. zuccarinii, Pleuropterus cuspidatus, Pleuropterus zuccarinii, Reynoutria japonica

The Plant:

  • An upright, herbaceous perennial with a shrub-like stature. Appears woody. Height ranges from 3 to 10 feet tall.

The Leaves:

  • broadly oblong-ovate
  • triangular
  • pointed and tapered at the tip

The Stem:

  • round
  • hollow
  • scales
  • smooth
  • swollen at joints

The Flowers:

  • White, five-petaled small flowers that are sometimes tinged with green. The flowers are attractive and grow on branched racemes. Blooms from August to September.

The Fruits:

  • small, winged fruits, beige to light brown
  • contain triangular, shiny, black, smooth, 3-angled seeds

The Habitat:

  • prefers full sun, tolerates light shade, salt, high temperatures, dry soil
  • intolerant of frost
  • thrives in wetlands, roadsides, disturbed areas

Dispersal:

  • winged fruits are wind dispersed
  • spreads mostly vegetatively via long rhizomes

Key ID Features:

  • shrub-like
  • triangular pointed leaves
  • greenish-white flowers
  • winged fruits
  • leaves zigzag on stem

Similar Species:

  • Giant knotweed has leaves greater than 30 cm long, deeply cordate leaf bases, and visible, multicellular hairs on the undersides of the leaves. The hybrid of Japanese and giant knotweed (P. x bohemica) has an intermediate leaf size, cordate to truncate leaf base, and stout, single-celled hairs on the undersides of the leaves, giving it a feel that is rough to the touch. Japanese knotweed has smooth leaf undersides with no hairs. Leaf hairs can be observed with a strong hand lens but on giant knotweed should even be visible to the naked eye.

Notes:

  • Due to its long rhizomes and winged fruits, this plant can travel long distances. This plant creates a monoculture, outcompeting native plants and destroying botanical diversity. Eradication can be quite difficult due to the plant's ability to regenerate from tiny rhizome fragments or even stem nodes.

Growth Form: Herb

Origin: Japan, China, Korea

Level of Invasiveness for Nantucket: Invasive

Level of Invasiveness for Massachusetts: Invasive

Massachusetts Cultivation Restrictions: importation ban, propagation ban




Credits: The Electronic Field Guide to the Invasive Plants of Nantucket (c) 2005-2006 Maria Mitchell Association, EFG Project, UMass Boston