WSU Clark County Extension
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Japanese Knotweed

Scientific name: Polygonum cuspidatum
Taxonomy
Family:Polygonacae
Type:Invasive plants
Native:NO
Plant Requirements
Zone:6 to 9
Moisture:Medium
Plant Characteristics
Height:5 ft
Width:10 ft
Bloom:Summer flowers
Bloom Time:June to September
Bloom Color:White
Additional Characteristics
Trees


Fruit

Leaves


Bark
Wildlife value

Poisonous
Description Japanese knotweed is designated a noxious weed in the State of Washington, as it displaces other plants while it spreads and is difficult to eradicate. This invasive perennial has extremely vigorous under-ground rhizomes (as deep as 3’) that form a deep, dense mat. An alternative name for knotweed is Mexican bamboo.

  Morphology:
Japanese knotweed is an upright, shrub like, herbaceous perennial that can grow to over 10 feet in height. As with all members of this family, the base of the stem above each joint is surrounded by a membranous sheath. Stems of Japanese knotweed are smooth, stout and swollen at joints where the leaf meets the stem.

Although leaves may vary in size, they are normally about 6” long by 3” – 4” wide, broadly oval to somewhat triangular, pointed at the tip and squared off at the base.

The minute greenish-white flowers occur in long, rather attractive, lacy clusters. Flowers give rise to small winged fruits containing very small seeds (1/10”) which are triangular and shiny.

  Adaptation:
Japanese knotweed can tolerate a variety of adverse conditions including full shade, high temperatures, high salinity and drought. It is found near water sources, such as along streams and rivers, in low-lying areas, waste places, utility rights-of-way and around old home sites. It can quickly become an invasive pest in natural areas after escaping from cultivated gardens. As little as a 1” piece of rhizome is enough to start a whole new plant!

  Pests:
No natural known pests. Recently, herbicide injections into the hollow stems at the base of plant have proven very effective in killing individual stems. However, each stem in a clump has to be injected.
For assistance, contact Dr. Charles Brun (brunc@wsu.edu), (360) 397-6060 5701
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