Japanese knotweed

(Reynoutria japonica)

Conservation Status
Japanese knotweed
  IUCN Red List

not listed


NNA - Not applicable

SNA - Not applicable


not listed

Weed Status

State Prohibited Noxious Weed, Prohibited – Control

Japanese knotweed is listed as an invasive terrestrial plant by the Minnesota Department of Natural Resources.

In 2004 the World Conservation Union (IUCN) put Japanese knotweed on a list of “100 of the World’s Worst Invasive Alien Species.”

Wetland Indicator Status
  Great Plains

FACU - Facultative upland


FACU - Facultative upland

  Northcentral & Northeast

FACU - Facultative upland


Japanese knotweed is a 3½ to 10 tall, erect, perennial forb that rises from a long, horizontal, spreading, underground stem (rhizome). It often forms a dense thicket. It spreads aggressively and can form colonies up to an acre in size.

The stems are annual, erect or arching, stout, hollow, and hairless. They are unbranched or few-branched, round in cross section, somewhat zigzagged, and swollen at the nodes. They are light green, often with reddish spots, or reddish-brown. Above each node the stem is surrounded by a papery, fringed sheath (ocrea). Older stems are finely ridged.

The leaves are alternate, deciduous, stalked, broadly oval, 3 to 6 long, and 2 to 4¾ wide. The base of the leaf is usually broad and straight across, sometimes slightly heart-shaped. The blade tapers to a point at the tip with concave sides along the tip. The upper surface is hairless. The lower surface is minutely rough-hairy along some of the veins. The hairs are not visible without a hand lens. The margins are untoothed.

The inflorescence is a branched, elongated, erect, 3 to 6 long cluster (panicle) of numerous flowers rising from upper leaf axils.

In its native range each plant produces either flowers with both fertile male and fertile female parts or flowers with infertile male and fertile female parts. In North America only the latter have been introduced.

The flowers are white or greenish-white and bell-shaped. There are 5 tepals consisting of 2 small inner petals and 3 larger, outer, keeled, petal-like sepals. The tepals are fused at the base. Male flowers have 6 to 8 reduced, nonfunctional stamens (staminodes). Female flowers have 3 spreading styles.

The fruit is a shiny, dark brown, hairless, three-angled, 1 16 to long nut (achene).




3½ to 10


Flower Color


White to greenish


Similar Species

  Giant knotweed (Reynoutria sachalinensis) is a vine, not an erect plant.  

Moist to moderate moisture. Woods, fence rows, roadsides. Full to partial sun.




August to September




In North America only plants with fertile female and infertile male plants have been introduced. The plant spreads by an extensive rhizome system. Having no fertile male flowers the plant cannot produce by reseeding. Fragments of stem or rhizome can produce new plants. Dispersal occurs when plant fragments are washed downstream or are inadvertently transported in dirt by humans.


Pests and Diseases






Distribution Map



2, 3, 4, 7, 22, 28, 29, 30.




Native to Japan, Korea, Taiwan, and China. Introduced and naturalized in the United States and Canada. Escaped cultivation.





  Kingdom Plantae (green algae and land plants)  
  Subkingdom Viridiplantae (green plants)  
  Infrakingdom Streptophyta (land plants and green algae)  
  Superdivision Embryophyta (land plants)  
  Division Tracheophyta (vascular plants)  
  Subdivision Spermatophytina (seed plants)  
  Class Magnoliopsida (flowering plants)  
  Superorder Caryophyllanae  


Caryophyllales (pinks, cactuses, and allies)  


Polygonaceae (knotweed)  
  Subfamily Polygonoideae  
  Tribe Polygoneae  
  Subtribe Reynoutriinae  



Subordinate Taxa






Fallopia japonica var. japonica

Pleuropterus cuspidatus

Pleuropterus zuccarinii

Polygonum cuspidatum

Polygonum cuspidatum var. compactum

Polygonum zuccarinii


Common Names



Japanese knotweed

Mexican bamboo

















A dry, one-chambered, single-seeded fruit, formed from a single carpel, with the seed attached to the membranous outer layer (wall) only by the seed stalk; the wall, formed entirely from the wall of the superior ovary, does not split open at maturity, but relies on decay or predation to release the contents.



The upper angle where a branch, stem, leaf stalk, or vein diverges.



Folded, as in a grass blade, or with a raised ridge, as in a grass sheath; like the keel of a boat.



The small swelling of the stem from which one or more leaves, branches, or buds originate.



A sheath around the stem at the base of a petiole formed from the stipules; a feature of many members of the Polygonaceae.



A pyramidal inflorescence with a main stem and branches. Flowers on the lower, longer branches mature earlier than those on the shorter, upper ones.



A horizontal, usually underground stem. It serves as a reproductive structure, producing roots below and shoots above at the nodes.



A modified stamen that produces no pollen. It often has no anther.



Refers to both the petals and the sepals of a flower when they are similar in appearance and difficult to tell apart. Tepals are common in lilies and tulips.

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    Japanese knotweed      


    Japanese knotweed   Japanese knotweed  


    Japanese knotweed      



  Fallopia japonica
Susanne Wiik
  Fallopia japonica  

Parkslirekne, Japanese knotweed

  Japanese Knotweed
Wez Smith
  Japanese Knotweed  

Japanese Knotweed (Fallopia japonica).

  Japanese Knotweed Identification
Invasive Weeds Agency

Published Jan 25, 2013

How to identify Japanese knotweed (Fallopia japonica) throughout the year. Find more info at Invasive Weeds Agency's website.




Visitor Videos

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Other Videos
  The plant that breaks through concrete
One Minute Environment

Uploaded Jun 30, 2011

The Japanese Knotweed (Fallopia japonica) is by law banned from Switzerland. It spreads nevertheless. It can breakt through concrete and pushes out native plants.

  Japanese Knotweed

Uploaded May 20, 2011

UNL Extension Educator Nicole Haxton shows us what Japanese Knotweed looks like and discusses strategies to control it.

  Invasive Species: Japanese Knotweed
Cindy Sandeno

Published on Jun 22, 2012

Potomac Highlands Cooperative Weed and Pest Management Area short film about the invasive species: Japanese Knotweed. Identification, the problems, and treatment of Tree of Heaven.




Visitor Sightings

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Location: Grant (Washington County)

Starting to see it all over the place!


Location: Shelton Washington

pain in the button, taken over acre

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