Japanese knotweed

(Reynoutria japonica)

Conservation Status
Japanese knotweed
 
  IUCN Red List

not listed

     
  NatureServe

NNA - Not applicable

SNA - Not applicable

     
  Minnesota

not listed

     
           
Weed Status
   
 

Specially Regulated Plant

Invasive

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

     
  Midwest

FACU - Facultative upland

     
  Northcentral & Northeast

FACU - Facultative upland

     
           
 
Description
 
 

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).

 
     
 

Height

 
 

3½ to 10

 
     
 

Flower Color

 
 

White to greenish

 
     
 

Similar Species

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

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

 
     
 
Biology
 
 

Flowering

 
 

August to September

 
     
 

Propagation

 
 

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.

 
     
 
Use
 
 

 

 
     
 
Distribution
 
 

Distribution Map

 

Sources

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

 
  3/27/2021      
         
 

Nativity

 
 

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

 
         
 

Occurrence

 
 

 

 
         
 
Taxonomy
 
  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 Asteranae  
 

Order

Solanales (nightshades, bindweeds, gooseweeds, and allies)  
 

Family

Polygonaceae (buckwheat)  
  Subfamily Polygonoideae  
  Tribe Polygoneae  
  Subtribe Reynoutriinae  
 

Genus

Reynoutria  
       
 

Subordinate Taxa

 
 

 

 
       
 

Synonyms

 
 

Pleuropterus cuspidatus

Pleuropterus zuccarinii

Polygonum cuspidatum

Polygonum cuspidatum var. compactum

Polygonum zuccarinii

Reynoutria japonica

 
       
 

Common Names

 
 

fleeceflower

Japanese knotweed

Mexican bamboo

 
       

 

 

 

 

 

 

 

 

 

 

 

 

Glossary

Achene

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.

 

Axil

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

 

Keeled

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

 

Node

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

 

Ocrea

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

 

Panicle

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

 

Rhizome

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

 

Staminode

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

 

Tepal

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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MinnesotaSeasons.com Photos
   

Plant

  Japanese knotweed    
       

Leaves

  Japanese knotweed   Japanese knotweed
       

Stem

  Japanese knotweed    
       
       

 

Camera

     
Slideshows
   
  Fallopia japonica
Susanne Wiik
 
  Fallopia japonica  
 
About

Parkslirekne, Japanese knotweed

 
     
  Japanese Knotweed
Wez Smith
 
  Japanese Knotweed  
 
About

Japanese Knotweed (Fallopia japonica).

 
     
  Japanese Knotweed Identification
Invasive Weeds Agency
 
   
 
About

Published Jan 25, 2013

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

 
     

 

slideshow

       
Visitor Videos
       

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Other Videos
 
  The plant that breaks through concrete
One Minute Environment
 
   
 
About

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
backyardfarmer
 
   
 
About

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
 
   
 
About

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.

   
       

 

Camcorder

         
Visitor Sightings
   

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Debbie
8/15/2018

Location: Grant (Washington County)

Starting to see it all over the place!


Tia
4/20/2016

Location: Shelton Washington

pain in the button, taken over acre


     
     
 
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