Reynoutria japonicaHoutt.

Japanese knotweed

WFO wfo-0000406106 Accepted WFO 2026-06 8 photographs CC0 / CC BY

Plate 1 figs. a–h

Reynoutria japonica, photographed by Jon Mortin
fig. a Jon Mortin, CC BY 4.0 / 2022-06-06 / obs. 203924863
Reynoutria japonica, photographed by Erin Lalime
fig. b Erin Lalime, CC BY 4.0 / obs. 203511984
Reynoutria japonica, photographed by Emily Summerbell
fig. c Emily Summerbell, CC BY 4.0 / obs. 205329168
Reynoutria japonica, photographed by Grzegorz Grzejszczak
fig. d Grzegorz Grzejszczak, CC BY-SA 4.0 / obs. 203788477
Reynoutria japonica, photographed by Yann Kemper
fig. e Yann Kemper, CC0 1.0 / obs. 203011237
Reynoutria japonica, photographed by Grzegorz Grzejszczak
fig. f Grzegorz Grzejszczak, CC BY-SA 4.0 / obs. 205795296
Reynoutria japonica, photographed by Larry Jensen
fig. g Larry Jensen, CC BY 4.0 / obs. 205564032
Reynoutria japonica, photographed by Ben Walters
fig. h Ben Walters, CC0 1.0 / obs. 205033180

Every figure is a research-grade observation under CC0, CC BY or CC BY-SA, rehosted with the photographer’s name, the licence and the observation it came from. Photographs under a NonCommercial licence are excluded from this site and are never stored, which costs us a great many pictures and is not negotiable.

Confused withby our own model

These are not lookalikes we guessed at. Each one is a species our identification model genuinely mistook for this plant, and how many times. The error rate is published.

Flowering n = 2,131 observations

Peak flowering in Sep, from 2,131 community-annotated observations worldwide. This is a global aggregate, not a forecast for your garden: the same species flowers on different dates in different hemispheres, and citizen-science records cluster near cities, at weekends, and in spring. Where a species has fewer than 30 annotated records we do not draw this chart at all.

Also published as 30 synonyms

A synonym is not an error. It is a record of botanists disagreeing, in print, about where this plant belongs. Each of these was somebody’s considered answer.

  • Fallopia compacta (Hook.f.) G.H.Loos & P.Keil
  • Fallopia japonica (Houtt.) Ronse Decr.
  • Fallopia japonica f. colorans (Makino) Yonek.
  • Fallopia japonica var. compacta (Hook.f.) J.P.Bailey
  • Fallopia japonica var. compacta J.Bailey
  • Fallopia japonica var. hachidyoensis (Makino) Yonek. & H.Ohashi
  • Fallopia japonica var. uzenensis (Honda) Yonek. & H.Ohashi
  • Pleuropterus cuspidatus (Siebold & Zucc.) H.Gross
  • Pleuropterus zuccarinii Small
  • Polygonum compactum Hook.f.
  • Polygonum cuspidatum Siebold & Zucc.
  • Polygonum cuspidatum var. spectabile Noter
  • Polygonum hachidyoense Makino
  • Polygonum reynoutria Makino
  • Polygonum reynoutria f. colorans Makino
  • Polygonum sieboldii de Vriese ex L.H.Bailey
  • Polygonum zuccarinii Small
  • Reynoutria compacta (Hook.f.) P.D.Sell
  • Reynoutria hachidyoensis (Makino) Nakai
  • Reynoutria hachidyoensis var. terminalis Honda
  • Reynoutria hachijoensis Nakai ex Jotani
  • Reynoutria hastata Nakai ex Ui
  • Reynoutria henryi Nakai
  • Reynoutria japonica var. compacta (Hook.f.) Buchheim

and 6 more.

Sourcesevery claim on this page

  1. World Flora Online Plant List. accepted name, authority, classification. CC0. Retrieved 2026-07-12.
  2. iNaturalist. photographs and flowering annotations, CC0 / CC BY / CC BY-SA only. per photograph. Retrieved 2026-06-27.

We publish what we can source and we say so when we cannot. This page has no care advice, no toxicity claim and no native range, because we do not yet have those from a source we can cite.