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Reynoutria japonica; photograph (c) arthur_haendler, some rights reserved (CC BY) CC BY(c) arthur_haendler, some rights reserved (CC BY) · observed growing wild · 2026-09-26

Reynoutria japonica Houtt.

Japanese knotweed; Mexican-bamboo; Asian knotweed; Asiatic Knotweed

Polygonaceae · Amur, Khabarovsk +12

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Reynoutria japonica, photograph author not stated, no rights reservedReynoutria japonica, photograph author not stated, no rights reservedReynoutria japonica, photograph (c) Patrice Dupont, some rights reserved (CC BY)Reynoutria japonica, photograph (c) Patrice Dupont, some rights reserved (CC BY)Reynoutria japonica, photograph (c) Patrice Dupont, some rights reserved (CC BY)Reynoutria japonica, photograph (c) Patrice Dupont, some rights reserved (CC BY) 77 photographs ›
Habitat figures
Coldest month, mean nightly low
-0.5°C
Jan at the habitat, not a floor; extremes not used (the weather cell's ground is too far off) CHELSA
Warmest month, mean daily high
29°C
Aug at the habitat CHELSA
Rain a year (median across the range)
2,068mm
12 months of 25 mm or more in the median year CHELSA
Open-sky light
16–32DLI
Lowest to highest month (DLI) CHELSA
The year in your months, northern hemisphere
  1. Jan
  2. Feb
  3. Mar
  4. Apr
  5. May
  6. Jun
  7. Jul
  8. Aug
  9. Sep
  10. Oct
  11. Nov
  12. Dec
Wet summers: the rain comes April to October, read as a summer growing season. rain rule, CHELSA
Northern hemisphere unless you set your site. The chart is in the habitat's months.

Summary

Reynoutria japonica, synonyms Fallopia japonica and Polygonum cuspidatum, is a species of herbaceous perennial plant in the knotweed and buckwheat family Polygonaceae. Common names include Japanese knotweed and Asian knotweed. It is native to East Asia in Japan, China and Korea. In North America and Europe, the species has successfully established itself in numerous habitats; it is classified as a pest and invasive species in several countries. More on Wikipedia ›

Quoted from Wikipedia, "Reynoutria japonica", CC BY-SA 4.0.

Cultivation

Seasons

The rain rule reads a summer growing season: 70% of the median year's rain (1,541 mm of 2,051 mm) falls in April to October, warmer than the year (wet-season mean 20.6 °C against a yearly mean of 15.0 °C). Months are the habitat's, northern hemisphere; shifted six months for a southern-hemisphere collection: October to April.

Rain

2,051 mm in the median year at the habitat, 1,541 mm of it April to October. Wettest month July at 269 mm, driest month December at 51 mm. Across the grid cells of the range the year's total runs 1,401 mm to 3,143 mm, the 10th to 90th percentile of each cell's own year. The median of the cells' own yearly totals, 2,068 mm, is the year's rain at the top of the page; the 2,051 mm here is the median year's, the twelve monthly medians added, which the rain rule reads. Habitat calendar, northern hemisphere; median year across the grid cells of the range, CHELSA.

Light

Open sky over the habitat: 16 to 32 mol/m²/day across the year (daily light integral, median year across the grid cells of the range, CHELSA; the lowest 10th-percentile month across the cells is 15, the highest 90th-percentile month 35).

Warmth and air
Temperature

Monthly means: the coldest month's mean nightly low -0.5 °C in January (across the grid cells of the range -1.5 °C to 2.5 °C), the warmest month's mean daily high 28.7 °C in August (median year across the grid cells of the range, CHELSA). Cold floor: none read. The nearest figure is -0.5 °C, the coldest month's mean nightly low, January, in the median year (CHELSA); a month's mean of its nightly lows is warmer than the nights a floor is read from. The daily extremes were read at a weather cell whose ground is too far above or below the typical spot to correct for, and are not used.

Humidity

Relative humidity at the habitat: 57 to 67% across the year (monthly means, median year across the grid cells of the range, CHELSA). A figure about the air, saying nothing about how the plant takes water.

How this section is made

Each card states this species' habitat figures with their sources, and what two fixed rules read from them; nothing here says what the plant does, wants or tolerates. The rules · the terms.

Seasons. Read from the median year of CHELSA monthly rainfall and mean temperature across the grid cells of the range: the rain rule takes the smallest set of months carrying 70% of the rain and calls it winter when its mean is more than half a degree below the year's, summer when more than that above, and neither in between; the temperature rule, used only under 120 mm, names the cooler six months when they are at least a degree cooler than the other six. Months are shifted for the other hemisphere only where the temperature curve moves 4 °C or more and the habitat lies 10° or more from the equator. A reading of the curves, not an observation of the plant.

Rain. CHELSA monthly precipitation, median year across the grid cells of the range. The rain that falls where the species is recorded; nothing about how the plant takes water.

Light. CHELSA shortwave radiation at each envelope cell, a daily-mean flux taken to a daily total and converted at 2.07 mol of PAR per MJ. The sky over the habitat, measured; what reaches a plant under a rock, a shrub or a shade cloth is not.

Temperature. CHELSA monthly means, median year across the grid cells of the range. No cold floor is read without a daily extremes series; the coldest month's mean nightly low named in its sentence is not one. Not a measured survival limit for any plant in a pot.

Humidity. CHELSA relative humidity, median year across the grid cells of the range.

Climate across the habitat

Habitat months, northern hemisphere

Habitat climate of Reynoutria japonica through the yearWarmest month August, mean daily high 29 °C; coldest month January, mean nightly low -0.5 °C; 2,051 mm of rain in the median year (the twelve monthly medians added). Medians across the range, from CHELSA. Habitat months, northern hemisphere.-10°0°10°20°30°40°frost°C · day and nightcold quarter0100200300400500600mm rainDLI 16–32RH 57–67%JanFebMarAprMayJunJulAugSepOctNovDec
day night rain medians and 10th–90th percentile across 11 habitat cells, CHELSA cold quarter: the three months around the coldest month's mean nightly low DLI, mol/m²/dayRH %each on its own scale
Figures by month
JanFebMarAprMayJunJulAugSepOctNovDec
Day °C8 / 6–98 / 7–1012 / 10–1317 / 14–1821 / 17–2224 / 18–2528 / 19–2829 / 19–2925 / 18–2620 / 16–2115 / 11–1711 / 8–11
Night °C-1 / -2–31 / -1–33 / 2–69 / 6–1014 / 9–1518 / 12–1922 / 12–2223 / 12–2319 / 12–2113 / 10–157 / 4–102 / 1–5
Rain mm66 / 44–102108 / 48–151168 / 96–207187 / 111–213195 / 123–341258 / 154–488269 / 156–468207 / 177–507253 / 200–409172 / 145–261117 / 77–15151 / 45–100
DLI18 / 17–2022 / 21–2327 / 25–2831 / 30–3332 / 30–3529 / 28–3232 / 29–3432 / 30–3426 / 23–2922 / 19–2617 / 16–2216 / 15–19
RH %57 / 54–6358 / 55–6359 / 58–6360 / 60–6362 / 61–6466 / 64–6967 / 65–7066 / 66–6965 / 63–6762 / 60–6560 / 57–6357 / 56–62

The daily extremes were not used: the correction for elevation, +1217 m, is beyond the 1,000 m either way that a lapse rate is applied over, so the weather cell's ground is other terrain than the typical spot's. So no cold floor is read for this species, and the coldest month's mean nightly low is not taken as one. What that cell gave, so corrected, over 44 years: absolute minimum -13.7 °C, 1st-percentile night -7.4 °C.

How this section is made
Each figure is the median across the 11 grid cells holding the 12 in-range records, with the 10th–90th percentile span across those cells after the slash where it differs. A dash is a month one or more of those cells has no figure for in the grid (a variable CHELSA does not carry there), so no median is taken rather than one over fewer cells. Extremes and elevation were read at a typical spot in the range, cell 1311:6026 (24.425, 121.325). Normals: CHELSA V2.1 1981–2010 climatology, 0.05° cells (each the mean of ~36 1 km pixels). Across the range: median and 10th–90th percentile of each month across the 11 distinct grid cells holding 12 of the 12 in-range records; extremes and elevation at the typical cell 1311:6026 (its coldest month's mean nightly low nearest the median across cells); its position is the cell centre. Extremes: NASA POWER (MERRA-2) daily 1981–2024, cell 24.50_121.250, lapse-corrected +1217 m at 6.5 °C/km. Elevation: ETOPO 2022, 2437 m (cell mean).

Natural habitat

Native range as published by WCVP; the marker is where the records are densest and decides nothing: the climate was read across the cells of the in-range records placed well enough to read one, not at the marker.
Openly licensed records inside the range, framed on where they fall.
NativeAmur, Khabarovsk, Kuril Is., Primorye, Sakhalin, China South-Central, Hainan, Manchuria, China North-Central, China Southeast, Japan, Korea, Nansei-shoto, Taiwan · introduced: Denmark, Finland, Føroyar, Great Britain, Ireland, Norway, Sweden, Austria, Belgium, Czechoslovakia, Germany, Hungary, Netherlands, Poland, Switzerland, France, Portugal, Spain, Bulgaria, Italy, Romania, Yugoslavia, Baltic States, Central European Russia, Ukraine, Altay, North Caucasus, Transcaucasus, Myanmar, Vietnam, Jawa, Tasmania, New Zealand North, New Zealand South, Alaska, British Columbia, Manitoba, New Brunswick, Newfoundland, Nova Scotia, Ontario, Prince Edward I., Québec, Colorado, Idaho, Montana, Oregon, Washington, Illinois, Iowa, Kansas, Minnesota, Missouri, Nebraska, Oklahoma, South Dakota, Wisconsin, Connecticut, Indiana, Maine, Massachusetts, Michigan, New Hampshire, New Jersey, New York, Ohio, Pennsylvania, Rhode I., Vermont, West Virginia, California, Utah, Alabama, Arkansas, Delaware, Florida, Georgia, Kentucky, Louisiana, Maryland, Mississippi, North Carolina, South Carolina, Tennessee, Virginia, District of Columbia · regions are the level-3 units of the World Geographical Scheme for Recording Plant Distributions, as Kew's World Checklist records ranges, some under historical names such as Yugoslavia or Czechoslovakia (how).
Map marker35.654, 139.444 · in the densest population, 8 records (67% of those in range)
Kew's description“perennial or rhizomatous geophyte” · “temperate” · quoted from WCVP, RBG Kew (CC BY 4.0); not derived here
Evidence usedThe map marker and the climate across the range rest on all 12 georeferenced records inside the native range; the map shows only the 4 openly licensed ones, which alone would put the marker 286 km away. 745 records outside the WCVP native range not used.
How this section is made
The map marker: the openly licensed record nearest the middle of the densest 1° population of the 12 in-range records; it decides nothing. Records outside the native range, living specimens and records marked introduced are not used; which records count.

Photographs

Reynoutria japonica, photograph 1did not loadReynoutria japonica, photograph 2did not loadReynoutria japonica, photograph 3did not loadReynoutria japonica, photograph 4did not loadReynoutria japonica, photograph 5did not loadReynoutria japonica, photograph 6did not load

Photographs: author not stated, no rights reserved · CC0 (2); (c) Patrice Dupont, some rights reserved (CC BY) (4).

Similar habitat climate The 6 nearest by mean daily high, mean nightly low and rain, month by month. How.

Diplazium chinense Diplazium chinense Asian twinsorus fern Photo: CC BY-SA, harum.kohCypripedium japonicum Cypripedium japonicum Japanese lady's-slipper Photo: CC BY, bemmaIris japonica Iris japonica Butterfly-flower Photo: CC BY, carabus123Rohdea japonica Rohdea japonica Japanese Sacred Lily Photo: CC BY, Saryu Mae 前 朝琉Rhododendron sanctum Rhododendron sanctum Ericaceae Photo: Public domain, Ole Jonny LarsenSedum makinoi Sedum makinoi Round-leaf stonecrop Photo: CC BY, 曾昱承 Yu-Cheng Zeng

Other Reynoutria 4 in the reference

Reynoutria × bohemica Reynoutria × bohemica Bohemian knotweed Photo: CC BY, E. A.Reynoutria compacta Reynoutria compacta Dwarf Japanese knotweed Photo: CC BY, BobReynoutria forbesii Reynoutria forbesii Forbes’ knotweed Photo: CC BY, 鄭雪雲Reynoutria sachalinensis Reynoutria sachalinensis Giant knotweed Photo: CC BY, Marcin Dyderski

Names & registers

Also known as Fallopia japonica; Fallopia japonica f. colorans; Fallopia japonica var. hachidyoensis; Fallopia japonica var. uzenensis; Pleuropterus cuspidatus and 38 more. (GBIF Backbone)

GBIFPOWOIPNIWFOWikidataWikipediaiNaturalist

The record

Family
Polygonaceae
Described by
Houtt.
Native to
Amur, Khabarovsk, Kuril Is. +11
Wild records
12 in range · 4 open

Every figure here is derived from public data by a stated rule and names its source; nothing about this species is written for its page by a person or by AI, apart from the marked, credited quotations. How →

Where this page came from built 2026-10-22
GBIF species record gbif.species answeredasked 2026-10-22
GBIF synonyms gbif.synonyms answeredasked 2026-10-22
GBIF common names gbif.vernacular answeredasked 2026-10-22names fetched 2026-10-22
WCVP native range wcvp.distribution answeredas of 2026-06-04
GBIF occurrence records gbif.occurrences answeredasked 2026-10-22GBIF occurrence search API; 43 records within 1 km of a botanic garden, herbarium or zoo left out (19 at a botanic garden or arboretum, 10 at a herbarium, 14 at a zoo; Wikidata's list)
Habitat climate climate answeredasked 2026-10-22
Daily extremes (NASA POWER) climate.extremes answeredasked 2026-10-22NASA POWER (MERRA-2) daily 1981–2024, cell 24.50_121.250, lapse-corrected +1217 m at 6.5 °C/km; read, and set aside: the correction for elevation, +1217 m, is beyond the 1,000 m either way that a lapse rate is applied over, so the weather cell's ground is other terrain than the typical spot's, so no cold floor is shown
Wikidata wikidata answeredasked 2026-10-22
Wikipedia wikipedia answeredasked 2026-10-22
iNaturalist photographs, wild inat.photos.wild answeredasked 2026-10-22from iNaturalist's open data of 2026-09-27: research-grade observations with an open-licence photograph, the 24 whose photographs iNaturalist received last (the open data holds no votes); filled 2026-10-22
iNaturalist photographs, cultivated inat.photos.cultivated not asked
GBIF media gbif.media answeredasked 2026-10-22
OpenAlex papers openalex not askednot yet asked for this reference; a later fill takes the papers from OpenAlex's snapshot

Every source is recorded as having answered, reported nothing, refused, failed, not been asked, or answered twice where one answer is taken (“not taken: two items answered”), and, for a source asked when this page was built, when: the figures are as the source held them that day; a source read from a file (Kew's World Checklist, a GBIF download) says "as of" the file's day instead; a row carried from an earlier build of the page names that build instead. Only “nothing to report” is ever shown as an absence. Records came from 16 datasets via GBIF.org; each carries its own licence.

Cultifolio

Opening your plants…