Polygonum cuspidatum - Sieb. & Zucc.
Japanese Knotweed
Other Common Names: Japanese knotweed
Synonym(s): Fallopia japonica (Houtt.) Dcne. ;Fallopia japonica var. japonica ;Reynoutria japonica Houtt.
Taxonomic Status: Accepted
Related ITIS Name(s): Polygonum cuspidatum Sieb. & Zucc. (TSN 20889)
French Common Names: renouée du Japon
Unique Identifier: ELEMENT_GLOBAL.2.135872
Element Code: PDPGN0L0U0
Informal Taxonomy: Plants, Vascular - Flowering Plants - Buckwheat Family
Kingdom Phylum Class Order Family Genus
Plantae Anthophyta Dicotyledoneae Polygonales Polygonaceae Polygonum
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Concept Reference
Concept Reference: Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. 2nd edition. 2 vols. Timber Press, Portland, OR.
Concept Reference Code: B94KAR01HQUS
Name Used in Concept Reference: Polygonum cuspidatum
Conservation Status

NatureServe Status

Global Status: GNR
Global Status Last Changed: 22Mar1994
Rounded Global Status: GNR - Not Yet Ranked
Nation: United States
National Status: NNA
Nation: Canada
National Status: NNA (02Nov2012)

U.S. & Canada State/Province Status
United States Arkansas (SNA), California (SNA), Colorado (SNR), Connecticut (SNA), Delaware (SNA), District of Columbia (SNA), Georgia (SNR), Idaho (SNA), Illinois (SNA), Indiana (SNA), Iowa (SNA), Kansas (SNA), Kentucky (SNA), Louisiana (SNA), Maine (SNA), Maryland (SNA), Massachusetts (SNR), Michigan (SNA), Minnesota (SNA), Mississippi (SNA), Missouri (SNA), Montana (SNA), New Hampshire (SNA), New Jersey (SNA), New York (SNA), North Carolina (SNA), Ohio (SNA), Oregon (SNA), Pennsylvania (SNA), Rhode Island (SNA), South Carolina (SNA), Tennessee (SNA), Utah (SNA), Vermont (SNA), Virginia (SNA), Washington (SNA), West Virginia (SNA), Wisconsin (SNA)
Canada Alberta (SNA), British Columbia (SNA), Labrador (SNA), Manitoba (SNA), New Brunswick (SNA), Newfoundland Island (SNA), Nova Scotia (SNA), Ontario (SNA), Prince Edward Island (SNA), Quebec (SNA)

Other Statuses

NatureServe Global Conservation Status Factors

Other NatureServe Conservation Status Information

U.S. States and Canadian Provinces
Color legend for Distribution Map
NOTE: The distribution shown may be incomplete, particularly for some rapidly spreading exotic species.

U.S. & Canada State/Province Distribution
United States ARexotic, CAexotic, CO, CTexotic, DCexotic, DEexotic, GA, IAexotic, IDexotic, ILexotic, INexotic, KSexotic, KYexotic, LAexotic, MA, MDexotic, MEexotic, MIexotic, MNexotic, MOexotic, MSexotic, MTexotic, NCexotic, NHexotic, NJexotic, NYexotic, OHexotic, ORexotic, PAexotic, RIexotic, SCexotic, TNexotic, UTexotic, VAexotic, VTexotic, WAexotic, WIexotic, WVexotic
Canada ABexotic, BCexotic, LBexotic, MBexotic, NBexotic, NFexotic, NSexotic, ONexotic, PEexotic, QCexotic

Range Map
No map available.

Ecology & Life History Not yet assessed
Economic Attributes Not yet assessed
Management Summary Not yet assessed
Population/Occurrence Delineation Not yet assessed
Population/Occurrence Viability
U.S. Invasive Species Impact Rank (I-Rank)
Disclaimer: While I-Rank information is available over NatureServe Explorer, NatureServe is not actively developing or maintaining these data. Species with I-RANKs do not represent a random sample of species exotic in the United States; available assessments may be biased toward those species with higher-than-average impact.

I-Rank: High/Medium
Rounded I-Rank: High
I-Rank Reasons Summary: Polygonum cuspidatum is an extremely competitive and aggressive invader of significant riparian and wetland habitats, as well as lower-quality sites. Infestations can replace native species and degrade aquatic habitat. Once established, control can be labor-intensive.
Subrank I - Ecological Impact: High/Medium
Subrank II - Current Distribution/Abundance: High
Subrank III - Trend in Distribution/Abundance: High/Low
Subrank IV - Management Difficulty: Medium
I-Rank Review Date: 12Jan2006
Evaluator: K. Maybury
Native anywhere in the U.S?
Native Range: China, Japan, Korea (IPANE, no date).

Download "An Invasive Species Assessment Protocol: Evaluating Non-Native Plants for their Impact on Biodiversity". (PDF, 1.03MB)
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Screening Questions

S-1. Established outside cultivation as a non-native? YES

S-2. Present in conservation areas or other native species habitat? Yes

Subrank I - Ecological Impact: High/Medium

1. Impact on Ecosystem Processes and System-wide Parameters:High/Moderate significance
Comments: Over the winter, the standing dead stems of this plant may create a fire hazard (Czarapata 2005; Adirondack Park Invasive Program, no date; Alaska Natural Heritage Program, no date). This material decomposes very slowly, and can form a deep soil organic layer (Alaska Natural Heritage Program, no date). Thickets can clog waterways causing local flooding (Czarapata 2005) and altering fish habitat (Alaska Natural Heritage Program, no date). The mass of dead stems may make the area more vulnerable to erosion as well as to flooding (Child et al. 1992 as cited in Shaw and Seiger 2002; Alaska Natural Heritage Program, no date). Flooding in turn spreds the plant by distributing stem and root pieces that can establish new colonies (K. Johnson, pers. comm. to B. Meyers-Rice, 2000; Shaw and Seiger 2002; Tu and Soll 2004; Adirondack Park Invasive Program, no date; Alaska Natural Heritage Program, no date).

2. Impact on Ecological Community Structure:Medium/Low significance
Comments: Forms dense, tall thickets, up to 10 feet in height (IPANE, no date); can become even taller in the Pacific Northwest, reaching 15 feet by June (Soll 2004, Tu and Soll 2004). Thickets can be so dense that human access to waterways can be severely impeded (Czarapata 2005; K. Sewak, pers. comm. to M. Esch, 2004).

3. Impact on Ecological Community Composition:High significance
Comments: This plant's early emergence in the spring, combined with extremely vigorous growth, allows it to shade out other vegetation and prevent regineration (Sukopp and Sukopp 1988 as cited in Seiger 1991; Soll 2004; Adirondack Park Invasive Program, no date). Forms nearly pure (monospecific) stands (Seiger 1991) and has displaced native flora in many riparian areas, e.g., along streambanks in western Pennsylvania (K. Sewak, pers. comm. to M. Esch, 2004).

4. Impact on Individual Native Plant or Animal Species:Moderate significance
Comments: In Alaska, reduces the food supply for juvenile salmon in the spring (Alaska Natural Heritage Program, no date). Hybridizes with other knotweeds. Zika and Jacobson (2003) pointed out that many formerly misidentified plants were actually hybrids of this species and another non-native, Polygonum sachalinense; the resulting hybrid is fertile (Bailey et al. 1996 as cited in FNA 2005 ).

5. Conservation Significance of the Communities and Native Species Threatened:High/Moderate significance
Comments: Often found in disturbed sites and waste places (FNA 2005) but also a serious concern in high quality riparian areas and wetlands, e.g. in the Adirondacks (Adirondack Park Invasive Plant Program, no date; in Great Smoky Mountains National Park (K. Johnson, pers. comm. to B. Meyers-Rice, 2000); in the Tongass National Forest (Alaska Natural Heritage Program, no date); and in Oregon's Sandy River (Tu and Soll 2004). Tu and Soll (2004) note that the Sandy River provides habitat for federally listed steelhead trout and chinook salmon.

Subrank II. Current Distribution and Abundance: High

6. Current Range Size in Nation:High significance
Comments: Found in most of the eastern half of the U.S. and the West Coast (Alaska to California); some areas in the interior West (Kartesz 1999, FNA 2005).

7. Proportion of Current Range Where the Species is Negatively Impacting Biodiversity:High significance
Comments: Impacting the Northeast (IPANE, no date), Midwest (Czarapata 2005), Pacific Northwest (Soll 2004), and Alaska (Alaska Natural Heritage Program). A state noxious weed in California, Oregon, and Washington (Kartesz 1999). Apparently uncommon in parts of the Southeast (Weakley 2005) but still having negative impacts (a state noxious weed in North Carolina [Kartesz 1999]). Fewer serious impacts probable in the Great Plains and other parts of the interior West as escapes not reported common there and generally in roadside ditches, irrigation canals, etc. (Great Plains Flora Association 1986, Welsh et al. 2003).

8. Proportion of Nation's Biogeographic Units Invaded:High significance
Comments: Based on widespread distribution.

9. Diversity of Habitats or Ecological Systems Invaded in Nation:High significance
Comments: Varied habitats; tolerant of high temperatures, dry soil and salt (IPANE, no date). Tolerates a wide variety of soils and moisture conditions; found in riparian areas, pond edges, woodland edges, as well as roadsides and yards (Czarapata 2005).

Subrank III. Trend in Distribution and Abundance: High/Low

10. Current Trend in Total Range within Nation:Medium/Low significance
Comments: Unknown but presumed not expanding extremely rapidly nor declining.

11. Proportion of Potential Range Currently Occupied:Low significance/Insignificant
Comments: Inferred.

12. Long-distance Dispersal Potential within Nation:High/Moderate significance
Comments: In riparian areas, the plants fragment and are spread by flood waters to new areas (K. Johnson, pers. comm. to B. Meyers-Rice, 2000; Shaw and Seiger 2002; Tu and Soll 2004; Adirondack Park Invasive Program, no date; Alaska Natural Heritage Program, no date). Root fragments as small as 1/2 inch can establish new plants (Soll 2004). Dispersal across marine waters has been documented as well (Beerling et al. 1994). Although this speices lost favor as a landscaping plant by the early 1900s because of its invasive qualities (IPANE, no date), it is occassionally still planted for screening and erosion control (Czarapata 2005).

13. Local Range Expansion or Change in Abundance:High/Low significance
Comments: In 1991 noted as spreading, particularly in the eastern U.S. (Seiger 1991). Degree of current spread uncertain.

14. Inherent Ability to Invade Conservation Areas and Other Native Species Habitats:Medium/Low significance
Comments: The degree of disturbance needed for establishment is somewhat unclear but most sources indicate that sunlight is critical. P. Dunwiddie (pers. comm. to B. Meyers-Rice, 1999) inidicated that it readily invades established vegetation of many sorts provided the area is reasonable sunny. Seiger (1991) says that it does not appear to be a threat in undisturbed forest and other low-light areas and notes that it requires high light conditions (effectively competing for light in these environments through early emergence in the spring and extremely rapid growth to great height). Alaska Natural Heritage Program (no date) indicates that this species can establish with little or no observable disturbance. However, Shaw and Seiger (2002) indicate that when the plants are found in interior forests, they represent colonies that established in sunlight and persisted as the forest canopy closed.

15. Similar Habitats Invaded Elsewhere:Low significance
Comments: A major problem in the British Isles and Europe in riparian areas and wetlands (Beerling et al. 1994).

16. Reproductive Characteristics:High significance
Comments: Grows faster than most other plant species---both natives and other exotics (Soll 2004), with growth rates exceeding 8 cm per day (Locandro 1973 as cited in Seiger 1991). Rhizomes can grow at least to 23 feet long and penetrate at least 7 feet into the soil (Soll 2004). Fragments easily during storms and fragments become established as new infestations (K. Johnson, pers. comm. to B. Meyers-Rice, 2000; Shaw and Seiger 2002; Tu and Soll 2004; Adirondack Park Invasive Program, no date; Alaska Natural Heritage Program, no date). Hybrid plants, at least, are fertile, so reproduction by seed can also occur.

Subrank IV. General Management Difficulty: Medium

17. General Management Difficulty:High significance
Comments: Difficult to eradicate or control once established. See Soll (2004) for a description of the extremely time-, labor-, and money-intensive processes needed for mechanical removal. Various herbicide and herbicide application techniques are also discussed by Soll (2004) but all involve trade-offs between time/money, degree of non-target damage, and efficacy. Some (stem injection of herbicides in particular) show promise but the author notes that this is time/labor intensive and that large patches will often require treatment with a combination of methods over several years.

18. Minimum Time Commitment:Low significance
Comments: Information based on Soll (2004): Large established patches will almost certainly require foliar herbicidal treatments over two or more years. Mechanical methods like cutting must be done assiduously over 3 consecutive field seasons. Injections of herbicide into the stems may control patches in only 1-2 treatments, although this is a labor-intensive process. A combination of treatments over several years may be needed.

19. Impacts of Management on Native Species:Low significance
Comments: Foliar herbicide application has a high risk of drift and is especially deliterious (and sometimes restricted) in riparian and wetland areas because of the risk to aquatic organisms. However, the stem-injection method and other direct-application methods will presumably be feasible in many of these situations and do not have this drawback.

20. Accessibility of Invaded Areas:Moderate significance
Comments: Infestations are on private property although many landowners may be willing to cooperate in removal as this species is generally not planted any longer or viewed as desirable.

Botanical data developed by NatureServe and its network of natural heritage programs (see Local Programs), The North Carolina Botanical Garden, and other contributors and cooperators (see Sources).

  • Adirondack Park Invasive Program. No date. Invasive plant profile: Japanese knotweed, Polygonum cuspidatum. In: Priority non-native plants in the Adirondack Park. Online: http://www.adkinvasives.com/documents/FULLSET.pdf. Accessed 2006.

  • Alaska Natural Heritage Program (AKNHP). No date. Polygonum cuspidatum Sieb. & Zucc. (Species profile) Online: http://akweeds.uaa.alaska.edu/pdfs/species_bios_pdfs/Species_bios_POCU.pdf. Accessed 2006.

  • Beerling, D. J., J. P. Bailey, A. P. Conolly. 1994. Fallopia japonica (Houtt.) Ronse Decraene, biological flora of the British Isles. J. Ecology 82:959-979

  • Czarapata, E. J. 2005. Invasive Plants of the Upper Midwest. The University of Wisconsin Press. Madison, WI. 215 pp.

  • Flora of North America Editorial Committee. 2005. Flora of North America North of Mexico. Vol. 5. Magnoliophyta: Caryophyllidae: Caryophyllales, Polygonales, and Plumbaginales. Oxford Univ. Press, New York. vii + 656 pp.

  • Great Plains Flora Association (R.L. McGregor, coordinator; T.M. Barkley, ed., R.E. Brooks and E.K. Schofield, associate eds.). 1986. Flora of the Great Plains. University Press of Kansas, Lawrence, Kansas. 1392 pp.

  • IPANE [Invasive Plant Atlas of New England]. No date. Polygonum cuspidatum. Online: http://webapps.lib.uconn.edu/ipane/browsing.cfm?descriptionid=86. Accessed 2006.

  • Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. 2nd edition. 2 vols. Timber Press, Portland, OR.

  • Kartesz, J.T. 1999. A synonymized checklist and atlas with biological attributes for the vascular flora of the United States, Canada, and Greenland. First edition. In: Kartesz, J.T., and C.A. Meacham. Synthesis of the North American Flora, Version 1.0. North Carolina Botanical Garden, Chapel Hill, N.C.

  • Meades, S.J. & Hay, S.G; Brouillet, L. 2000. Annotated Checklist of Vascular Plants of Newfoundland and Labrador. Memorial University Botanical Gardens, St John's NF. 237pp.

  • Seiger, L. 1991. Element stewardship abstract for Polygonum cuspidatum. The Nature Conservancy, Arlington, VA

  • Shaw, R. H. and L. A. Seiger. 2002. Japanese knotweed. In: Van Driesche, R. et. al. Biological control of invasive plants in the eastern United States. USDA Forest Service Publication FHTET-2002-04.

  • Soll, J. 2004. Controlling knotweed in the Pacific Northwest. The Nature Conservancy. Version current as of Jan. 16, 2004. Available online: http://tncweeds.ucdavis.edu/moredocs/polspp01.pdf

  • Tu, M. and J. Soll. 2004. Sandy River, northern Oregon: Knotweed eradication at a watershed in the Pacific Northwest - a success story. The Nature Conservancy, Oregon Chapter. Available online: http://tncweeds.ucdavis.edu/success/or002/or002.pdf. Accessed 2006.

  • Weakley, A. S. 2005. Flora of the Carolinas, Virginia, and Georgia. Draft as of June 10, 2005. UNC Herbarium, North Carolina Botanical Garden, Chapel Hill. Available online: http://www.herbarium.unc.edu/flora.htm. Accessed 2006.

  • Welsh, S.L., N.D. Atwood, S. Goodrich and L.C. Higgins. (Eds.) 2003. A Utah Flora. 3rd edition. Brigham Young University, Provo, Utah, U.S.A. 912 pp.

  • Zika, P.F. and A.L. Jacobson. 2003. An overlooked hybrid Japanese knotweed (Polygonum cuspidatum × sachalinense; Polygonaceae) in North America. Rhodora 105(922): 143-152.

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