Persicaria wallichii_Dean Kelch_CDFA
Photo: Dean Kelch

Persicaria wallichii Risk Assessment

Common names: Himalayan knotweed

Persicaria wallichii -- California

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Created by: Ramona Robison
Created on: Tuesday, Aug 9th, 2016
Evaluation Summary
Summary: 
General Evaluation Information
Date of Evaluation: 
September 2, 2016
Evaluation Time (hrs): 
3 Hours
Evaluation Status: 
Completed
Plant Information
Plant: 
Persicaria wallichii
Plant Material: 
If the plant is a cultivar, and if the cultivar's behavior differs from its parent's (behavior), explain how: 
Regional Information
Region Name: 
California
Climate Matching Map
These maps were built using a toolkit created in collaboration between GreenInfo Network, PlantRight, Cal-IPC, and Agricultural Sustainability Institute at UC Davis.
Link to Climate Matching Map: 
Climate Matching Maps PDF: 
Invasive History and Climate Matching
1. Has the species (or cultivar or variety, if applicable; applies to subsequent "species" questions) become naturalized where it is not native?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
P. wallichii is naturalized in California in 3 counties according to Calflora. It is also naturalized in Washington, Oregon, Monatana and Massachusetts in the United States, and Canada, Europe and New Zealand.
Reference(s): 
2. Is the species (or cultivar or variety) noted as being naturalized in the US or world in a similar climate?
Yes or No: 
Yes
Points: 
2
Confidence Level: 
High
Answer / Justification: 
P. wallichii is naturalized in California in 3 counties according to Calflora. It is also naturalized in Washington, Oregon, Monatana and Massachusetts in the United States, and Canada, Europe and New Zealand. It occurs in a climate that matches California in Washington and Oregon.
Reference(s): 
3. Is the species (or cultivar or variety) noted as being invasive in the U.S. or world?
Yes or No: 
Yes
Points: 
2
Confidence Level: 
High
Answer / Justification: 
P. wallichii is reported as invasive in its native range in northern India (Kala and Shrivastava, 2004), as well as in its non-native range in Belgium and the UK (CABI). In the western USA it is a declared noxious weed in the states of Montana, California, Washington and Oregon (CABI).
Reference(s): 
4. Is the species (or cultivar or variety) noted as being invasive in the US or world in a similar climate?
Yes or No: 
Yes
Points: 
3
Confidence Level: 
High
Answer / Justification: 
P. wallichii is reported as invasive in its native range in northern India (Kala and Shrivastava, 2004), as well as in its non-native range in Belgium and the UK (CABI). In the western USA it is a declared noxious weed in the states of Montana, California, Washington and Oregon (CABI). In Washington "It grows vigorously into dense colonies that exclude native vegetation, alters natural ecosystems and is difficult to eradicate. It poses a significant threat to riparian areas, where it can survive severe floods. Small fragments can form new plants." Some of the Washington P. wallichii distribution matches the climate in California.
Reference(s): 
5. Are other species of the same genus (or closely related genera) invasive in a similar climate?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
Fallopia japonica (Polygonum cuspidatum), Fallopia sachalinensis (P. sachalinense), and P. polystachyum (former name of Persicaria wallichii) are all considered invasive in the Pacific Northwest and occur in climates which match California's.
Reference(s): 
6. Is the species (or cultivar or variety) found predominately in a climate matching the region of concern?
Yes or No: 
No
Points: 
0
Confidence Level: 
High
Answer / Justification: 
Although there are locations in Washington and Oregon which match California's climate, most of the other locations (northern Europe and New Zealand), do not match California's climate.
Reference(s): 
Impact on Native Plants and Animals
7. Does this plant displace native plants and dominate (overtop or smother) the plant community in areas where it has established?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
It grows vigorously and creates large and dense stands that exclude native vegetation and prevent tree seedlings from growing. P. wallichii can greatly alter natural ecosystems and promotes the erosion of river banks. In Washington "It grows vigorously into dense colonies that exclude native vegetation, alters natural ecosystems and is difficult to eradicate. It poses a significant threat to riparian areas, where it can survive severe floods. Small fragments can form new plants."
Reference(s): 
8. Is the plant noted as promoting fire and/or changing fire regimes?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
There is no information on whether P. wallichii promotes or changes fire regimes.
Reference(s): 
9. Is the plant a health risk to humans or animals/fish? Has the species been noted as impacting grazing systems?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
P. wallichii is not listed as toxic on reference websites. In some cases goat grazing is used for control of the species.
Reference(s): 
10. Does the plant produce impenetrable thickets, blocking or slowing movement of animals, livestock, or humans?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Medium
Answer / Justification: 
P. wallichii grows as high as 6 feet and can form impenetrable thickets along streams and in wet areas. Canes from previous years can persist and could also block movement.
Reference(s): 
Reproductive Strategies
11. Does this species (or cultivar or variety) reproduce and spread vegetatively?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Small fragments can form new plants.
Reference(s): 
12. If naturally detached fragments from this plant are capable of producing new plants, is this a common method of reproduction for the plant?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Small fragments can form new plants.
Reference(s): 
13. Does the species (or cultivar or variety) commonly produce viable seed?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
P. wallichii has bisexual, insect-pollinated flowers and reproduces sexually by seeds in its native range. Several sources (DiTomaso and Healy, 2007; Ison, 2011) report that seed production is rare in California, British Columbia and the UK.
Reference(s): 
14. Does this plant produce copious viable seeds each year (> 1000)?
Yes or No: 
No
Points: 
0
Confidence Level: 
High
Answer / Justification: 
Several sources (DiTomaso and Healy, 2007; Ison, 2011) report that seed production is rare in California, British Columbia and the UK. Therefore seed production would be low if it occurs in California.
Reference(s): 
15. Is there significant germination (>25%) of seeds the next growing season, with no requirement of an infrequent environmental condition for seeds to germinate (i.e. fire) or long dormancy period?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
Several sources (DiTomaso and Healy, 2007; Ison, 2011) report that seed production is rare in California, British Columbia and the UK. Therefore chance of significant germination is low.
Reference(s): 
16. Does this plant produce viable seed within the first three years (for an herbaceous species) to five years (for a woody species) after germination?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
Several sources (DiTomaso and Healy, 2007; Ison, 2011) report that seed production is rare in California, British Columbia and the UK. There is no information available on when viable seed is production begins in California or elsewhere.
Reference(s): 
17. Does this plant continuously produce seed for >3 months each year or does seed production occur more than once a year?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
The flowering period of P. wallichii is August to September, and it sets fruit from September until October.
Reference(s): 
Dispersal
18. Are the plant’s propagules frequently dispersed long distance (>100 m) by mammals or birds or via domestic animals?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Medium
Answer / Justification: 
Means of dispersal include wind, water, animal movements, translocation by humans or accidental transport by human agency.
Reference(s): 
19. Are the plant’s propagules frequently dispersed long distance (>100 m) by wind or water?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Seeds of P. wallichii are dispersed by wind and water, while rhizome and stem fragments are dispersed in waterways or by flooding.
Reference(s): 
20. Are the plant’s propagules frequently dispersed via contaminated seed (agriculture or wildflower packets), equipment, vehicles, boats or clothing/shoes?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Medium
Answer / Justification: 
Means of dispersal include wind, water, animal movements, translocation by humans or accidental transport by human agency. Stem or root fragments can be spread in contaminated fill material (Soll, 2004).
Reference(s): 
Evaluation Notes

Washington: http://www.nwcb.wa.gov/detail.asp?weed=104

CABI: http://www.cabi.org/isc/datasheet/120210

Invasive Plant Atlas: http://www.invasiveplantatlas.org/subject.html?sub=6230

Reviewed by Eric Wrubel.

Total PRE Score

  • < 13 : accept (low risk of invasiveness)
  • 13 - 15 : evaluate further
  • > 15 : reject (high risk of invasiveness)

PRE Score: 
17
Number of questions answered: 
20
Screener Confidence (%): 
74.0
PRE Content Access and Privacy
Organization: 
California Invasive Plant Council (Cal-IPC)
Evaluation visibility: 
Public - accessible to all site users

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