Fallopia_xbohemica_Bohemian knotweed_John Bailey_cropped
Photo: John Bailey

Fallopia ×bohemica Risk Assessment

Common names: Bohemian knotweed

Fallopia ×bohemica -- California

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Created by: Ramona Robison
Created on: Tuesday, Sep 20th, 2016
Evaluation Summary
Summary: 
General Evaluation Information
Date of Evaluation: 
September 30, 2016
Evaluation Time (hrs): 
3 Hours
Evaluation Status: 
Completed
Plant Information
Plant: 
Fallopia ×bohemica
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: 
High
Answer / Justification: 
Since Fallopia xbohemica is a hybrid, it is not considered to be native to its current distribution in Europe (DAISIE) or North America (FNA). Fallopia ×bohemica is a widespread hybrid between F. japonica and F. sachalinensis. These species are native to Japan, China and Korea, and Japan and the Russian Federation Kurile Island Sakhalin (GRIN, GBIF). It occurs in California in one County according to Calflora. It is also present in Washington, Oregon, Canada and Europe.
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: 
Fallopia xbohemica occurs in Washington and areas of Europe which match the climate of California based on the Cal-IPC climate map.
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: 
It is considered invasive in Washington and Europe, as well as elsewhere where it occurs.
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: 
It is considered invasive in Washington and Europe, as well as elsewhere where it occurs. It occurs in areas of Washington and Europe which match the climate of California according to the Cal-IPC climate map.
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: 
Other species of knotweed (Japanese, Sakhalin and Himalayan) are considered invasive in California and elsewhere. They occur in the United States and in Europe in areas which match the climate of California based on the Cal-IPC climate map.
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: 
Medium
Answer / Justification: 
The distribution of Fallopia xbohemica in GBIF is much smaller than that indicated by the distribution map of countries available in DAISIE. The map on the USDA NRCS Plants website also does not show any occurrences in the United States, but there is information available on Washington and California locations (Washington, Calflora). Another research study in the Eastern US collected samples from CT, MA, ME, RI, and VT (Gammon 2007). Given the data available for northern Europe and areas of the eastern US, the majority of the reported distribution does not fall in areas which match the climate of California based on the Cal-IPC climate map.
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: 
Very High
Answer / Justification: 
They [giant knotweed species] form dense colonies, almost eliminating other plants within these stands.
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 F. xbohemica promotes or alters 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: 
High
Answer / Justification: 
"None of these knotweeds [including Bohemian] are known to be toxic to animals, and, in fact Japanese knotweed is edible to humans. Grazing by livestock may provide some growth reduction, but has not been shown to eliminate colonies (DiTomaso et al. 2013)."
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: 
Very High
Answer / Justification: 
F. xbohemica can grow up to 2.5 meters tall and grows in dense thickets so could block the movement of humans or animals.
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: 
Bohemian Knotweed reproduces vegetatively from rhizomes and by seed. Each node on the plant stock is able to produce roots and new plants. New plants can sprout from small fragments.
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: 
Bohemian Knotweed reproduces vegetatively from rhizomes and by seed. Each node on the plant stock is able to produce roots and new plants. New plants can sprout from small fragments.
Reference(s): 
13. Does the species (or cultivar or variety) commonly produce viable seed?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Medium
Answer / Justification: 
Although earlier references report that Bohemian Knotweed reproduces only vegetatively from rhizomes, according to the Washington Noxious Weed Board it is able to reproduce by seed and a paper by Soll (2004) also confirms viable seed production.
Reference(s): 
14. Does this plant produce copious viable seeds each year (> 1000)?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Low
Answer / Justification: 
Although earlier references report that Bohemian Knotweed reproduces only vegetatively from rhizomes, according to the Washington Noxious Weed Board it is able to reproduce by seed. An examination of images online shows that a single inflorescence typically contains considerably more than 100 flowers. Each plant/patch can easily produce as many as 100 inflorescences. Even with a very low rate of fertility these plants likely would produce many more than 1000 seeds per plant. Of course there is likely less flower production in shaded settings, but it seems likely that in many cases this plant produces >1000 seeds each year.
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: 
This question can be answered NO, based on the information on the Invasive Species Compendium datasheet for the taxon http://www.cabi.org/isc/datasheet/108332 Reproductive Biology Like its parents, this is a gynodieocious taxon (hermaphrodite and male-sterile individuals), the hermaphrodites being self-incompatible (Bailey, 1994). The hermaphrodite plants do not make such good female parents even when they have regular meiosis and access to compatible pollen. This seems to be because the development of the gynoecium is not usually so advanced as in the male-sterile plants, although it has been noted that towards the end of the season, some of the hermaphrodite flowers exhibit much greater stigmatal development and do produce seed (Bailey, 1994). Whatever ploidy level is considered, all taxa are capable of producing viable seed giving rise to vigorous plants (usually with aneuploid chromosome numbers), which currently do not become established spontaneously in Europe; the situation in the USA may well be different and is in need of urgent investigation (Forman and Kesseli, 2003; Bailey et al., 2008).
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: 
Points: 
Confidence Level: 
Answer / Justification: 
Although viable seed production in the Pacific Northwest has been confirmed (Soll 2004), there is no information on the time required for germinated seedlings to reach reproductive maturity.
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: 
Points: 
Confidence Level: 
Answer / Justification: 
Although seed production in the Pacific Northwest has been confirmed (Soll 2004), there is no information on the length of seed production.
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: 
No
Points: 
0
Confidence Level: 
High
Answer / Justification: 
F. xbohemica is dispersed along watercourses through movement of rhizome fragments. It is not likely to be dispersed by animals or birds since the seeds are small and not adapted to attach to animals.
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: 
Very High
Answer / Justification: 
F. xbohemica is dispersed along watercourses through movement of rhizome fragments.
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: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
Although the plant is dispersed through disposal of yard waste, it does not have special adaptations to allow it to attach to vehicles, boats, clothing or shoes.
Reference(s): 
Evaluation Notes

Washington Noxious Weed Board: http://www.nwcb.wa.gov/weeds/bohemian-knotweed

UC Davis WRIC: http://wric.ucdavis.edu/information/natural%20areas/wr_P/Polygonum_bohem...

CABI: http://www.cabi.org/isc/datasheet/108332

DAISIE: http://www.europe-aliens.org/speciesFactsheet.do?speciesId=8037#

Reviewed by Barbara Castro, Eric Wrubel and Tim Hyland.

General comment by Eric Wrubel:

The invasion risk for Japanese and boheminan knotweed in upland habitats in California is likely to be medium to low, but I feel the risk of invasion and severe impacts to California's riparian systems is high. Japanese and bohemian knotweed would score medium to low in the PRE, because their global distribution does not have a 50% overlap with California's climate, and seed production/viability is low - or nonexistent. However, these taxa are capable of rapid clonal spread where they are not water-limited.

 

Total PRE Score

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

PRE Score: 
16
Number of questions answered: 
18
Screener Confidence (%): 
76.7
PRE Content Access and Privacy
Organization: 
California Invasive Plant Council (Cal-IPC)
Evaluation visibility: 
Public - accessible to all site users

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