Giant Knotweeds | Fallopia japonica, F. sachalinensis, & F. X bohemica

California Pest Rating for
Giant knotweeds |  Fallopia japonica, F. sachalinensis, & F. X bohemica
Pest Rating: A |  Proposed Seed Rating: R

Initiating Event:

Japanese and giant knotweeds have been listed as A rated plants by CDFA for  many years. Bohemian knotweed has a Q rating.

History & Status:

Background: The giant knotweeds comprise 2 species and their hybrid: Japanese knotweed (F. japonica), giant knotweed (F. sachalinensis), and Bohemian knotweed (F. X bohemica).  They are large bamboo-like herbs to over 4 m tall under ideal conditions. They spread via underground rhizomes and can form extensive patches that exclude other vegetation. Where well adapted, they can spread via seed as well.  They produce copious biomass in areas with a year-long supply of soil moisture, and often overtop surrounding vegetation. This changes the shade profile, competitive environment, and hydrology of the community. Older stands are quite dense and can impede water flow along streams. Originally imported as an ornamental screen or hedge plant, giant knotweeds are native to Asia. In North America, this plant is not held in check by natural enemies and is capable of thriving and spreading in a wide range of conditions, especially riverbanks, roadsides and other moist, disturbed areas.

Giant knotweed is the biggest of the three species, sometimes exceeding 16 feet in height. The stems are smooth, hollow and light green, resembling the canes of bamboo, and sparingly branched. The leaves are 6 to 16″ long, with a deeply heart-shaped base and a blunt leaf tip. Diagnostic hairs on the leaf underside are long, thin and wavy. Giant knotweed has been declared noxious in California, Oregon, and Washington. Japanese knotweed is smaller (to 8’) and has hairless leaves. It has has been declared noxious in Alabama, California, Oregon, Vermont, and Washington. Bohemian knotweed is intermediate between the parents. Despite its hybrid origin, Bohemian knotweed produces fertile seed that can spread the plant. As it has often been misidentified, the range of Bohemian knotweed is likely larger than is recognized currently.

Official Control: Several counties in California have controlled giant knotweeds where they are found.

Worldwide Distribution: Giant knotweeds are native to Japan and Eastern Asia. They  heave been widely planted and escaped in all but the driest regions of North America.

California Distribution: Giant knotweeds are known from detected populations scattered throughout the state. Most of these represent planted plants that have spread vegetatively. Giant knotweeds can spread best, both vegetatively and by seed, in Northwestern California, especially Humboldt and Siskiyou Counties.

This risk giant knotweeds pose to California is evaluated below.

Consequences of Introduction: 

1) Climate/Host Interaction: Evaluate if the pest would have suitable hosts and climate to establish in California. Score: 2

Low (1) Not likely to establish in California; or likely to establish in very limited areas.

Medium (2) may be able to establish in a larger but limited part of California.

High (3) likely to establish a widespread distribution in California.

Risk is Medium (2) as illustrated by the moist habitat of the plant in states where it occurs. Giant knotweeds are expected to colonize riparian areas, pond margins, wetlands, roadside ditches, irrigation canal banks, and moist forest edges. They will also spread in neglected gardens and urban waste areas.

2) Known Pest Host Range: Evaluate the host range of the pest. Score: 3

Low (1) has a very limited host range.

Medium (2) has a moderate host range.

High (3) has a wide host range.

Risk is High (3) as weeds do not require any one host, but grow wherever ecological conditions are favorable.

3) Pest Dispersal Potential: Evaluate the natural and artificial dispersal potential of the pest. Score: 3

Low (1) does not have high reproductive or dispersal potential.

Medium (2) has either high reproductive or dispersal potential.

High (3) has both high reproduction and dispersal potential.

Risk is High (3) as the plant spreads via water flow, human dispersal, and by seed in favorable regions.

4) Economic Impact: Evaluate the likely economic impacts of the pest to California using the criteria below. Score: 2

A. The pest could lower crop yield.

B. The pest could lower crop value (includes increasing crop production costs).

C. The pest could trigger the loss of markets (includes quarantines).

D. The pest could negatively change normal cultural practices.

E. The pest can vector, or is vectored, by another pestiferous organism.

F. The organism is injurious or poisonous to agriculturally important animals.

G. The organism can interfere with the delivery or supply of water for agricultural uses.

Low (1) causes 0 or 1 of these impacts.

Medium (2) causes 2 of these impacts.

High (3) causes 3 or more of these impacts.

Risk is Medium (2) as the plants regrow very rapidly and often invade fallow fields and meadows. Pulling or digging out the weed has some effect if repeated regularly. All waste plant material must be burnt, as Japanese knotweed can survive composting. Burning the plant in situ is ineffective. Sheep, goats, cattle and horses will graze the young shoots.

5) Environmental Impact:  Evaluate the environmental impact of the pest on California using the criteria below.

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.

B. The pest could directly affect threatened or endangered species.

C. The pest could impact threatened or endangered species by disrupting critical habitats.

D. The pest could trigger additional official or private treatment programs.

E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Score the pest for Environmental Impact. Score: 3

Low (1) causes none of the above to occur.

Medium (2) causes one of the above to occur.

High (3) causes two or more of the above to occur.

Risk is High (3) as the plant can dominate wetland habitats that are particularly important for native species, including sensitive species. In addition, the plant changes the profile and disrupts natural communities. It often triggers treatment programs. It is highly invasive in urban areas and in ornamental plantings.

Consequences of Introduction to California for giant knotweeds:

Add up the total score and include it here. (13)

Low = 5-8 points

Medium = 9-12 points

High = 13-15 points

6) Post Entry Distribution and Survey Information: Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included. (-1)

Not established (0) Pest never detected in California, or known only from incursions.

Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).

Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.

High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Final Score:

7) The final score is the consequences of introduction score minus the post entry distribution and survey information score: (12)


Giant and Japanese knotweeds are invasive in favorable habitats in CA. Bohemian knotweed can be expected to act like its parents. Therefore, uncertainty is low.

Conclusion and Rating Justification:

Bad weeds of horticultural areas, waste areas, and marginal wetlands. They deserve an A rating. The chance of eradication is moderate to high.


Baldwin, B. G., D. H. Goldman, D. J. Keil, R. Patterson, T. J. Rosatti, and D. H. Wilken, editors. 2012. The Jepson manual: vascular plants of California, second edition. University of California Press, Berkeley.

Beerling, D. J., J. P. Bailey, and A. P. Conolly. 1994. Fallopia japonica (Houtt.) Ronse Decraene. J. Ecol. 82: 959-979.

Consortium of California Herbaria (

Flora of North America Editorial Committee, eds.  1993+.  Flora of North America North of Mexico.  16+ vols.  New York and Oxford.

Stone, Katharine R. 2010. Polygonum sachalinense, P. cuspidatum, P. × bohemicum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer):

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

Responsible Party:

Dean G. Kelch, Primary Botanist; California Department of Food and Agriculture; 1220 N Street, Sacramento, CA 95814; Tel. (916) 654-0312;[@]

Pest Rating: A |  Proposed Seed Rating: R

Posted by ls