California Invasive Plant Council

Fallopia japonica

Synonyms: Polygonum cuspidatum; Reynoutria japonica, Fallopia baldschuanica

Common names: Japanese knotweed; Mexican bamboo

Fallopia japonica (=Polygonum cuspidatum) (Japanese knotweed) is a perennial forb/herb (family Polygonaceae ). Plants grow vigorously and create dense colonies that exclude other vegetation. Established colonies are extremely difficult to eradicate. It inhabits disturbed moist sites, roadsides, riparian and wetland areas. Plants typically grow in open, sunny areas on moist soils in cool temperate climates. There is very little of this species in California, but it is spreading rapidly in the Northwest and has been a major problem in the southern and northern US, as well as Europe.

Cal-IPC Rating: Moderate — Alert?

Explanation of Cal-IPC ratings

• High – These species have severe ecological impacts on physical processes, plant and animal communities, and vegetation structure. Their reproductive biology and other attributes are conducive to moderate to high rates of dispersal and establishment. Most are widely distributed ecologically.
• Moderate – These species have substantial and apparent-but generally not severe-ecological impacts on physical processes, plant and animal communities, and vegetation structure. Their reproductive biology and other attributes are conducive to moderate to high rates of dispersal, though establishment is generally dependent upon ecological disturbance. Ecological amplitude and distribution may range from limited to widespread.
• Limited – These species are invasive but their ecological impacts are minor on a statewide level or there was not enough information to justify a higher score. Their reproductive biology and other attributes result in low to moderate rates of invasiveness. Ecological amplitude and distribution are generally limited, but these species may be locally persistent and problematic.
• Alert – An Alert is listed on species with High or Moderate impacts that have limited distribution in California, but may have the potential to spread much further.
• Watch – These species have been assessed as posing a high risk of becoming invasive in the future in California.

CDFA Rating: A*?

Explanation of CDFA ratings

• A – A pest of known economic or environmental detriment and is either not known to be established in California or it is present in a limited distribution that allows for the possibility of eradication or successful containment.
• B – A pest of known economic or environmental detriment and, if present in California, it is of limited distribution.
• C – A pest of known economic or environmental detriment and, if present in California, it is usually widespread.
• D – An organism known to be of little or no economic or environmental detriment, to have an extremely low likelihood of weediness, or is known to be a parasite or predator.
• Q – An organism or disorder suspected to be of economic or environmental detriment, but whose status is uncertain because of incomplete identification or inadequate information.
• * – An asterisk next to the rating indicates that a plant is included in the CCR Section 4500 list of California State Noxious Weeds.
• -* – This plant is included in the CCR Section 4500 list of California State Noxious Weeds, but is otherwise not rated.

More details

Assessment(s)

Plant Assessment Form - Information gathered by Cal-IPC on the impacts, rate of spread, and distribution of invasive plants in California. Does not include management information.

Species ID Card

Invasive Species ID Card - To support field identification of early detection species, Cal-IPC has designed a set of Species ID cards that can be downloaded, printed double-sided, and trimmed to size.

Special Reports

Weed Management Notes

• Management Notes - Information on management techniques and effectiveness from the University of California Cooperative Extension’s Weed Research & Information Center.

Cal-IPC Newsletter Articles

• Wildland Weed News. Cal-IPC. Vol 31, Issue 2
• Knocking out knotweed in Marin County. Dirkse, Anna. Vol 28, Issue 2
• The cost of inaction: An example from the Toutle River. Cal-IPC. Vol 27, Issue 3
• Eradicating knotweeds and other new invaders on California’s North Coast. Eberwein, Amy. Vol 24, Issue 3
• Working on weeds in the eastern US. Meyer, Tanya. Vol 22, Issue 1
• Tracking eradication progress . Guise, Vince. Vol 21, Issue 3
• Jepson Manual 2nd ed. invasive plant name changes. Brusati, Elizabeth. Vol 20, Issue 2
• Chasing weeds from Maine to California. Stack, Lois Berg. Vol 16, Issue 4
• Japanese knotweed (Polygonum cuspidatum) and other knotweeds. Brusati, Elizabeth. Vol 13, Issue 1

Cal-IPC Symposium Presentations

Presentations are linked where available. Where a presentation is not available, find more information by reading the abstract in the Cal-IPC Symposia Archive.

• Biological control agents of weeds in California: Overview of rearing, releases, and monitoring by the CDFA Biological Control Program. Moran, Patrick (2023)
• North coast invasive weeds – early detection rapid response eradication project. Reynolds, Candace (2023)
• Regional EDRR with Bay Area WMAs. Johnson, Doug; Valentine, Nicole (2022)
• North Coast invasive weeds: Early detection rapid response eradication project. Ferson, Susannah; Avila-Martinez, Carla (2021)
• The role of public gardens as sentinels of plant invasion. Culley, Theresa; Dreisilker, Kurt; Ryan, Clair; Landel, Hans; Cavallin, Nadia; Arcate Schuler, Jessica; Shultz, Brittany; Gettig, Roger; Havens, Kayri (2020)
• 2019 Weed Alerts. Burger, Jutta; Price, Bob (2019)
• Communicating the threat of Japanese knotweed (Fallopia japonica) to private landowners along San Geronimo Creek. Dirkse, Anna (2019)
• Partnerships lifeline: early detection and eradication of Japanese knotweed across boundaries. Simpson, Bobbi (2018)
• 2003 Cal-IPC red alert! New invasions, recent expansions, and a few others to be on the look-out For…. Tu, Mandy; Randall, John M. (2003)

Other Fallopia japonica Information

• CalPhotos - Images of plants taken mostly in California.
• Calflora - Distribution map and records of this species in California.
• CalWeedMapper - Distribution map of this species in California with ability to determine regional priorities.
• EDDMapS - Distribution of this species in North America.
• Jepson Interchange - Information on this plant's taxonomy, biology, and distribution from UC Berkeley's Jepson Herbarium.
• USDA PLANTS Database - Information on identification and distribution, with links to websites in individual states.

Additional photos