Calflora, a 501c3 non-profit
Distribution by County   10891    
Fallopia japonica  (Houtt.) Ronse Decr.
Japanese knotweed
Fallopia japonica is a perennial herb that is not native to California.
Cal-IPC rating: moderate
Record Count by County

Alameda: 9 Los Angeles: 2 San Francisco: 10 Shasta: 3
Butte: 3 Marin: 413 San Luis Obispo: 1 Siskiyou: 10
Contra Costa: 5 Mendocino: 4 Santa Barbara: 1 Sonoma: 16
Del Norte: 17 Nevada: 2 Santa Clara: 5 Ventura: 1
Humboldt: 57 Plumas: 1 Santa Cruz: 13

Suggested Citation
Calflora: Information on California plants for education, research and conservation, with data contributed by public and private institutions and individuals, including the Consortium of California Herbaria. [web application]. 2026. The Calflora Database [a non-profit organization]. Available: https://www.calflora.org/   (Accessed: 08/19/2026).