California Invasive Plant Council

Persicaria wallichii

Common names: Himalayan knotweed

Persicaria wallichii (Himalayan knotweed) is a perennial herb/ (family Polygonaceae) with white flowers and narrow leaves found in the north and central coast ranges of California. It is native to south-central Asia. It favors grasslands, wetlands, bog and marsh, and riparian and bottomland habitat. It spreads via small fragments that form new plants. Means of dispersal include wind, water, animals and humans.

Cal-IPC Rating: Watch

CDFA Rating: -*?

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 Risk Assessment - An evaluation of the potential for a plant to be invasive in California.

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.

Weed Management Notes

Cal-IPC Newsletter Articles

• Snapshots from the field. Lindgren, Jackie; Eberwein, Amy; Parish, Julie; Chapple, Tanya. Vol 26, Issue 1
• Eradicating knotweeds and other new invaders on California’s North Coast. Eberwein, Amy. Vol 24, Issue 3
• 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.

• North coast invasive weeds – early detection rapid response eradication project. Reynolds, Candace (2023)
• North Coast invasive weeds: Early detection rapid response eradication project. Ferson, Susannah; Avila-Martinez, Carla (2021)

Other Persicaria wallichii Information

• CalPhotos - Images of plants taken mostly in California.
• Calflora - Distribution map and records of this species in California.
• 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