Japanese knotweed

Reynoutria japonica Houtt.

Family: Polygonaceae

Pet & Human Safety

People: Skin irritant to peoplePets: Not on the ASPCA lists as we read them

Hazardous parts

Not yet checked.

Preparation

n/a

Hazard type

Not yet checked.

Hazard route

Not stated by our sources (not a clearance). Detail: NC State has no poison section for this plant, so no NC State severity or dermatitis fields are recorded

How complete this information is

The safety information on this page is partial. Where a source covers a point, it is shown; where none does, the page says so.

Toxic to pets

No ASPCA entry found.

No ASPCA page found for Reynoutria / Polygonum cuspidatum. Do not treat as pet-safe.

PlotAndPetal cannot tell you whether a plant you found can be eaten. Only someone who can see the plant can identify it.

Look-alike warning

Could be mistaken for Other plants with a similar name or look. To tell them apart: Match on the scientific name, Reynoutria japonica Houtt. (Ontario Invasive Plant Council, Ontario.ca, NC State and Kew WCVP agree). The name "Polygonum cuspidatum" is an older name for the same plant; Kew WCVP also lists Fallopia japonica as a synonym., not on the common name.. Several knotweeds are regulated together in Ontario: Japanese knotweed (Reynoutria japonica), Bohemian knotweed (R. bohemica), giant knotweed (R. sachalinensis) and Himalayan knotweed (Koenigia polystachya) (Ontario.ca). The name "Polygonum cuspidatum" is an older name for Japanese knotweed; Kew WCVP also holds an unrelated invalid "Polygonum cuspidatum" name for a Persicaria acuminata plant of the Americas, ignored. Check the botanical name.

Poison control

If someone has collapsed, cannot be woken, is having a seizure or has trouble breathing, call 911 now. For any other possible poisoning, call Poison Help: 1-800-222-1222.

Emergency number source: U.S. federal law (47 U.S.C. § 251(e)(3)), read 2026-10-07.

Naturalist Lore

Historical Background & Breeding Lore

The history of this plant and its name, and how it was bred, where we have them.

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Care

Hardiness zones: Not checked yet
Light: Full sun per NC State; also grows in shade per Ontario.ca.
Water needs: high high is a plain-language reading: NC State lists drainage as moist or occasionally wet, and Ontario.ca says it grows in many soil types provided there is enough moisture. This is not an instruction to water it. Always finger-test two inches down.
Mature size: US: NC State: 10-13 ft tall and 3-20 ft wide; rapid growth; CA: Ontario.ca: can grow larger than 2 m (about 6.5 ft) tall

Soil & Drainage

Pruning & Flush Rhythm

Feeding Schedule

How to prepare the soil, when and what to feed, and how and when to prune where that applies.

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