Japanese knotweed

Discuss this plant with the guide — choose this profile as a possible match and prepare your observations for Colorado Plants & Practical Care.

Reynoutria japonica

Colorado noxious weed · List A — Eradicate. CO-rules-2025

Uncommon in Colorado. All populations are designated for eradication statewide. This is a management category, separate from danger ratings. Category source · What the lists mean.

Root and stem fragments regrow. Exposure help.

Identification caveat: Colorado’s rule uses Fallopia japonica. Giant and Bohemian knotweeds need separate identification; broad leaves and bamboo-like stems are insufficient to distinguish them.

Still researching this plant

This is a sourced working profile. Evidence is incomplete for People, Dogs & cats, Other animals. Identification and source-photo limits are explained below. Independent botanical and veterinary review is pending. Uncertainty is not a finding that the plant is safe.

Three views of this plant

Source-identified reference photographs; these are not a complete identification key. A source location outside Colorado does not establish local occurrence.

Reynoutria japonica: Broad pointed leaves and pale flower clusters viewed against the light. — **leaves and flowers** — Broad pointed leaves and pale flower clusters viewed against the light.
Michael Gasperl ( Migas ) · CC BY-SA 3.0
Photo source and identification
Original record · Original image
Source scientific identification plus visual morphology review of the selected photograph. This is a source-identified reference image, not a specimen-based taxonomic determination. Japanese, giant, and Bohemian knotweeds can overlap in overall appearance. These photographs retain the source identification; a field determination should use mature leaf bases, underside hairs, and other diagnostic characters.
Wikimedia server-resized derivative; no local crop, colour change, or generative editing. Gallery display preserves the full image; the catalog preview uses a CSS crop.

Reynoutria japonica: A dense leafy stand with abundant pale flower clusters. — **flowering habit** — A dense leafy stand with abundant pale flower clusters.
User:BotBln · CC BY-SA 3.0
Photo source and identification
Original record · Original image
Source scientific identification plus visual morphology review of the selected photograph. This is a source-identified reference image, not a specimen-based taxonomic determination. Japanese, giant, and Bohemian knotweeds can overlap in overall appearance. These photographs retain the source identification; a field determination should use mature leaf bases, underside hairs, and other diagnostic characters.
Wikimedia server-resized derivative; no local crop, colour change, or generative editing. Gallery display preserves the full image; the catalog preview uses a CSS crop.

Reynoutria japonica: Close view of small cream-white flowers spaced along a slender flowering branch. — **flower detail** — Close view of small cream-white flowers spaced along a slender flowering branch.
Udo Schmidt from Deutschland · CC BY-SA 2.0
Photo source and identification
Original record · Original image
Source scientific identification plus visual morphology review of the selected photograph. This is a source-identified reference image, not a specimen-based taxonomic determination. Japanese, giant, and Bohemian knotweeds can overlap in overall appearance. These photographs retain the source identification; a field determination should use mature leaf bases, underside hairs, and other diagnostic characters.
Wikimedia server-resized derivative; no local crop, colour change, or generative editing. Gallery display preserves the full image; the catalog preview uses a CSS crop.

Recognize

Tall hollow, jointed stems carry alternate, broad leaves with generally flattened bases and small, pale flower clusters. N-reynoutria-japonica

Plants & habitat

Rhizomes form dense stands and cut stems can root in wet conditions. Growth can displace streambank vegetation; avoid moving contaminated soil or plant pieces. N-reynoutria-japonica

People

The reviewed sources do not establish species-specific ingestion effects or a safe exposure amount for people. Missing evidence does not establish safety.

Dogs & cats

The reviewed sources do not establish species-specific ingestion effects or a safe exposure amount for dogs or cats. Missing evidence does not establish safety.

Other animals

The reviewed sources do not establish species-specific ingestion effects or a safe exposure amount for livestock. Missing evidence does not establish safety.

Before taking action

Confirm identification and the requirements of your county weed program. For chemical treatment, follow the current product label, including restrictions around animals and water. S27

This is a statewide eradication priority. Contact your county weed program promptly for identification and an approved management and disposal plan. CO-rules-2025

References

CO-rules-2025 — Colorado Noxious Weed Act rules, 8 CCR 1206-2, Colorado Secretary of State / Department of Agriculture. Accessed 2026-09-07T01:07:48+00:00. Locator: Parts 3.1, 4.1, 5.1; management provisions; effective May 15, 2025.
N-reynoutria-japonica — reynoutria japonica — Plant Toolbox, NC State Extension. Accessed 2026-09-07T01:04:52+00:00. Locator: Description; Attributes; Problems; Poisonous to Humans.
S27 — 2103 – Colorado Noxious Weeds, PlantTalk Colorado · CSU Extension. Accessed 2026-09-07T00:23:57+00:00. Locator: List A, B and C management categories.

Content revised 2026-09-07 03:50:00Z · Source checks dated separately in the references.