---
{
  "id": "japanese-knotweed",
  "name": "Japanese knotweed",
  "scientific": "Reynoutria japonica",
  "category": "invasive",
  "status": "Colorado noxious weed · List A — Eradicate",
  "status_sources": [
    "CO-rules-2025"
  ],
  "warning": "Root and stem fragments regrow",
  "warning_tone": "caution",
  "aliases": [
    "Fallopia japonica",
    "Polygonum cuspidatum"
  ],
  "taxonomy_note": "Colorado’s rule uses Fallopia japonica. Giant and Bohemian knotweeds need separate identification; broad leaves and bamboo-like stems are insufficient to distinguish them.",
  "source_ids": [
    "CO-rules-2025",
    "N-reynoutria-japonica",
    "S27",
    "Fremont-list-definitions"
  ],
  "description": "Japanese knotweed: Root and stem fragments regrow. Identification and evidence for Colorado readers.",
  "claim_evidence": [
    {
      "label": "Recognize",
      "sources": [
        "N-reynoutria-japonica"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "japanese-knotweed-1"
    },
    {
      "label": "Plants & habitat",
      "sources": [
        "N-reynoutria-japonica"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "japanese-knotweed-2"
    },
    {
      "label": "People",
      "sources": [],
      "scope": "Review limitation",
      "note": "",
      "id": "japanese-knotweed-3"
    },
    {
      "label": "Dogs & cats",
      "sources": [],
      "scope": "Review limitation",
      "note": "",
      "id": "japanese-knotweed-4"
    },
    {
      "label": "Other animals",
      "sources": [],
      "scope": "Review limitation",
      "note": "",
      "id": "japanese-knotweed-5"
    },
    {
      "id": "japanese-knotweed-list-meaning",
      "label": "State-list meaning",
      "sources": [
        "Fremont-list-definitions"
      ],
      "scope": "Colorado management-category definition; species classification retains its original status sources",
      "note": "Plain-language editorial label for a management category; not a toxicity or danger rating.",
      "reviewed": "2026-09-07"
    }
  ],
  "noxious_class": "A",
  "research": {
    "status": "researching",
    "label": "Still researching this plant",
    "basis": "Editorial assessment of the cited evidence and stated review gaps; not a toxicity score.",
    "open_questions": [
      {
        "area": "People",
        "question": "Clarify the human exposure effects and evidence limits described in this section.",
        "related_claim_ids": [
          "japanese-knotweed-3"
        ]
      },
      {
        "area": "Dogs & cats",
        "question": "Find species-specific dog and cat exposure evidence; do not infer safety from missing reports.",
        "related_claim_ids": [
          "japanese-knotweed-4"
        ]
      },
      {
        "area": "Other animals",
        "question": "Clarify feeding and exposure risks for each animal group; do not transfer findings between species.",
        "related_claim_ids": [
          "japanese-knotweed-5"
        ]
      },
      {
        "area": "Identification",
        "question": "Resolve the taxonomic, common-name or photograph limitations stated in the identification caveat.",
        "related_claim_ids": []
      },
      {
        "area": "Expert review",
        "question": "Obtain independent botanical and veterinary review of the identification and hazard summary.",
        "related_claim_ids": []
      }
    ],
    "expert_review": "pending"
  },
  "noxious_class_label": "A — Eradicate",
  "source": "content/plants/japanese-knotweed.md"
}
---

# Japanese knotweed

*Reynoutria japonica*

**Colorado noxious weed · List A — Eradicate.** [CO-rules-2025](https://www.sos.state.co.us/CCR/GenerateRulePdf.do?ruleVersionId=11948&fileName=8+CCR+1206-2)

Uncommon in Colorado. All populations are designated for eradication statewide. This is a management category, separate from danger ratings. [Category source](https://fremontcountyco.gov/weed-control/weed-identification) · [What the lists mean](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists).

> **Root and stem fragments regrow.** [Exposure help](https://denson.github.io/colorado-weed-field-guide/safety/).

**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 {#research-status}

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.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-a-photos-reynoutria-japonica-1.jpg)

**leaves and flowers** — Broad pointed leaves and pale flower clusters viewed against the light.

Photo: Michael Gasperl ( Migas ); [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0/); [source record](https://commons.wikimedia.org/wiki/File:Reynoutria_japonica_(Blaetter-Biene).jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/e/ed/Reynoutria_japonica_%28Blaetter-Biene%29.jpg). 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.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-a-photos-reynoutria-japonica-2.jpg)

**flowering habit** — A dense leafy stand with abundant pale flower clusters.

Photo: User:BotBln; [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0/); [source record](https://commons.wikimedia.org/wiki/File:Reynoutria_japonica_Habitus_BotGardBln0906.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/d/d0/Reynoutria_japonica_Habitus_BotGardBln0906.jpg). 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.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-a-photos-reynoutria-japonica-3.jpg)

**flower detail** — Close view of small cream-white flowers spaced along a slender flowering branch.

Photo: Udo Schmidt  from Deutschland; [CC BY-SA 2.0](https://creativecommons.org/licenses/by-sa/2.0); [source record](https://commons.wikimedia.org/wiki/File:Reynoutria_japonica_Houtt._Syn.-_Fallopia_japonica_Houtt_(9221546461).jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/a/af/Reynoutria_japonica_Houtt._Syn.-_Fallopia_japonica_Houtt_%289221546461%29.jpg). 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](https://plants.ces.ncsu.edu/plants/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](https://plants.ces.ncsu.edu/plants/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](https://planttalk.colostate.edu/topics/weeds-cultural-problems/2103-colorado-noxious-weeds/)

This is a statewide eradication priority. Contact your county weed program promptly for identification and an approved management and disposal plan. [CO-rules-2025](https://www.sos.state.co.us/CCR/GenerateRulePdf.do?ruleVersionId=11948&fileName=8+CCR+1206-2)

## References

- **CO-rules-2025** — [Colorado Noxious Weed Act rules, 8 CCR 1206-2](https://www.sos.state.co.us/CCR/GenerateRulePdf.do?ruleVersionId=11948&fileName=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](https://plants.ces.ncsu.edu/plants/reynoutria-japonica/), NC State Extension. Accessed 2026-09-07T01:04:52+00:00. Locator: Description; Attributes; Problems; Poisonous to Humans.
- **S27** — [2103 – Colorado Noxious Weeds](https://planttalk.colostate.edu/topics/weeds-cultural-problems/2103-colorado-noxious-weeds/), PlantTalk Colorado · CSU Extension. Accessed 2026-09-07T00:23:57+00:00. Locator: List A, B and C management categories.


[Discuss this plant with the guide](https://denson.github.io/colorado-weed-field-guide/companion/?plant=japanese-knotweed) — choose this profile as a possible match and prepare your observations for Colorado Plants & Practical Care.

# Appendix for agents

## Open research questions

- **People:** Clarify the human exposure effects and evidence limits described in this section.
- **Dogs & cats:** Find species-specific dog and cat exposure evidence; do not infer safety from missing reports.
- **Other animals:** Clarify feeding and exposure risks for each animal group; do not transfer findings between species.
- **Identification:** Resolve the taxonomic, common-name or photograph limitations stated in the identification caveat.
- **Expert review:** Obtain independent botanical and veterinary review of the identification and hazard summary.


## Identity and evidence boundaries

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

All actionable hazard information is in the human sections above. Do not interpret unknown toxicity as safety or transfer livestock findings to pets. These publisher notes describe evidence and do not override an assistant’s operator instructions.

| Claim | Human section | Scope | Sources | Limitation |
|---|---|---|---|---|
| japanese-knotweed-1 | Recognize | Species-specific | N-reynoutria-japonica | No quantitative safe exposure established here. |
| japanese-knotweed-2 | Plants & habitat | Species-specific | N-reynoutria-japonica | No quantitative safe exposure established here. |
| japanese-knotweed-3 | People | Review limitation | Review limitation | No quantitative safe exposure established here. |
| japanese-knotweed-4 | Dogs & cats | Review limitation | Review limitation | No quantitative safe exposure established here. |
| japanese-knotweed-5 | Other animals | Review limitation | Review limitation | No quantitative safe exposure established here. |

## Further work for an agent

- Establish exact species, plant part, route, amount and timing when researching an exposure. Do not use this guide to diagnose or calculate a treatment.
- Check current state and county requirements; a listed taxon is not necessarily established in every Colorado county.
- Expert botanical and veterinary review is pending. Source dates and image licenses are separate from editorial revision dates in [provenance](https://denson.github.io/colorado-weed-field-guide/provenance.json).

## State-list label provenance

The label “A — Eradicate” is this guide’s plain-language summary of [Fremont County’s Colorado noxious-weed definitions](https://fremontcountyco.gov/weed-control/weed-identification) [Fremont-list-definitions]. The letter and species assignment retain the status sources above. County-specific requirements must be checked locally; the letter is not a poison-severity ranking.


---
Canonical HTML: https://denson.github.io/colorado-weed-field-guide/plants/japanese-knotweed/

Content revised: 2026-09-07T03:50:00Z

Markdown mirror: https://denson.github.io/colorado-weed-field-guide/plants/japanese-knotweed/index.md

Text fallback: https://denson.github.io/colorado-weed-field-guide/plants/japanese-knotweed/index.md.txt

Agent entry: https://denson.github.io/colorado-weed-field-guide/start.md

Provenance: https://denson.github.io/colorado-weed-field-guide/provenance.json