---
{
  "id": "eurasian-watermilfoil",
  "name": "Eurasian watermilfoil",
  "scientific": "Myriophyllum spicatum",
  "category": "invasive",
  "status": "Colorado noxious weed · List B — Stop the spread",
  "status_sources": [
    "CO-rules-2025"
  ],
  "warning": "Fragments spread dense aquatic growth",
  "warning_tone": "caution",
  "aliases": [],
  "taxonomy_note": "Native milfoils and hybrids can look similar. Confirm identification before management; a feathery leaf is not enough.",
  "source_ids": [
    "CO-rules-2025",
    "N-myriophyllum-spicatum",
    "S27",
    "Fremont-list-definitions"
  ],
  "description": "Eurasian watermilfoil: Fragments spread dense aquatic growth. Identification and evidence for Colorado readers.",
  "claim_evidence": [
    {
      "label": "Recognize",
      "sources": [
        "N-myriophyllum-spicatum"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "eurasian-watermilfoil-1"
    },
    {
      "label": "Plants & habitat",
      "sources": [
        "N-myriophyllum-spicatum"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "eurasian-watermilfoil-2"
    },
    {
      "label": "People",
      "sources": [
        "N-myriophyllum-spicatum"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "eurasian-watermilfoil-3"
    },
    {
      "label": "Dogs & cats",
      "sources": [],
      "scope": "Review limitation",
      "note": "",
      "id": "eurasian-watermilfoil-4"
    },
    {
      "label": "Other animals",
      "sources": [],
      "scope": "Review limitation",
      "note": "",
      "id": "eurasian-watermilfoil-5"
    },
    {
      "id": "eurasian-watermilfoil-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": "B",
  "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": "Dogs & cats",
        "question": "Find species-specific dog and cat exposure evidence; do not infer safety from missing reports.",
        "related_claim_ids": [
          "eurasian-watermilfoil-4"
        ]
      },
      {
        "area": "Other animals",
        "question": "Clarify feeding and exposure risks for each animal group; do not transfer findings between species.",
        "related_claim_ids": [
          "eurasian-watermilfoil-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": "B — Stop the spread",
  "source": "content/plants/eurasian-watermilfoil.md"
}
---

# Eurasian watermilfoil

*Myriophyllum spicatum*

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

Established in some areas. Requirements vary by location: eradication, containment or suppression to stop further spread. 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).

> **Fragments spread dense aquatic growth.** [Exposure help](https://denson.github.io/colorado-weed-field-guide/safety/).

**Identification caveat:** Native milfoils and hybrids can look similar. Confirm identification before management; a feathery leaf is not enough.

## Still researching this plant {#research-status}

This is a sourced working profile. Evidence is incomplete for **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.

![A watermilfoil shoot at the water surface with a small flower spike rising above the water.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-bc-photos-myriophyllum-spicatum-02.jpg)

**emergent flowering shoot** — A watermilfoil shoot at the water surface with a small flower spike rising above the water.

Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org; [CC BY 3.0 us](https://creativecommons.org/licenses/by/3.0/us/deed.en); [source record](https://commons.wikimedia.org/wiki/File:Myriophyllum_spicatum_5447200_(cropped).jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/1/1c/Myriophyllum_spicatum_5447200_%28cropped%29.jpg). Source-identified photograph with visual consistency check Eurasian water-milfoil - Myriophyllum spicatum L. - United States Source labeling and visible consistency checked; not an independent specimen determination. Wikimedia-generated thumbnail rescaling only; downloaded bytes unchanged. Gallery display preserves the full image; the catalog preview uses a CSS crop.

![Fine, whorled watermilfoil foliage forming submerged branching growth in shallow water.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-bc-photos-myriophyllum-spicatum-11.jpg)

**submerged foliage** — Fine, whorled watermilfoil foliage forming submerged branching growth in shallow water.

Photo: Donald Hobern from Copenhagen, Denmark; [CC BY 2.0](https://creativecommons.org/licenses/by/2.0); [source record](https://commons.wikimedia.org/wiki/File:Myriophyllum_spicatum_(7914509568).jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/7/71/Myriophyllum_spicatum_%287914509568%29.jpg). Source-identified photograph with visual consistency check Myriophyllum spicatum L., Spiked Water-milfoil, Øregaard Museum grounds, Hellerup, Denmark, 1 September 2012 Source labeling and visible consistency checked; not an independent specimen determination. Wikimedia-generated thumbnail rescaling only; downloaded bytes unchanged. Gallery display preserves the full image; the catalog preview uses a CSS crop.

![Macro view of small female flowers in a whorl around a watermilfoil stem, with a 2 mm scale.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-bc-photos-myriophyllum-spicatum-12.jpg)

**female flowers** — Macro view of small female flowers in a whorl around a watermilfoil stem, with a 2 mm scale.

Photo: Stefan.lefnaer; [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0); [source record](https://commons.wikimedia.org/wiki/File:Myriophyllum_spicatum_sl3.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/a/a8/Myriophyllum_spicatum_sl3.jpg). Source-identified photograph with visual consistency check Female flowers Taxonym: Myriophyllum spicatum ss Fischer et al. EfÖLS 2008 ISBN 978-3-85474-187-9 Location: Alte Donau, Vienna-Floridsdorf - ca. 160 m a.s.l. Habitat: oxbow lake Source labeling and visible consistency checked; not an independent specimen determination. Wikimedia-generated thumbnail rescaling only; downloaded bytes unchanged. Gallery display preserves the full image; the catalog preview uses a CSS crop.


## Recognize

Submerged, feather-like leaves occur in whorls; small flowering spikes emerge above the water. [N-myriophyllum-spicatum](https://plants.ces.ncsu.edu/plants/myriophyllum-spicatum/)

## Plants & habitat

Stem fragments spread readily. Dense growth crowds native plants and obstructs recreation and water infrastructure. [N-myriophyllum-spicatum](https://plants.ces.ncsu.edu/plants/myriophyllum-spicatum/)

## People

Dense growth interferes with swimming and boating; avoid entering heavily vegetated water. [N-myriophyllum-spicatum](https://plants.ces.ncsu.edu/plants/myriophyllum-spicatum/)

## 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/)

## 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-myriophyllum-spicatum** — [myriophyllum spicatum — Plant Toolbox](https://plants.ces.ncsu.edu/plants/myriophyllum-spicatum/), NC State Extension. Accessed 2026-09-07T01:04:54+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=eurasian-watermilfoil) — choose this profile as a possible match and prepare your observations for Colorado Plants & Practical Care.

# Appendix for agents

## Open research questions

- **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

Native milfoils and hybrids can look similar. Confirm identification before management; a feathery leaf is not enough.

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 |
|---|---|---|---|---|
| eurasian-watermilfoil-1 | Recognize | Species-specific | N-myriophyllum-spicatum | No quantitative safe exposure established here. |
| eurasian-watermilfoil-2 | Plants & habitat | Species-specific | N-myriophyllum-spicatum | No quantitative safe exposure established here. |
| eurasian-watermilfoil-3 | People | Species-specific | N-myriophyllum-spicatum | No quantitative safe exposure established here. |
| eurasian-watermilfoil-4 | Dogs & cats | Review limitation | Review limitation | No quantitative safe exposure established here. |
| eurasian-watermilfoil-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 “B — Stop the spread” 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/eurasian-watermilfoil/

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

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

Text fallback: https://denson.github.io/colorado-weed-field-guide/plants/eurasian-watermilfoil/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