---
{
  "id": "hydrilla",
  "name": "Hydrilla",
  "scientific": "Hydrilla verticillata",
  "category": "invasive",
  "status": "Colorado noxious weed · List A — Eradicate",
  "status_sources": [
    "CO-rules-2025"
  ],
  "warning": "Dense aquatic growth; wildlife disease association",
  "warning_tone": "caution",
  "aliases": [],
  "taxonomy_note": "Separate hydrilla from native waterweeds using a botanical key. Disease association is not proof that every plant or waterbody is toxic.",
  "source_ids": [
    "CO-rules-2025",
    "N-hydrilla-verticillata",
    "S27",
    "Fremont-list-definitions"
  ],
  "description": "Hydrilla: Dense aquatic growth; wildlife disease association. Identification and evidence for Colorado readers.",
  "claim_evidence": [
    {
      "label": "Recognize",
      "sources": [
        "N-hydrilla-verticillata"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "hydrilla-1"
    },
    {
      "label": "Plants & habitat",
      "sources": [
        "N-hydrilla-verticillata"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "hydrilla-2"
    },
    {
      "label": "People",
      "sources": [],
      "scope": "Review limitation",
      "note": "",
      "id": "hydrilla-3"
    },
    {
      "label": "Dogs & cats",
      "sources": [],
      "scope": "Review limitation",
      "note": "",
      "id": "hydrilla-4"
    },
    {
      "label": "Other animals",
      "sources": [
        "N-hydrilla-verticillata"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "hydrilla-5"
    },
    {
      "id": "hydrilla-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": [
          "hydrilla-3"
        ]
      },
      {
        "area": "Dogs & cats",
        "question": "Find species-specific dog and cat exposure evidence; do not infer safety from missing reports.",
        "related_claim_ids": [
          "hydrilla-4"
        ]
      },
      {
        "area": "Other animals",
        "question": "Clarify feeding and exposure risks for each animal group; do not transfer findings between species.",
        "related_claim_ids": [
          "hydrilla-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/hydrilla.md"
}
---

# Hydrilla

*Hydrilla verticillata*

**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).

> **Dense aquatic growth; wildlife disease association.** [Exposure help](https://denson.github.io/colorado-weed-field-guide/safety/).

**Identification caveat:** Separate hydrilla from native waterweeds using a botanical key. Disease association is not proof that every plant or waterbody is toxic.

## 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.

![Hydrilla verticillata: A wet stem tip with short leaves in whorls and toothed leaf margins.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-a-photos-hydrilla-verticillata-1.jpg)

**leaf-whorl detail** — A wet stem tip with short leaves in whorls and toothed leaf margins.

Photo: Stanislav Murashkin; [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/); [source record](https://www.inaturalist.org/observations/175178376); [original image](https://inaturalist-open-data.s3.amazonaws.com/photos/304416621/original.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. Other submerged waterweeds can look similar. Identification follows the sources; the close view shows toothed leaves in whorls, while the whole-shoot photographs show overall structure. iNaturalist large server-resized image; no local crop, colour change, or generative editing. Gallery display preserves the full image; the catalog preview uses a CSS crop.

![Hydrilla verticillata: Several long, branching shoots laid against a plain background, showing spaced leaf whorls and lower root-like structures.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-a-photos-hydrilla-verticillata-2.jpg)

**whole shoots** — Several long, branching shoots laid against a plain background, showing spaced leaf whorls and lower root-like structures.

Photo: Darkmax; [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0); [source record](https://commons.wikimedia.org/wiki/File:Hydrilla_Verticillata_3.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/3/3d/Hydrilla_Verticillata_3.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. Other submerged waterweeds can look similar. Identification follows the sources; the close view shows toothed leaves in whorls, while the whole-shoot photographs show overall structure. 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.

![Hydrilla verticillata: A long hydrilla stem held above a stream, showing repeated dense leaf whorls.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-a-photos-hydrilla-verticillata-3.jpg)

**whole stem** — A long hydrilla stem held above a stream, showing repeated dense leaf whorls.

Photo: Alex Abair; [CC BY 4.0](https://creativecommons.org/licenses/by/4.0); [source record](https://commons.wikimedia.org/wiki/File:Hydrilla_verticillata_54198908.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/4/47/Hydrilla_verticillata_54198908.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. Other submerged waterweeds can look similar. Identification follows the sources; the close view shows toothed leaves in whorls, while the whole-shoot photographs show overall structure. 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

A submerged plant with whorls of small, toothed leaves, often with teeth beneath the midrib. It can form dense surface growth. [N-hydrilla-verticillata](https://plants.ces.ncsu.edu/plants/hydrilla-verticillata/)

## Plants & habitat

Fragments and overwintering tubers support persistent infestations. Dense growth displaces native aquatic vegetation. [N-hydrilla-verticillata](https://plants.ces.ncsu.edu/plants/hydrilla-verticillata/)

## 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

NC State reports an association with bacteria involved in a neurological disease of waterfowl. The plant’s presence alone does not establish toxin exposure at a particular Colorado waterbody. [N-hydrilla-verticillata](https://plants.ces.ncsu.edu/plants/hydrilla-verticillata/)

## 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-hydrilla-verticillata** — [hydrilla verticillata — Plant Toolbox](https://plants.ces.ncsu.edu/plants/hydrilla-verticillata/), 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=hydrilla) — 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

Separate hydrilla from native waterweeds using a botanical key. Disease association is not proof that every plant or waterbody is toxic.

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 |
|---|---|---|---|---|
| hydrilla-1 | Recognize | Species-specific | N-hydrilla-verticillata | No quantitative safe exposure established here. |
| hydrilla-2 | Plants & habitat | Species-specific | N-hydrilla-verticillata | No quantitative safe exposure established here. |
| hydrilla-3 | People | Review limitation | Review limitation | No quantitative safe exposure established here. |
| hydrilla-4 | Dogs & cats | Review limitation | Review limitation | No quantitative safe exposure established here. |
| hydrilla-5 | Other animals | Species-specific | N-hydrilla-verticillata | 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/hydrilla/

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

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

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