Hydrilla

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

Hydrilla verticillata

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.

Dense aquatic growth; wildlife disease association. Exposure help.

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

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. — **leaf-whorl detail** — A wet stem tip with short leaves in whorls and toothed leaf margins.
Stanislav Murashkin · CC BY 4.0
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. 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. — **whole shoots** — Several long, branching shoots laid against a plain background, showing spaced leaf whorls and lower root-like structures.
Darkmax · 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. 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. — **whole stem** — A long hydrilla stem held above a stream, showing repeated dense leaf whorls.
Alex Abair · CC BY 4.0
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. 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

Plants & habitat

Fragments and overwintering tubers support persistent infestations. Dense growth displaces native aquatic vegetation. N-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

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-hydrilla-verticillata — hydrilla verticillata — 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.