Eurasian watermilfoil

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

Myriophyllum spicatum

Colorado noxious weed · List B — Stop the spread. CO-rules-2025

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 · What the lists mean.

Fragments spread dense aquatic growth. Exposure help.

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

Still researching this plant

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.

**emergent flowering shoot** — A watermilfoil shoot at the water surface with a small flower spike rising above the water.
Leslie J. Mehrhoff, University of Connecticut, Bugwood.org · CC BY 3.0 us
Photo source and identification
Original record · Original image
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.

**submerged foliage** — Fine, whorled watermilfoil foliage forming submerged branching growth in shallow water.
Donald Hobern from Copenhagen, Denmark · CC BY 2.0
Photo source and identification
Original record · Original image
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.

**female flowers** — Macro view of small female flowers in a whorl around a watermilfoil stem, with a 2 mm scale.
Stefan.lefnaer · CC BY-SA 4.0
Photo source and identification
Original record · Original image
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

Plants & habitat

Stem fragments spread readily. Dense growth crowds native plants and obstructs recreation and water infrastructure. N-myriophyllum-spicatum

People

Dense growth interferes with swimming and boating; avoid entering heavily vegetated water. N-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

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-myriophyllum-spicatum — myriophyllum spicatum — Plant Toolbox, NC State Extension. Accessed 2026-09-07T01:04:54+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.