Horseweed

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

Erigeron canadensis

Native Colorado plant · not on state Lists A/B/C. C-floristic-2020 CO-rules-2025

Fast windborne spread; some populations resist herbicides. Exposure help.

Identification caveat: CNHP treats Conyza canadensis as native in its 2020 assessment and records historical disagreement over western nativity. NC State uses Erigeron canadensis. Horseweed is also a common name for unrelated plants.

Still researching this plant

This is a sourced working profile. Evidence is incomplete for People, Dogs & cats. 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.

Erigeron canadensis: A tall upright stem topped by a broad, finely branched pale mature head cluster. — **mature habit** — A tall upright stem topped by a broad, finely branched pale mature head cluster.
Enrico Blasutto · 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, not an independent specimen-based taxonomic determination. Source file titles use Erigeron canadensis and source descriptions use its synonym Conyza canadensis. Similar horseweed species were not substituted; source names are preserved.
Wikimedia server-resized derivative; no local crop, colour change or generative editing. Gallery display preserves the full image; catalog preview may use a CSS crop.

Erigeron canadensis: A tall branched flowering stem with many small heads and numerous narrow leaves below. — **flowering stem** — A tall branched flowering stem with many small heads and numerous narrow leaves below.
Enrico Blasutto · 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, not an independent specimen-based taxonomic determination. Source file titles use Erigeron canadensis and source descriptions use its synonym Conyza canadensis. Similar horseweed species were not substituted; source names are preserved.
Wikimedia server-resized derivative; no local crop, colour change or generative editing. Gallery display preserves the full image; catalog preview may use a CSS crop.

Erigeron canadensis: Close view of a small mature head releasing fine pale pappus hairs, beside green and spent heads. — **seed head** — Close view of a small mature head releasing fine pale pappus hairs, beside green and spent heads.
Enrico Blasutto · 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, not an independent specimen-based taxonomic determination. Source file titles use Erigeron canadensis and source descriptions use its synonym Conyza canadensis. Similar horseweed species were not substituted; source names are preserved.
Wikimedia server-resized derivative; no local crop, colour change or generative editing. Gallery display preserves the full image; catalog preview may use a CSS crop.

Identification

Starts as a hairy leaf rosette, then rises into a narrow upright plant with many small pale flower heads near the top. Leaves become smaller up the stem. N-erigeron-canadensis

People

NC State reports mucosal irritation. Avoid ingestion; the source does not establish a safe amount. N-erigeron-canadensis

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 same source reports mucosal irritation in livestock. It gives no validated safe feeding rate. N-erigeron-canadensis

Plants & habitat

A prolific disturbance weed with windborne seed; flowers and hollow stems also support native bees. Glyphosate resistance is documented in some populations, not assumed for every Colorado patch. N-erigeron-canadensis

Management & timing

Remove plants completely before seed release; broken stems can regrow. Seek local advice when repeated treatment fails. N-erigeron-canadensis

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

C-floristic-2020 — Colorado Floristic Quality Assessment: 2020 Update, Colorado Natural Heritage Program, Colorado State University. Accessed 2026-09-07T02:27:33+00:00. Locator: Appendix B, Colorado species list: scientific name and native/non-native fields.
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-erigeron-canadensis — Erigeron canadensis, NC State Extension. Accessed 2026-09-07T02:23:12+00:00. Locator: Description, identification attributes and plant problems; toxicity section where present.
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 02:46:00Z · Source checks dated separately in the references.