--- { "id": "horseweed", "name": "Horseweed", "scientific": "Erigeron canadensis", "category": "native", "status": "Native Colorado plant · not on state Lists A/B/C", "status_sources": [ "C-floristic-2020", "CO-rules-2025" ], "warning": "Fast windborne spread; some populations resist herbicides", "warning_tone": "caution", "aliases": [ "Conyza canadensis", "marestail" ], "taxonomy_note": "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.", "source_ids": [ "C-floristic-2020", "CO-rules-2025", "N-erigeron-canadensis", "S27" ], "description": "Horseweed: Fast windborne spread; some populations resist herbicides. Identification and evidence for Colorado readers.", "claim_evidence": [ { "label": "Identification", "sources": [ "N-erigeron-canadensis" ], "scope": "Species-specific", "note": "", "id": "horseweed-1" }, { "label": "People", "sources": [ "N-erigeron-canadensis" ], "scope": "Species-specific", "note": "", "id": "horseweed-2" }, { "label": "Dogs & cats", "sources": [], "scope": "Review limitation", "note": "", "id": "horseweed-3" }, { "label": "Other animals", "sources": [ "N-erigeron-canadensis" ], "scope": "Species-specific", "note": "", "id": "horseweed-4" }, { "label": "Plants & habitat", "sources": [ "N-erigeron-canadensis" ], "scope": "Species-specific", "note": "", "id": "horseweed-5" }, { "label": "Management & timing", "sources": [ "N-erigeron-canadensis" ], "scope": "Species-specific", "note": "", "id": "horseweed-6" } ], "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": [ "horseweed-2" ] }, { "area": "Dogs & cats", "question": "Find species-specific dog and cat exposure evidence; do not infer safety from missing reports.", "related_claim_ids": [ "horseweed-3" ] }, { "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" }, "colorado_relevance": { "source_id": "C-floristic-2020", "locator": "Appendix B; PDF page 46; Conyza canadensis", "basis": "Included in the 2020 statewide flora assessment. This is not a current county abundance survey.", "source_taxon": "Conyza canadensis" }, "source": "content/plants/horseweed.md" } --- # Horseweed *Erigeron canadensis* **Native Colorado plant · not on state Lists A/B/C.** [C-floristic-2020](https://cnhp.colostate.edu/download/documents/2020/CO_FQA_2020_Final_Report.pdf) [CO-rules-2025](https://www.sos.state.co.us/CCR/GenerateRulePdf.do?ruleVersionId=11948&fileName=8+CCR+1206-2) > **Fast windborne spread; some populations resist herbicides.** [Exposure help](https://denson.github.io/colorado-weed-field-guide/safety/). **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 {#research-status} 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.](https://denson.github.io/colorado-weed-field-guide/assets/photos/coverage-hazard-photos-erigeron-canadensis-1.jpg) **mature habit** — A tall upright stem topped by a broad, finely branched pale mature head cluster. Photo: Enrico Blasutto; [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0); [source record](https://commons.wikimedia.org/wiki/File:Erigeron_canadensis_ENBLA01.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/f/fb/Erigeron_canadensis_ENBLA01.jpg). 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.](https://denson.github.io/colorado-weed-field-guide/assets/photos/coverage-hazard-photos-erigeron-canadensis-2.jpg) **flowering stem** — A tall branched flowering stem with many small heads and numerous narrow leaves below. Photo: Enrico Blasutto; [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0); [source record](https://commons.wikimedia.org/wiki/File:Erigeron_canadensis_ENBLA04.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/c/c0/Erigeron_canadensis_ENBLA04.jpg). 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.](https://denson.github.io/colorado-weed-field-guide/assets/photos/coverage-hazard-photos-erigeron-canadensis-3.jpg) **seed head** — Close view of a small mature head releasing fine pale pappus hairs, beside green and spent heads. Photo: Enrico Blasutto; [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0); [source record](https://commons.wikimedia.org/wiki/File:Erigeron_canadensis_ENBLA07.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/1/19/Erigeron_canadensis_ENBLA07.jpg). 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](https://plants.ces.ncsu.edu/plants/erigeron-canadensis/) ## People NC State reports mucosal irritation. Avoid ingestion; the source does not establish a safe amount. [N-erigeron-canadensis](https://plants.ces.ncsu.edu/plants/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](https://plants.ces.ncsu.edu/plants/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](https://plants.ces.ncsu.edu/plants/erigeron-canadensis/) ## Management & timing Remove plants completely before seed release; broken stems can regrow. Seek local advice when repeated treatment fails. [N-erigeron-canadensis](https://plants.ces.ncsu.edu/plants/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](https://planttalk.colostate.edu/topics/weeds-cultural-problems/2103-colorado-noxious-weeds/) ## References - **C-floristic-2020** — [Colorado Floristic Quality Assessment: 2020 Update](https://cnhp.colostate.edu/download/documents/2020/CO_FQA_2020_Final_Report.pdf), 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](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-erigeron-canadensis** — [Erigeron canadensis](https://plants.ces.ncsu.edu/plants/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](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=horseweed) — choose this profile as a possible match and prepare your observations for Colorado Plants & Practical Care. # Appendix for agents Colorado inclusion: [C-floristic-2020](https://cnhp.colostate.edu/download/documents/2020/CO_FQA_2020_Final_Report.pdf), Appendix B; PDF page 46; Conyza canadensis. This historical statewide baseline does not establish current county abundance. ## 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. - **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 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. 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 | |---|---|---|---|---| | horseweed-1 | Identification | Species-specific | N-erigeron-canadensis | No quantitative safe exposure established here. | | horseweed-2 | People | Species-specific | N-erigeron-canadensis | No quantitative safe exposure established here. | | horseweed-3 | Dogs & cats | Review limitation | Review limitation | No quantitative safe exposure established here. | | horseweed-4 | Other animals | Species-specific | N-erigeron-canadensis | No quantitative safe exposure established here. | | horseweed-5 | Plants & habitat | Species-specific | N-erigeron-canadensis | No quantitative safe exposure established here. | | horseweed-6 | Management & timing | Species-specific | N-erigeron-canadensis | 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). --- Canonical HTML: https://denson.github.io/colorado-weed-field-guide/plants/horseweed/ Content revised: 2026-09-07T02:46:00Z Markdown mirror: https://denson.github.io/colorado-weed-field-guide/plants/horseweed/index.md Text fallback: https://denson.github.io/colorado-weed-field-guide/plants/horseweed/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