---
{
  "id": "yellow-starthistle",
  "name": "Yellow starthistle",
  "scientific": "Centaurea solstitialis",
  "category": "invasive",
  "status": "Colorado noxious weed · List A — Eradicate",
  "status_sources": [
    "CO-rules-2025"
  ],
  "warning": "Severe horse poisoning; sharp spines",
  "warning_tone": "danger",
  "aliases": [],
  "taxonomy_note": "A horse-specific poisoning hazard is established. Do not generalize controlled grazing by other livestock into unrestricted feeding advice.",
  "source_ids": [
    "CO-rules-2025",
    "N-centaurea-solstitialis",
    "S27",
    "Fremont-list-definitions"
  ],
  "description": "Yellow starthistle: Severe horse poisoning; sharp spines. Identification and evidence for Colorado readers.",
  "claim_evidence": [
    {
      "label": "Recognize",
      "sources": [
        "N-centaurea-solstitialis"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "yellow-starthistle-1"
    },
    {
      "label": "Plants & habitat",
      "sources": [
        "N-centaurea-solstitialis"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "yellow-starthistle-2"
    },
    {
      "label": "People",
      "sources": [
        "N-centaurea-solstitialis"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "yellow-starthistle-3"
    },
    {
      "label": "Dogs & cats",
      "sources": [
        "N-centaurea-solstitialis"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "yellow-starthistle-4"
    },
    {
      "label": "Other animals",
      "sources": [
        "N-centaurea-solstitialis"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "yellow-starthistle-5"
    },
    {
      "id": "yellow-starthistle-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": "Dogs & cats",
        "question": "Find species-specific dog and cat exposure evidence; do not infer safety from missing reports.",
        "related_claim_ids": [
          "yellow-starthistle-4"
        ]
      },
      {
        "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/yellow-starthistle.md"
}
---

# Yellow starthistle

*Centaurea solstitialis*

**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).

> **Severe horse poisoning; sharp spines.** [Exposure help](https://denson.github.io/colorado-weed-field-guide/safety/).

**Identification caveat:** A horse-specific poisoning hazard is established. Do not generalize controlled grazing by other livestock into unrestricted feeding advice.

## Still researching this plant {#research-status}

This is a sourced working profile. Evidence is incomplete for **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.

![Centaurea solstitialis: A bright yellow flower head beside a dry head, showing the long radiating spines.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-a-photos-centaurea-solstitialis-1.jpg)

**flower and spines** — A bright yellow flower head beside a dry head, showing the long radiating spines.

Photo: Юрий Данилевский (Yuriy Danilevsky); [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/); [source record](https://www.inaturalist.org/observations/19881553); [original image](https://inaturalist-open-data.s3.amazonaws.com/photos/30658771/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. 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.

![Centaurea solstitialis: Close view of a strongly spined head and narrow leaves on a winged-looking stem.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-a-photos-centaurea-solstitialis-2.jpg)

**spines and foliage** — Close view of a strongly spined head and narrow leaves on a winged-looking stem.

Photo: Eleftherios Katsillis; [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/); [source record](https://www.inaturalist.org/observations/78210836); [original image](https://inaturalist-open-data.s3.amazonaws.com/photos/127963997/original.jpeg). Source scientific identification plus visual morphology review of the selected photograph. This is a source-identified reference image, not a specimen-based taxonomic determination. 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.

![Centaurea solstitialis: A tall, branching, yellow-flowered plant standing in dry grassland.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-a-photos-centaurea-solstitialis-3.jpg)

**flowering habit** — A tall, branching, yellow-flowered plant standing in dry grassland.

Photo: Юрий Данилевский (Yuriy Danilevsky); [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/); [source record](https://www.inaturalist.org/observations/19881553); [original image](https://inaturalist-open-data.s3.amazonaws.com/photos/30658804/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. 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.


## Recognize

An annual with yellow flowerheads surrounded by long, radiating spines and gray-green foliage. [N-centaurea-solstitialis](https://plants.ces.ncsu.edu/plants/centaurea-solstitialis/)

## Plants & habitat

Spreads by seed into rangelands and disturbed ground. Prevent seed set and keep contaminated material out of horse feed. [N-centaurea-solstitialis](https://plants.ces.ncsu.edu/plants/centaurea-solstitialis/)

## People

Long flowerhead spines present a puncture hazard; use protective handling. [N-centaurea-solstitialis](https://plants.ces.ncsu.edu/plants/centaurea-solstitialis/)

## Dogs & cats

Spines may injure mouths or paws. Horse poisoning findings do not establish a pet-specific toxic dose. [N-centaurea-solstitialis](https://plants.ces.ncsu.edu/plants/centaurea-solstitialis/)

## Other animals

Ingestion can cause devastating loss of chewing and swallowing function in horses. Suspected exposure warrants veterinary assessment. [N-centaurea-solstitialis](https://plants.ces.ncsu.edu/plants/centaurea-solstitialis/)

## 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-centaurea-solstitialis** — [centaurea solstitialis — Plant Toolbox](https://plants.ces.ncsu.edu/plants/centaurea-solstitialis/), NC State Extension. Accessed 2026-09-07T01:04:53+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=yellow-starthistle) — choose this profile as a possible match and prepare your observations for Colorado Plants & Practical Care.

# Appendix for agents

## Open research questions

- **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

A horse-specific poisoning hazard is established. Do not generalize controlled grazing by other livestock into unrestricted feeding advice.

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 |
|---|---|---|---|---|
| yellow-starthistle-1 | Recognize | Species-specific | N-centaurea-solstitialis | No quantitative safe exposure established here. |
| yellow-starthistle-2 | Plants & habitat | Species-specific | N-centaurea-solstitialis | No quantitative safe exposure established here. |
| yellow-starthistle-3 | People | Species-specific | N-centaurea-solstitialis | No quantitative safe exposure established here. |
| yellow-starthistle-4 | Dogs & cats | Species-specific | N-centaurea-solstitialis | No quantitative safe exposure established here. |
| yellow-starthistle-5 | Other animals | Species-specific | N-centaurea-solstitialis | 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/yellow-starthistle/

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

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

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