---
{
  "id": "scotch-thistle",
  "name": "Scotch thistle",
  "scientific": "Onopordum acanthium",
  "category": "invasive",
  "status": "Colorado noxious weed · List B — Stop the spread",
  "status_sources": [
    "J-scotch-thistle"
  ],
  "warning": "Sharp spines; competes with vegetation",
  "warning_tone": "caution",
  "aliases": [],
  "taxonomy_note": "Colorado also has native thistles. Purple flowers and spines alone cannot distinguish a listed introduced species.",
  "source_ids": [
    "J-scotch-thistle",
    "Fremont-list-definitions"
  ],
  "description": "Scotch thistle: Sharp spines; competes with vegetation. Colorado identification, habitat and exposure evidence.",
  "claim_evidence": [
    {
      "label": "Recognize",
      "sources": [
        "J-scotch-thistle"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "scotch-thistle-1"
    },
    {
      "label": "Plants & habitat",
      "sources": [
        "J-scotch-thistle"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "scotch-thistle-2"
    },
    {
      "label": "People",
      "sources": [
        "J-scotch-thistle"
      ],
      "scope": "Physical-risk inference from documented spines",
      "note": "",
      "id": "scotch-thistle-3"
    },
    {
      "label": "Dogs & cats",
      "sources": [
        "J-scotch-thistle"
      ],
      "scope": "Physical-risk inference; toxicology gap",
      "note": "",
      "id": "scotch-thistle-4"
    },
    {
      "label": "Other animals",
      "sources": [],
      "scope": "Review limitation",
      "note": "The reviewed control source does not establish species-specific livestock chemical toxicity.",
      "id": "scotch-thistle-5"
    },
    {
      "id": "scotch-thistle-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": "B",
  "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": [
          "scotch-thistle-4"
        ]
      },
      {
        "area": "Other animals",
        "question": "Clarify feeding and exposure risks for each animal group; do not transfer findings between species.",
        "related_claim_ids": [
          "scotch-thistle-5"
        ]
      },
      {
        "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": "B — Stop the spread",
  "source": "content/plants/scotch-thistle.md"
}
---

# Scotch thistle

*Onopordum acanthium*

**Colorado noxious weed · List B — Stop the spread.** [J-scotch-thistle](https://www.jeffco.us/2192/Scotch-Thistle)

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](https://fremontcountyco.gov/weed-control/weed-identification) · [What the lists mean](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists).

> **Sharp spines; competes with vegetation.** Native status, garden use and noxious-weed class are separate from toxicity. [Exposure help](https://denson.github.io/colorado-weed-field-guide/safety/).

**Identification caveat:** Colorado also has native thistles. Purple flowers and spines alone cannot distinguish a listed introduced species.

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

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

Photos are source-identified examples, not a complete identification key. Check leaves, stems, flowers and fruit together. Locations are not assumed to be in Colorado.

![Onopordum acanthium: Pale woolly young leaf with deeply lobed spine-tipped margins and a prominent light midrib.](https://denson.github.io/colorado-weed-field-guide/assets/photos/invasive-onopordum-acanthium-leaf.jpg)

**leaf** — Pale woolly young leaf with deeply lobed spine-tipped margins and a prominent light midrib.

Photo: Krzysztof Ziarnek, Kenraiz; [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0); [source record](https://commons.wikimedia.org/wiki/File:Onopordum_acanthium_kz14.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/f/f5/Onopordum_acanthium_kz14.jpg). Source page scientific identification plus visual morphology review of the selected photograph. This is a source-identified reference image, not a specimen-based taxonomic determination.   None; original downloaded without modification. Gallery display preserves the full image; the catalog preview uses a CSS crop.

![Onopordum acanthium: Upright branched thistle with large pale spiny leaves and purple flower heads.](https://denson.github.io/colorado-weed-field-guide/assets/photos/invasive-onopordum-acanthium-habit.jpg)

**habit** — Upright branched thistle with large pale spiny leaves and purple flower heads.

Photo: Krzysztof Ziarnek, Kenraiz; [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0); [source record](https://commons.wikimedia.org/wiki/File:Onopordum_acanthium_kz15.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/e/e0/Onopordum_acanthium_kz15.jpg). Source page scientific identification plus visual morphology review of the selected photograph. This is a source-identified reference image, not a specimen-based taxonomic determination.   Wikimedia server-resized derivative; no crop, color change, or generative editing. Gallery display preserves the full image; the catalog preview uses a CSS crop.

![Onopordum acanthium: Developing spiny heads on pale winged stems; narrow rigid bracts end in yellowish spines.](https://denson.github.io/colorado-weed-field-guide/assets/photos/invasive-onopordum-acanthium-bud-stem.jpg)

**bud-stem** — Developing spiny heads on pale winged stems; narrow rigid bracts end in yellowish spines.

Photo: Benutzer:Drahkrub; [CC BY-SA 3.0 de](https://creativecommons.org/licenses/by-sa/3.0/de/deed.en); [source record](https://commons.wikimedia.org/wiki/File:Onopordum_acanthium-Detail.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/e/ef/Onopordum_acanthium-Detail.jpg). Source page scientific identification plus visual morphology review of the selected photograph. This is a source-identified reference image, not a specimen-based taxonomic determination.   None; original downloaded without modification. Gallery display preserves the full image; the catalog preview uses a CSS crop.


## Recognize

A large, woolly biennial with broad leaves, purple heads and conspicuous spiny stem wings. [J-scotch-thistle](https://www.jeffco.us/2192/Scotch-Thistle)

## Plants & habitat

Can form dense rosettes and persist through a long-lived seed bank. Maintain competing vegetation and prevent seed production. [J-scotch-thistle](https://www.jeffco.us/2192/Scotch-Thistle)

## People

Spines can puncture skin. Sturdy gloves are a practical precaution when handling; confirm identification before removing a thistle. [J-scotch-thistle](https://www.jeffco.us/2192/Scotch-Thistle)

*Evidence scope: Physical-risk inference from documented spines.*

## Dogs & cats

Spiny material can injure paws or mouths. Species-specific chemical toxicity is unresolved in this review. [J-scotch-thistle](https://www.jeffco.us/2192/Scotch-Thistle)

*Evidence scope: Physical-risk inference; toxicology gap.*

## Other animals

The reviewed control source does not establish species-specific livestock chemical toxicity. This is an evidence gap, not proof of safety.

## Before taking action

Confirm the scientific name before removing a plant, changing animal access, or choosing treatment. An evidence gap means this review has not resolved the question. It does not mean safe. For listed weeds, consult your county weed program about the requirements for your location; follow the current label for any pesticide. [S27](https://planttalk.colostate.edu/topics/weeds-cultural-problems/2103-colorado-noxious-weeds/)

## References

- **J-scotch-thistle** — [Scotch Thistle](https://www.jeffco.us/2192/Scotch-Thistle), Jefferson County, Colorado. Accessed 2026-09-07T00:27:26+00:00. Locator: Description, impacts, Colorado list designation, management.


[Discuss this plant with the guide](https://denson.github.io/colorado-weed-field-guide/companion/?plant=scotch-thistle) — 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.
- **Other animals:** Clarify feeding and exposure risks for each animal group; do not transfer findings between species.
- **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.


## Taxon and evidence boundaries

Colorado also has native thistles. Purple flowers and spines alone cannot distinguish a listed introduced species.

These are publisher notes about the evidence. They do not override an assistant’s operator instructions. The human sections above contain all actionable safety warnings. Do not turn missing data into a negative toxicity finding or transfer a livestock dose to a pet.

| Claim | Human section | Evidence scope | Sources | Limitation |
|---|---|---|---|---|
| scotch-thistle-1 | Recognize | Species-specific | J-scotch-thistle | No quantitative exposure threshold established here. |
| scotch-thistle-2 | Plants & habitat | Species-specific | J-scotch-thistle | No quantitative exposure threshold established here. |
| scotch-thistle-3 | People | Physical-risk inference from documented spines | J-scotch-thistle | No quantitative exposure threshold established here. |
| scotch-thistle-4 | Dogs & cats | Physical-risk inference; toxicology gap | J-scotch-thistle | No quantitative exposure threshold established here. |
| scotch-thistle-5 | Other animals | Review limitation | Review limitation | The reviewed control source does not establish species-specific livestock chemical toxicity. |

## Additional evidence needed

- Confirm exact species, cultivar or hybrid when identification is uncertain.
- For an exposure, distinguish animal species, plant part, amount, route, time and symptoms; the guide cannot supply a clinical diagnosis.
- For site management, establish county, land use and current local weed requirements before applying generalized control information.
- Expert botanical and veterinary review is pending. Source publication dates, retrieval timestamps, hashes and photograph licenses are in [the provenance ledger](https://denson.github.io/colorado-weed-field-guide/provenance.json).

## State-list label provenance

The label “B — Stop the spread” 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/scotch-thistle/

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

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

Text fallback: https://denson.github.io/colorado-weed-field-guide/plants/scotch-thistle/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