---
{
  "id": "squarrose-knapweed",
  "name": "Squarrose knapweed",
  "scientific": "Centaurea virgata",
  "category": "invasive",
  "status": "Colorado noxious weed · List A — Eradicate",
  "status_sources": [
    "CO-rules-2025"
  ],
  "warning": "Spiny seedheads; loss of rangeland forage",
  "warning_tone": "caution",
  "aliases": [
    "Centaurea squarrosa",
    "Centaurea virgata subsp. squarrosa"
  ],
  "taxonomy_note": "The state listing uses C. virgata. The NPS account and photographs concern the narrower taxon C. virgata subsp. squarrosa. Do not transfer Russian-knapweed horse toxicity to this species.",
  "source_ids": [
    "CO-rules-2025",
    "NPS-squarrose",
    "S27",
    "Fremont-list-definitions"
  ],
  "description": "Squarrose knapweed: Spiny seedheads; loss of rangeland forage. Identification and evidence for Colorado readers.",
  "claim_evidence": [
    {
      "label": "Recognize",
      "sources": [
        "NPS-squarrose"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "squarrose-knapweed-1"
    },
    {
      "label": "Plants & habitat",
      "sources": [
        "NPS-squarrose"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "squarrose-knapweed-2"
    },
    {
      "label": "People",
      "sources": [
        "NPS-squarrose"
      ],
      "scope": "Physical-risk inference",
      "note": "",
      "id": "squarrose-knapweed-3"
    },
    {
      "label": "Dogs & cats",
      "sources": [
        "NPS-squarrose"
      ],
      "scope": "Attachment and physical-risk inference",
      "note": "",
      "id": "squarrose-knapweed-4"
    },
    {
      "label": "Other animals",
      "sources": [],
      "scope": "Review limitation",
      "note": "",
      "id": "squarrose-knapweed-5"
    },
    {
      "id": "squarrose-knapweed-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": [
          "squarrose-knapweed-4"
        ]
      },
      {
        "area": "Other animals",
        "question": "Clarify feeding and exposure risks for each animal group; do not transfer findings between species.",
        "related_claim_ids": [
          "squarrose-knapweed-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": "A — Eradicate",
  "source": "content/plants/squarrose-knapweed.md"
}
---

# Squarrose knapweed

*Centaurea virgata*

**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).

> **Spiny seedheads; loss of rangeland forage.** [Exposure help](https://denson.github.io/colorado-weed-field-guide/safety/).

**Identification caveat:** The state listing uses C. virgata. The NPS account and photographs concern the narrower taxon C. virgata subsp. squarrosa. Do not transfer Russian-knapweed horse toxicity to this 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

Source-identified reference photographs; these are not a complete identification key. A source location outside Colorado does not establish local occurrence.

![Centaurea virgata: A small pink-purple flower head and slender leafy stem held in front of a hand.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-a-photos-centaurea-virgata-1.jpg)

**flower and head** — A small pink-purple flower head and slender leafy stem held in front of a hand.

Photo: Alexander Dubynin; [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/); [source record](https://www.inaturalist.org/observations/238399194); [original image](https://inaturalist-open-data.s3.amazonaws.com/photos/424414396/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. The selected observations are identified more narrowly as Centaurea virgata squarrosa. The source subspecies name is preserved. 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 virgata: A slender branching stalk with dry flower heads held for viewing.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-a-photos-centaurea-virgata-2.jpg)

**dry heads** — A slender branching stalk with dry flower heads held for viewing.

Photo: Alexander Dubynin; [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/); [source record](https://www.inaturalist.org/observations/274426317); [original image](https://inaturalist-open-data.s3.amazonaws.com/photos/493336493/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. The selected observations are identified more narrowly as Centaurea virgata squarrosa. The source subspecies name is preserved. 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 virgata: A low, spreading, finely branched plant with narrow divided foliage in dry rocky ground.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-a-photos-centaurea-virgata-3.jpg)

**habit** — A low, spreading, finely branched plant with narrow divided foliage in dry rocky ground.

Photo: Alexander Dubynin; [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/); [source record](https://www.inaturalist.org/observations/274426317); [original image](https://inaturalist-open-data.s3.amazonaws.com/photos/493336428/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. The selected observations are identified more narrowly as Centaurea virgata squarrosa. The source subspecies name is preserved. 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

A branching perennial with a woody base, pink-purple heads and backward-curved spines on the flowerhead bracts. [NPS-squarrose](https://www.nps.gov/articles/squarrose-knapweed.htm)

## Plants & habitat

Dense stands reduce forage. Seed-bearing heads catch on animal hair, wool and clothing, spreading infestations. [NPS-squarrose](https://www.nps.gov/articles/squarrose-knapweed.htm)

## People

Hooked bract spines warrant gloves and careful handling. [NPS-squarrose](https://www.nps.gov/articles/squarrose-knapweed.htm)

*Evidence scope: Physical-risk inference.*

## Dogs & cats

Hooked heads can catch in fur; inspect animals after contact. Pet ingestion effects are not established in this review. [NPS-squarrose](https://www.nps.gov/articles/squarrose-knapweed.htm)

*Evidence scope: Attachment and physical-risk inference.*

## 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](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.
- **NPS-squarrose** — [Exotic Species: Squarrose Knapweed](https://www.nps.gov/articles/squarrose-knapweed.htm), US National Park Service. Accessed 2026-09-07T01:18:57+00:00. Locator: Species or hybrid identity, description and ecology.
- **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=squarrose-knapweed) — 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.


## Identity and evidence boundaries

The state listing uses C. virgata. The NPS account and photographs concern the narrower taxon C. virgata subsp. squarrosa. Do not transfer Russian-knapweed horse toxicity to this species.

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 |
|---|---|---|---|---|
| squarrose-knapweed-1 | Recognize | Species-specific | NPS-squarrose | No quantitative safe exposure established here. |
| squarrose-knapweed-2 | Plants & habitat | Species-specific | NPS-squarrose | No quantitative safe exposure established here. |
| squarrose-knapweed-3 | People | Physical-risk inference | NPS-squarrose | No quantitative safe exposure established here. |
| squarrose-knapweed-4 | Dogs & cats | Attachment and physical-risk inference | NPS-squarrose | No quantitative safe exposure established here. |
| squarrose-knapweed-5 | Other animals | Review limitation | Review limitation | 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/squarrose-knapweed/

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

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

Text fallback: https://denson.github.io/colorado-weed-field-guide/plants/squarrose-knapweed/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