---
{
  "id": "diffuse-knapweed",
  "name": "Diffuse knapweed",
  "scientific": "Centaurea diffusa",
  "category": "invasive",
  "status": "Colorado noxious weed · List B — Stop the spread",
  "status_sources": [
    "CO-rules-2025"
  ],
  "warning": "Skin irritation; tumbling seed spread",
  "warning_tone": "caution",
  "aliases": [],
  "taxonomy_note": "Spotted knapweed and spotted × diffuse hybrids require separate identification. Russian-knapweed horse poisoning must not be transferred merely because the common names overlap.",
  "source_ids": [
    "CO-rules-2025",
    "J-diffuse-knapweed",
    "S27",
    "Fremont-list-definitions"
  ],
  "description": "Diffuse knapweed: Skin irritation; tumbling seed spread. Identification and evidence for Colorado readers.",
  "claim_evidence": [
    {
      "label": "Recognize",
      "sources": [
        "J-diffuse-knapweed"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "diffuse-knapweed-1"
    },
    {
      "label": "Plants & habitat",
      "sources": [
        "J-diffuse-knapweed"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "diffuse-knapweed-2"
    },
    {
      "label": "People",
      "sources": [
        "J-diffuse-knapweed"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "diffuse-knapweed-3"
    },
    {
      "label": "Dogs & cats",
      "sources": [
        "J-diffuse-knapweed"
      ],
      "scope": "Physical-risk inference; chemical-toxicity gap",
      "note": "",
      "id": "diffuse-knapweed-4"
    },
    {
      "label": "Other animals",
      "sources": [],
      "scope": "Review limitation",
      "note": "",
      "id": "diffuse-knapweed-5"
    },
    {
      "id": "diffuse-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": "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": [
          "diffuse-knapweed-4"
        ]
      },
      {
        "area": "Other animals",
        "question": "Clarify feeding and exposure risks for each animal group; do not transfer findings between species.",
        "related_claim_ids": [
          "diffuse-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": "B — Stop the spread",
  "source": "content/plants/diffuse-knapweed.md"
}
---

# Diffuse knapweed

*Centaurea diffusa*

**Colorado noxious weed · List B — Stop the spread.** [CO-rules-2025](https://www.sos.state.co.us/CCR/GenerateRulePdf.do?ruleVersionId=11948&fileName=8+CCR+1206-2)

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).

> **Skin irritation; tumbling seed spread.** [Exposure help](https://denson.github.io/colorado-weed-field-guide/safety/).

**Identification caveat:** Spotted knapweed and spotted × diffuse hybrids require separate identification. Russian-knapweed horse poisoning must not be transferred merely because the common names overlap.

## 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.

![Dense, widely branched diffuse knapweed bearing many small pale flower heads; photographed at Williams Fork Reservoir, Colorado.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-bc-photos-centaurea-diffusa-02.jpg)

**habit** — Dense, widely branched diffuse knapweed bearing many small pale flower heads; photographed at Williams Fork Reservoir, Colorado.

Photo: Kenraiz; [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0); [source record](https://commons.wikimedia.org/wiki/File:Centaurea_diffusa_kz06.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/8/87/Centaurea_diffusa_kz06.jpg). Source-identified photograph with visual consistency check Centaurea diffusa at Williams Fork Reservoir, Colorado, USA Source labeling and visible consistency checked; not an independent specimen determination. Wikimedia-generated thumbnail rescaling only; downloaded bytes unchanged. Gallery display preserves the full image; the catalog preview uses a CSS crop.

![Branching stems carrying small knapweed heads with pale spiny bracts.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-bc-photos-centaurea-diffusa-06.jpg)

**heads and bracts** — Branching stems carrying small knapweed heads with pale spiny bracts.

Photo: Stefan.lefnaer; [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0); [source record](https://commons.wikimedia.org/wiki/File:Centaurea_diffusa_sl40.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/5/5e/Centaurea_diffusa_sl40.jpg). Source-identified photograph with visual consistency check Inflorescence with capitula Taxonym: Centaurea diffusa ss Fischer et al. EfÖLS 2008 ISBN 978-3-85474-187-9 Location: Kühberg east of Wieselsfeld next to Hollabrunn, district Hollabrunn, Lower Austria - ca. 325 m a.s.l. Habitat: fallow land Source labeling and visible consistency checked; not an independent specimen determination. Wikimedia-generated thumbnail rescaling only; downloaded bytes unchanged. Gallery display preserves the full image; the catalog preview uses a CSS crop.

![Two pale knapweed achenes with short bristles, shown at close range with a 3 mm scale.](https://denson.github.io/colorado-weed-field-guide/assets/photos/list-bc-photos-centaurea-diffusa-08.jpg)

**achenes** — Two pale knapweed achenes with short bristles, shown at close range with a 3 mm scale.

Photo: Stefan.lefnaer; [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0); [source record](https://commons.wikimedia.org/wiki/File:Centaurea_diffusa_sl12.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/2/25/Centaurea_diffusa_sl12.jpg). Source-identified photograph with visual consistency check Achenes with short pappus Taxonym: Centaurea diffusa ss Fischer et al. EfÖLS 2008 ISBN 978-3-85474-187-9 Location: sand pit 0,4 south-eastern of Schleinbach, district Mistelbach, Lower Austria - ca. 270 m a.s.l. Habitat: extraction face of a sand pit Source labeling and visible consistency checked; not an independent specimen determination. Wikimedia-generated thumbnail rescaling only; downloaded bytes unchanged. Gallery display preserves the full image; the catalog preview uses a CSS crop.


## Recognize

Much-branched stems carry small white to pale purple flowerheads with long terminal spines on their bracts. [J-diffuse-knapweed](https://www.jeffco.us/2466/Diffuse-Knapweed)

## Plants & habitat

Dry plants break off and tumble, spreading seeds through disturbed rangeland and roadsides. [J-diffuse-knapweed](https://www.jeffco.us/2466/Diffuse-Knapweed)

## People

The county reports skin irritation. Wear gloves when handling plants. [J-diffuse-knapweed](https://www.jeffco.us/2466/Diffuse-Knapweed)

## Dogs & cats

Spiny bracts present a physical contact concern. The reviewed source does not establish pet ingestion effects. [J-diffuse-knapweed](https://www.jeffco.us/2466/Diffuse-Knapweed)

*Evidence scope: Physical-risk inference; chemical-toxicity gap.*

## 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/)

## 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.
- **J-diffuse-knapweed** — [Diffuse Knapweed](https://www.jeffco.us/2466/Diffuse-Knapweed), Jefferson County, Colorado. Accessed 2026-09-07T00:27:22+00:00. Locator: Description, impacts, Colorado list designation, management.
- **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=diffuse-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

Spotted knapweed and spotted × diffuse hybrids require separate identification. Russian-knapweed horse poisoning must not be transferred merely because the common names overlap.

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 |
|---|---|---|---|---|
| diffuse-knapweed-1 | Recognize | Species-specific | J-diffuse-knapweed | No quantitative safe exposure established here. |
| diffuse-knapweed-2 | Plants & habitat | Species-specific | J-diffuse-knapweed | No quantitative safe exposure established here. |
| diffuse-knapweed-3 | People | Species-specific | J-diffuse-knapweed | No quantitative safe exposure established here. |
| diffuse-knapweed-4 | Dogs & cats | Physical-risk inference; chemical-toxicity gap | J-diffuse-knapweed | No quantitative safe exposure established here. |
| diffuse-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 “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/diffuse-knapweed/

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

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

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