Squarrose knapweed
Discuss this plant with the guide — choose this profile as a possible match and prepare your observations for Colorado Plants & Practical Care.
Centaurea virgata
Colorado noxious weed · List A — Eradicate. CO-rules-2025
Uncommon in Colorado. All populations are designated for eradication statewide. This is a management category, separate from danger ratings. Category source · What the lists mean.
Spiny seedheads; loss of rangeland forage. Exposure help.
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
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.
Alexander Dubynin · CC BY 4.0
Photo source and identification
Original record · Original image
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.
Alexander Dubynin · CC BY 4.0
Photo source and identification
Original record · Original image
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.
Alexander Dubynin · CC BY 4.0
Photo source and identification
Original record · Original image
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
Plants & habitat
Dense stands reduce forage. Seed-bearing heads catch on animal hair, wool and clothing, spreading infestations. NPS-squarrose
People
Hooked bract spines warrant gloves and careful handling. NPS-squarrose
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
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
This is a statewide eradication priority. Contact your county weed program promptly for identification and an approved management and disposal plan. CO-rules-2025
References
- CO-rules-2025 — Colorado Noxious Weed Act rules, 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, 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, PlantTalk Colorado · CSU Extension. Accessed 2026-09-07T00:23:57+00:00. Locator: List A, B and C management categories.