Diffuse knapweed

Discuss this plant with the guide — choose this profile as a possible match and prepare your observations for Colorado Plants & Practical Care.

Centaurea diffusa

Colorado noxious weed · List B — Stop the spread. CO-rules-2025

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 · What the lists mean.

Skin irritation; tumbling seed spread. Exposure help.

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

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.

**habit** — Dense, widely branched diffuse knapweed bearing many small pale flower heads; photographed at Williams Fork Reservoir, Colorado.
Kenraiz · CC BY-SA 4.0
Photo source and identification
Original record · Original image
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.

**heads and bracts** — Branching stems carrying small knapweed heads with pale spiny bracts.
Stefan.lefnaer · CC BY-SA 4.0
Photo source and identification
Original record · Original image
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.

**achenes** — Two pale knapweed achenes with short bristles, shown at close range with a 3 mm scale.
Stefan.lefnaer · CC BY-SA 4.0
Photo source and identification
Original record · Original image
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

Plants & habitat

Dry plants break off and tumble, spreading seeds through disturbed rangeland and roadsides. J-diffuse-knapweed

People

The county reports skin irritation. Wear gloves when handling plants. J-diffuse-knapweed

Dogs & cats

Spiny bracts present a physical contact concern. The reviewed source does not establish pet ingestion effects. J-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

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.
J-diffuse-knapweed — 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, PlantTalk Colorado · CSU Extension. Accessed 2026-09-07T00:23:57+00:00. Locator: List A, B and C management categories.

Content revised 2026-09-07 03:50:00Z · Source checks dated separately in the references.