Diffuse Knapweed | Nebraska Invasive Species Council

Credit: Carey Minteer, University of Florida, Bugwood.org

General Information

Species Name: Centaurea diffusa

Also Known As: bushy star-thistle, spreading knapweed, tumble knapweed, white knapweed

Family: Asteraceae (Sunflower)

Growth Form: Forb

Life Span: Biennial

Flowering Dates: June-September

Origin: Europe

Toxic: No, but some people develop a rash after handling the plants.

Noxious: Yes. Diffuse knapweed is listed as a State Noxious Weed under the Noxious Weed Control Act in Nebraska, which means it is required by law for it to be removed from public and private land in the state.

Why Is It Invasive?

It has little value as forage for cattle and limited seasonal value for big game. Infestations increase production costs for ranchers, decrease plant diversity and wildlife habitat, increase soil erosion rates and pose wildfire hazards.

What Does It Look Like?

General Characteristics

It is an annual, biennial or perennial growing to 3.9 to 31.5 inches. It has a long taproot and generally has one main stem that branches freely. Stems, leaves and flower head bracts covered in hairs. Basal leaves typically die back before plant flowers.

Flowers

Flowerheads numerous and generally narrow. Flowers white or sometimes purplish. Bracts at the base of flowerheads are leathery, have obvious veins and are edged with a fringe of spines plus a longer spreading spine at the tip.

Leaves

Leaves covered with short dense hairs. Basal leaves (leaves at stem base) are short stalked and often twice divided into narrow lobes, growing to 7.8 inches. Stem leaves reduce in size up the stem with top leaves being stalkless, smaller and less divided.

Stems

Stems have high degree of branching and are typically covered in short, dense hairs.

Seeds

Seeds are dark brown and small without a pappus or it is minute, less than 0.5 mm.

Photos

A closeup of diffuse knapweed flowerheads in varying shades of purple and white

Credit: K. George Beck and James Sebastian, Colorado State University, Bugwood.org

A closeup of diffuse knapweed cauline leaves

Credit: K. George Beck and James Sebastian, Colorado State University, Bugwood.org

A diffuse of two white diffuse knapweed flowers

Credit: Dave Powell, USDA Forest Service (retired), Bugwood.org

Diffuse knapweed stem and cauline leaves

Credit: K. George Beck and James Sebastian, Colorado State University, Bugwood.org

Two color forms (white and purple) of diffuse knapweed

Credit: Joseph M. DiTomaso, University of California - Davis, Bugwood.org

Cluster of Canada thistle plants with tall, spiky stems and purple flower heads in a wild field.

Credit: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Fall germinated diffuse knapweed seedlings

Credit: K. George Beck and James Sebastian, Colorado State University, Bugwood.org

The bolting growth stage of diffuse knapweed

Credit: K. George Beck and James Sebastian, Colorado State University, Bugwood.org

Where Does It Grow?

Diffuse knapweed grows in disturbed and undisturbed prairies, riparian areas, roadsides, waste areas, and open woodlands. It is most common in sandy soils. Grows best in dry, well-drained soils.

How Does It Spread?

Diffuse knapweed reproduces primarily by seed but may also regenerate from the crown. A single flower stalk can produce 1,200 seeds. When the plant is broken off at the base, it can be blown around like a tumbleweed and disperse its seed.

How Do I Control It?

Mechanical

Cultivation will eliminate infestations. Mowing may delay flower production, therefore potentially decreasing seed production.

Contact your county weed control authority to determine appropriate removal methods.

Biological

Biological control agents available for diffuse knapweed are: Agapeta zoegana, the sulfur knapweed moth that have root mining larvae; Bangasternus fausti, the broad-nosed seedhead weevil that have seed-feeding larvae; Cyphocleonus achates, the knapweed root weevil, which has root mining larvae (larvae feeding within the roots); Sphenoptera jugoslavica, the bronze knapweed root borer that have larvae that bore into and tunnel within knapweed roots; Larinus minutus, the seedhead feeding weevil. For more information about the biological control of diffuse knapweed, please contact your local county weed control authority.

Cultural

When buying hay or forage, check to make sure it is weed-free certified. Management should include proper grazing and rotational grazing techniques that would maintain rangelands and prevent invasion. Disturbed areas should be re-seeded with desirable species to prevent spread.

One way that invasive plant seeds and fragments can spread is in soil. Sometimes plants are planted purposefully. You can prevent the spread of invasive plants.

PlayCleanGo: Stop Invasive Species in Your Tracks

REMOVE plants, animals and mud from boots, gear, pets and vehicles.
CLEAN your gear before entering and leaving the recreation site.
STAY on designated roads and trails.
PLANT non-invasive species.

Chemical

Please refer to the 2025 Guide for Weed, Disease and Insect Management in Nebraska or contact your county weed control authority.

What Should I Do If I See It in Nebraska?

If you see diffuse knapweed in Nebraska, you should report it to your county weed control authority. For guidance on what information to include in your report, check out our reporting tips.