Diffuse Knapweed

What is diffuse knapweed?

Diffuse knapweed is an aster-family weed from the eastern Mediterranean and western Asia that now covers large stretches of western range. Dependable diffuse knapweed identification rests on the bracts under each small flower head, which carry a comb of spines and one longer, stiff tip spine. That detail and its habit of breaking off and tumbling in winter separate it from spotted knapweed, its closest look-alike.

It pushes out forage grasses and native wildflowers on dry range, offers little feed for cattle or big game, raises erosion and fire risk, and lowers land value. Montana ranchers rank diffuse and spotted knapweed third among weeds for livestock losses. It may also release compounds that suppress other plants, though how much that matters is debated.

Diffuse Knapweed identification: what to look for

Rosettes are easiest to find in spring and late summer, but the flower heads settle it. Look closely at the bracts: diffuse knapweed bracts are straw-colored with a comb of tiny spines and a sharp tip spine, while spotted knapweed bracts have dark tips. The two can hybridize, so if a plant looks in-between, ask your county weed office.

• Rosette: Low and grayish, with short-haired leaves cut once or twice into narrow lobes.
• Stems: Usually one rough, hairy main stem branching heavily into a rounded bush.
• Flower heads: Numerous slender heads about 1/2 inch long, mostly white, sometimes pink, lavender or purplish.
• Bracts: Leathery and veined, with a comb-like fringe and a longer, stiff spine a few millimeters long at the tip.
• Taproot: A single long taproot; pulling must take the whole crown or the plant can regrow.
• Winter habit: After seeding, the plant dies, breaks off at the soil line and rolls, often piling against fences.

Where it grows in the United States

Its main range runs from Washington, Idaho and Montana south to New Mexico and Arizona, with Washington, Oregon, Idaho and Montana hit hardest. It has also appeared in several Midwestern states and in New Jersey, Connecticut and Massachusetts. A 2003 estimate put it on about 1.8 million acres in western states and only about 5,000 acres in the East.

It is built for semiarid country with roughly 6 to 35 inches of yearly precipitation and favors light, well-drained soils; it tolerates neither flooding nor shade. Look for it in shortgrass and mixedgrass prairie, sagebrush steppe, open ponderosa pine and Douglas-fir stands and pinyon-juniper woodland, from sea level in eastern Washington to about 9,200 feet in Colorado. Disturbance lets it in, and in Colorado it is worst along the Front Range.

Diffuse Knapweed look-alikes

A few plants get mixed up with it:

• Spotted knapweed (Centaurea stoebe): A short-lived perennial with lavender to pinkish-purple flowers. Its bracts have dark tips with a soft fringe and no long, stiff tip spine.
• Russian knapweed (Rhaponticum repens): Forms dense colonies from creeping black roots and has pinkish-purple, pineapple-shaped heads instead of small white ones.
• Yellow starthistle (Centaurea solstitialis): Also spiny, but its flowers are bright yellow and its spines are long and yellowish.

How to get rid of diffuse knapweed

Pulling, digging and mowing: Small patches can be pulled or dug if you get the whole root crown and return several times a season. Persistence pays: in an Oregon trial, pulling three times a year for five years cut density by 98 percent. Pull before flowering and bag plants with seed heads. Mowing alone rarely kills it; about a fifth of plants mowed monthly to 2 inches in a Washington study were still alive after four years, and Washington's weed board advises against mowing because taproots resprout. If you do mow, time it for late bud to early bloom to cut seed. In tilled ground, regular cultivation can eliminate it.

Cultural control matters most, because the right conditions keep it from coming back.

• Graze cattle twice in spring while plants are bolting and 6 to 12 inches tall; Colorado State research cut seed set by half and reduced tumbling off-site.
• Release biological control insects such as Urophora seed-head flies, the knapweed seed-head weevil and root-feeding weevils and moths; several are distributed by state agriculture departments.
• Reseed degraded range with competitive perennial grasses, forbs and shrubs; heavy grazing of grasses increases knapweed, so manage grazing to keep cover dense.
• Where possible, irrigation helps grasses outcompete it.

Herbicides: Colorado State University Extension lists several selective active ingredients: picloram (spring rosette to mid-bolt, or fall), aminopyralid (spring rosette to bolting, or fall), clopyralid alone or with 2,4-D (rosette to early bud after all shoots emerge, or fall) and dicamba (spring rosette or fall). Adding 2,4-D broadens control but can injure native wildflowers and shrubs that help resist reinvasion. USDA's fire effects review also names aminocyclopyrachlor, indaziflam, chlorsulfuron and glyphosate. Spraying buys time but does not change what let knapweed in, so pair it with reseeding and grazing management. Choose a product labeled for your site (pasture, rangeland, roadside or yard); the label is the law. If you hire someone to spray, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Keeping it from coming back

Diffuse knapweed spreads only by seed, so stop seed. Pull or treat plants before flowering and collect dead plants before they tumble. Check fence lines, ditches and road edges where tumbleweeds collect.

Keep range and pasture in good condition with rest periods so grasses stay thick; overgrazed and bare ground is where it starts. Never move plants or seed in Washington, where it is quarantined.

Questions we get about diffuse knapweed

Similar weeds

Spotted KnapweedCentaurea stoebeRussian KnapweedRhaponticum repensYellow StarthistleCentaurea solstitialis