Weed Guide > Diffuse Knapweed

Diffuse Knapweed

DSC 1973

Scientific Name: Centaurea diffusa

Family: Asteraceae (Aster or Sunflower Family)

Other common names: barnaby (many folks in the Methow call it this), white knapweed, tumble knapweed

Life Cycle: Biennial; spreads only from seed

Toxicity: Not known to be toxic to animals or human, but all knapweeds contain carcinogens, so it is best to wear gloves when pulling more than one plant.

Description Bloom
(varies by elevation)
What to Do When? Invades Undisturbed Land State Class
Takes 2 years to grow; starts with a “rosette” then grows 2-3' tall; tiny flowers on branching stalks; gets prickly as it dries up. Little white, pink or light purple thistle-like flowers bloom July-Sept. Hand pull second-year plants; get the whole root. Biocontrol is effective on large populations. June - Aug. best when ground is wet, before it goes to seed; bag and throw away seed-heads anytime. Yes! Your pristine land is not safe; be on the lookout. B
Weeds diffuse knapweed11

General Description

  • Diffuse knapweed is a biennial thistle that can mature and flower at a wide variety of heights - anywhere from a few inches to a few feet tall.
  • First-year plants stay low to the ground with a “rosette” of deeply divided fern-like leaves.
  • Second-year plants have one stiff and upright stalk/stem with numerous spreading branches giving it a rounded top.
  • Flowers are white or pink to light purple, with small, sharp spines that are soft early in the season then get prickly.
  • Stems, leaves and flower heads are covered in tiny hairs, giving the plant a grayish appearance.
  • Knapweed can flower continuously from early summer into the fall.
Young Knapweed


  • Being a biennial, knapweed completes its life cycle in two years.
  • The first year, it establishes a taproot - a single long root that goes straight down into the ground - and a basal rosette (a little ring of leaves).
  • The second year, the plant grows upward with a stalk; it flowers, produces seeds and then dies.
  • Diffuse knapweed reproduces only by seed (not roots).

Prevention & Control


  • When driving or walking or moving with animals through infested areas, inspect and clean clothing and shoes, vehicles, and animals to remove any seeds before going into uninfested areas.
  • Saturated or well irrigated soils typically kill knapweed.


  • The best control for any biennial, including diffuse knapweed, is hand pulling the second year plant; the long stalk gives you leverage on the long tap root.
  • It’s important to get the entire long taproot, so pull when the soil is moist.
  • Pulling in May & June is ideal because plants are usually not prickly yet; can be left on the ground; and the plant has not produced seeds yet.
  • It's important to get the entire long taproot, so pull when the soil is moist.
  • If there’s any chance flowers have “gone to seed,” put plants in a sealed bag and throw them away, or burn them in a hot and controlled fire.
  • Wear gloves when pulling; knapweed contains carcinogen diffuse knapweed, is hand pulling the second year plant; the long stalk gives you leverage on the long tap root.


  • Mowing or weed-whacking, if done during budding or very early flowering, can impede growth by hindering seed production, but it does not eliminate them entirely. And if mowed at the wrong time, it can actually encourage low-to-the ground-blooming.


  • The knapweed seed head weevil (Larinus minutus) is a tiny beetle that is effective at reducing large knapweed infestations.
  • The Regional Biocontrol Project run through the Ferry County Extension office for both Okanogan and Ferry Counties, is the source of several free biocontrols, including the knapweed seed head weevil. Follow the above link to the website for more information.
  • Any method requires repeated sessions over several seasons because seeds stay viable in the soil for many years.

See the whole “Toolbox of Weed Control Methods” for more details.

Interesting Tidbits

  • Knapweed can grow anywhere, but it tends to not like cultivated soil or excessive moisture such as irrigated pasture.
  • Knapweed is an excellent source of nectar for bees and other insects in mid to late summer when other sources are scarce but there's plenty of knapweed for them - you don't need to save any!
  • Diffuse knapweed, like other knapweeds, releases allelochemicals into the ground around it, preventing the growth of other plants.
  • The presence of knapweed can serve as an indicator species for soil/range degradation.