Diffuse Knapweed(Centaurea diffusa)

Status:

Listed Noxious Weed In Montana & North Dakota

History:

First Montana Specimen Collected 1

County:

Mineral

Year:

1951

“Diffuse knapweed was first reported in Washington state in 1907. Today the plant has spread to more than 3.21 million acres across Washington, Idaho, Oregon, Montana, and British Columbia.” (Elpel)

Habitat:

Growth Habit:

Annual or biennial, bushy, up to 2 ft. tall. Rosette formed first year, flowering stalk
elongates second year.

Leaves:

Grayish-green, alternate, basal leaves whorled, upper leaves much reduced. Covered with
fine hair.

Stem:

Hairy, erect, single main stem from a rootstock, branched near or above the base.

Flower:

Solitary, usually white, sometimes pink, rose or lavender; seed head bracts end as
sharp, rigid spines.

Roots:

Elongated taproot.

Seeds:

Oblong, dark brown or gray with longitudinal lines.

Other Notes:

“Diffuse knapweed is a tap-rooted annual, biennial or short-lived perennial forb that reproduces by seed. Plants over winter as a rosette that resembles spotted knapweed. Plants usually produce a single main stem that divides into numerous branches about
halfway up the stem, giving it a ball-shaped, tumbleweed appearance and
mobility. A single flower head is at the end of each branch. Flowers are
usually white, but can range to light purple. Bloom period is usually from
mid-July through September."

"Bracts on diffuse knapweed have a rigid terminal spine about one-quarter to one-third of an inch long with four to five pairs of shorter lateral spines. Bracts can have dark-colored tips
but lack the dark fringe present on spotted knapweed."

"Diffuse knapweed spreads mainly by wind.Mature plants break off at ground level and tumble in the wind or become attached to the undercarriage of vehicles and equipment. ”(Duncan, Story & Sheley, 9)