DIFFUSE KNAPWEED — Centaurea diffusa Lam.

Asteraceae — (Sunflower family)

The first year, the plant begins as a rosette of airy, deep cut (carrot like), greenish grey leaves covered with short hairs or nap developing a deep tap root. 

The second year, a stiff central stem with narrow sparse leaves shoots up branching off to support white flower heads (maybe pink or purplish).  The base of the “petals” sit in a vase shaped group of bracts.  The papery bracts have a spine at the flower end and seem to weave together towards the bottom of the “vase”.  

This competitive plant can produce about 18,000 seeds that are mostly spread as the plant breaks off at the base becoming a “tumble weed”.   The knapweed crowds out other desirable plants, invading ranges, pastures, and disturbed areas.

The following is courtesy of Weeds of the West:

Diffuse knapweed was introduced from the Mediterranean Region of Eurasia and is a biennial or short-lived perennial forb which reproduces only by seed. The plant usually produces a single main much-branched stem that is one to two feet tall and rough to the touch. Leaves are pinnately divided; the reduced leaves of the inflorescence are mostly entire.

A basal rosette of leaves is present in young plants with each leaf divided into narrow segments. Seedlings of diffuse knapweed have finely divided leaves covered with short hair. Stem leaves in the mature plant become much reduced as you ascend to the tip and alternate one per node. Flowering heads are numerous and narrow. Flowers are white to rose or sometimes purplish and are located on each branch tip; margins of involucral bracts are divided like the teeth of a comb.

The bracts surrounding each flower bear four to five pairs of lateral spines and one, long terminal spine. Diffuse knapweed can resemble spotted knapweed, which has black-tipped bracts. The difference is the sharp spine at the end of the bract that is characteristic of diffuse knapweed.

Centaurea is a large genus of over four hundred species, most originating in the Mediterranean region. A number of the knapweed species have been introduced from Eurasia and now represent a threat to pastures, riparian areas and rangelands. Diffuse knapweed infests roadsides, waste areas and dry rangelands, and as a highly competitive plant, threatens to exclude many desirable species. It is a tough competitor on dry sites and rapidly invades and dominates disturbed areas. Flowering occurs from July to September.

Growth Habit: Annual or biennial, bushy, up to 2` 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 grey with longitudinal lines.

Other: May seriously reduce productive potential of infested rangelands.(Courtesy of Weeds of the West)

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