Diffuse Knapweed
Family Asteraceae
Centaura difusa
Designation/Status
Arizona Noxious Weed List – Prohibited
Federal Noxious Weed List – Not Listed
Origins
Native to grasslands and shrub steppes of the Eastern
Mediterranean and western Asia.
Plant Characteristics
Life Cycle: Normally a
biennial, but may behave as an annual or short-lived perennial.
Flowering occurs from June to September. Seeds germinate in the
fall or spring and develop into low-lying, tap-rooted rosettes.
Seed production by diffuse knapweed averages 11,200 to 48,100 per
square meter.
Visual Appearance: Grows 1 to 3
feet tall from a deep taproot. Upright stems have numerous
spreading branches, which give the plant a ball-shaped appearance
and tumble-weed mobility when broke off. Basal leaves, which form
rosettes on a central crown, are borne on short stalks and are
deeply divided into lobes on both sides of the midrib. Stem leaves
are stalk-less, becoming progressively smaller and less divided
higher up the stem, with the uppermost small leaves bract-like.
Urn-shaped flower heads are 3/16 to 1/4 in diameter, and 5/16 to
7/16 inch long. Heads are solitary or borne in clusters of two or
three at the ends of the branches. Bracts surrounding the flower
heads are yellowish green with a buff or brown margin.
Habitat
Diffuse Knapweed prefers shrub
steppe type areas, commonly on light well-drained soils such as
sandy or gravelly loams or loamy fine sands. Diffuse Knapweed does
not grow well in dense shade or poorly drained soils.
Control Measures
Mechanical and Cultural: Pulling or digging is
feasible for control of scattered plants. Mechanical control is
more effective if enough of the taproot is removed to discourage
sprouting. Mowing can be used to reduce seed production.
Biological: Gall-forming flies, the Peacock fly and
some weevils are effective in controlling Diffuse Knapweed
Chemical: Picloram, Clopyralid, Curtail, 2,4-D have
been used with success in the past. These herbicides can be used
in the spring and fall and work best when applied to the rosette
stage of growth.
Other Points of Interest
The earliest record of diffuse knapweed in western North America
is from an alfalfa field at Bingen, Washington, in 1907. Diffuse
Knapweed is ideally suited to be spread by vehicles and by
tumbling in the wind. It evolved to spread by the wind blowing
the ball shaped plants in the same manner as tumbleweed. The
seeds, held in urn-shaped heads which do not open widely, are lost
gradually as it rolls, giving the plant the advantage of distant
distribution.
