Spotted Knapweed
Family Asteraceae
Centaura maculosa
Designation/Status
Arizona Noxious Weed List -
Prohibited
Federal Noxious Weed List – Not Listed
Origins
Central Europe and east to central Russia.
Plant Characteristics
Life Cycle: Perennial with a stout
non-branching taproot capable of living up to 9 years. Flowering
occurs from June to October. Seeds germinate in the fall and early
spring when moisture and temperature are suitable. Some evidence
that spotted knapweeds release chemicals which inhibit surrounding
vegetation.
Visual Appearance: One or more stems,
branched 1 to 3 feet tall. Basal leaves up to 6 inches long,
blades narrowly elliptic to oblanceolate, entire to pinnately
divided. First year's growth leaves are deeply lobed. Flowering
heads are solitary at the end of branches. Involucres bracts stiff
and tipped with a dark comb-like fringe giving the appearance of
spots. Bracts have obvious vertical veins below the tipped area
and a reduced central spine. Ray flowers are pinkish-purple or
rarely cream-colored. Fruits are about 1/8 inch long, tipped with
a tuft of persistent bristles. Seeds are 1/8-inch long, oval,
brown to black, with pale longitudinal lines.
Habitat
Rangeland and disturbed deep, well developed to dry soils, but can
form dense stands in moist areas on well-drained soils.
Control Measures
Mechanical and Cultural:
Persistent and careful hand pulling can control spotted knapweed.
Since re-growth can occur from crowns and viable seeds in the
soil, entire plants must be removed before they produce seeds each
year.
Biological: Two seed head-feeding flies, Urophora
affinis and U. quadrifasciata produce galls on flower
heads, where they reduce seed production by 50%.
Chemical: Picloram, Clopyralid, Dicamba, and 2,4-D effectively control
spotted knapweed on rangeland. Periods of control with herbicides
grow shorter on coarse soils.
Other Points of Interest
Introduced to America
through contaminated Alfalfa seed in the late 1800s. Early
introductions were also through discarded soil used as ship
ballast.
