Russian knapweed invades county
By Tim D'Amato
Larimer County Department of Natural Resources
Russian knapweed (Acroptilon repens) is a perennial weed species of the
composite family becoming increasingly more common in pastures and rangeland
of Larimer County. This deep-rooted invasive plant is toxic to horses and
toxic to other vegetation. The plant contains herbicide-like chemicals
that inhibit the growth of competing plants, resulting in fairly solid
stands of Russian knapweed.
Worse yet, a toxin contained in Russian knapweed causes an irreversible
nervous disorder in horses called chewing disease. Chewing disease causes
symptoms similar to Parkinson's disease in humans, for which no treatment
is available. When diagnosed in horses, euthanasia is usually recommended.
Russian knapweed grows 1 to 4 feet tall and reproduces by seed and a creeping
root system. Leaves and stems are a bluish green color and the flowers
are pink-lavender, present on the plants from June through August. A surefire
identification characteristic for Russian knapweed is to pull a stalk and
look for the black upper root section, a characteristic unique to this
weed species.
Russian knapweed is listed as a noxious weed species under the Colorado
State Noxious Weed Act, and on the Larimer County Noxious Weed List. Landowners
in Larimer County are obligated to control this species and prevent spread
to neighboring properties.
Effective control of Russian knapweed requires herbicide application and
subsequent revegetation of grasses. There are currently no insect biocontrol
agents available for this weed species, and hand-pulling a perennial plant
such as Russian knapweed requires a degree of persistence not feasible
for the size of infestations typically found in range or pasture sites.
Best timing for controlling Russian knapweed is herbicide application at
the flowering stage, usually mid-summer. If the knapweed stand has been
established for multiple years then disking the site in the fall prior
to seeding may be necessary to dilute the plant toxins in the soil. Seeding
new pasture grass is most successful when conducted between November and
April.
For specific herbicide and seeding recommendations, contact the Larimer
County Weed District office at 498-5768.
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