All plant descriptions and control methods courtesy University of Nevada Cooperative Extension
Spotted knapweed, introduced from Eurasia as a contaminant in alfalfa and clover seeds, is one of the leading problem weeds in the United States. This biennial plant can dominate rangelands that receive less than eight inches of annual precipitation. Difficult to control because of its prolific seed production and early seedling emergence, it flourishes in areas with more moisture or at higher elevations where moisture is more reliable. It suppresses the growth of other plants by releasing inhibiting chemicals from its roots. Large plants may produce up to 25,000 seeds that remain viable in the soil for five years or more. Its seed survives rangeland fires to recolonize after the first moisture following the fire, before most native species. Consistently eliminating new seedlings is key to reducing the soil seed bank and controlling spotted knapweed. Missing one year of control will negate any previous efforts to manage it.
Distinguishing features:
CONTROL MEASURES
Most Effective Strategy:
Once established, this plant is difficult to control. The most effective
treatments combine herbicides,
biological control agents, and cultural controls. Consistent, uniform herbicide
applications in spring and early summer control rosettes and mature plants. Plant bare soil with competitive vegetation.
Retreatment is necessary when new spotted knapweed
seedlings emerge. A long-term effort must be continued until the soil seed bank
is depleted.
Mechanical Control:
Persistent hand-pulling, before plants produce seed,
controls small infestations. The entire plant must be removed. Properly dispose of flowering plants after removal. Repeated tillage, 7
inches deep, reduces spotted knapweed and encourages the growth of grasses. Fire may increase the establishment of this
plant. Repeated mowing can reduce seed
production if it is delayed until seeds are near maturity but before they reach
full maturity and are viable.
Cultural Control:
Carefully managed, repeated grazing by sheep and goats early
in the spring when grasses are dormant reduces spotted knapweed seedlings and rosettes.
Biological Control:
Several biological control agents are available for
suppressing spotted knapweed. Two gall flies (Urophora
affinis and U. quadrifasciata) redirect plant energy from seed production to
formation of galls to encompass the fly’s eggs. The
sulfur knapweed moth (Agapeta zoegana) mines within the root and in Nevada has
been found to be quite effective in
reducing plant vigor. However, it does not spread rapidly. Also a root boring
beetle (Cyphocleonus achates), a seed head
moth (Metzneria paucipunctella), a peacock fly (Chaetorellia acroliphi), the
bronze root borer (Sphenoptera jogoslavica), the
green clearwing seed head fly (Terellia virens), the blister mite (Aceria
centaureae), 2 root moths (Pelochrista medullano and
Pterolonche inspersa), and 3 seed head weevils (Larinus minutus, L. obtusus, and
Bangasternus fausti) reduce plant vigor with
varying degrees of success. The soil fungus, Sclerotinia sclerotiorum, infects
spotted knapweed and can cause wilting or death
of individual plants. The bacteria, Pseudomonas syringae pv. syringae, also
infects this plant species. Each biological agent is
effective only under specific environmental conditions.
Read and follow the label. It is the law!
Chemical Control:
Picloram (Tordon®)
Rate: Apply Tordon 22K® at 1 to 2 pints per acre or 1/4 to 1/2 pound a.e. per
acre. Lower application rates may require
annual spot treatment that may be improved by tank mixing with 1 pound a.e. per
acre 2,4-D.
Time: The optimum application of picloram is from the rosette to midbolting
stages of growth or to fall regrowth.
Remarks: Up to 4-years control of spotted knapweed seedlings has been
demonstrated in picloram research plots. Adjustment
of the application rate will affect the selectivity of picloram. At the
recommended rate, picloram will not
damage established perennial grasses.
Caution: This restricted-use herbicide must be applied by, or its application
supervised by, a certified applicator. Follow the
label directions. Picloram remains active in the soil for a long time. Do not
contaminate water.
Clopyralid (Transline® or Stinger®)
Rate: Apply at 1 to 1 1/3 pints per acre or 6 to 8 ounces a.e. per acre. Use a
nonionic surfactant at a rate of 1 to 2 quarts
per 100 gallons of water carrier.
Time: Apply anytime plants are actively growing, including any fall regrowth.
Optimum time to apply is after most
rosettes have emerged but before flower bud formation.
Remarks: Clopyralid, a selective herbicide, effectively kills plants in the
Asteraceae family but does not harm most plant
species that compete with spotted knapweed.
Caution: Use Transline® in noncroplands only and Stinger® in croplands, but do
not rotate to any crop other than wheat,
barley, oats, or grass for 1 year after treatment. Do not apply by airplane when
a temperature inversion exists.
Follow label directions. These products remain active in the soil and may
contaminate water.
Metsulfuron (Escort®) (Tank Mix)
Rate: Use 1/2 to 1 ounce metsulfuron combined with 8 fluid ounces dicamba and 16
fluid ounces 2,4-D per acre.
Time: Apply from rosette to full bloom stages of growth.
Remarks: Use a nonionic surfactant to increase the herbicide’s effectiveness. In
alkaline soils, above pH 7.5, reestablishment
of competitive vegetation may be difficult due to residual herbicidal action on
germinating seedlings.
Caution: Apply only to pasture, rangeland, and noncrop sites. Follow the label
directions. Do not contaminate water.
Dicamba (Clarity™, Diablo™, Fuego™, Vanquish™, Veteran™, or Banvel®)
Rate: Use 1/2 to 1 pound of Dicamba per acre.
Time: Apply when plants are in the rosette stage of growth.
Remarks: Use higher rates when treating dense or tall vegetative growth. Dicamba
is frequently used in a tank mix with other
herbicides to increase its effectiveness. Check label for tank mixing
instructions.
Caution: Avoid drift to sensitive crops. Read and follow the label directions.
2,4-D amine or ester
Rate: Use 1 to 2 quarts per acre or 1 to 2 pounds a.e. per acre.
Time: Apply prior to flower bud formation. Repeated herbicide applications may
be necessary during the growing season
to kill newly germinating seeds. When spotted knapweed is present in cereal
crops, apply 2,4-D before the cereal
crop reaches the boot stage.
Remarks: Annual treatments are needed to control spotted knapweed seedlings or
rosettes.
Caution: It is not lawful to apply more than 2 pounds a.i. per acre per year.
Avoid drift to crops, especially with a volatile
ester formulation. Follow the label directions. Do not contaminate water unless
using a product specifically formulated
for use in water.
Clopyralid plus 2,4-D (Curtail®)
Rate: Apply 2 to 4 quarts per acre or 0.19 pound a.e. clopyralid and 1 pound a.e.
2,4-D per acre or 0.38 pound a.e.
clopyralid and 2.0 pounds a.e. 2,4-D per acre.
Time: Apply after most of the rosettes have emerged but before flower bud
formation.
Remarks: Only apply in rangeland, pastures, and noncropland.
Caution: Do not permit lactating dairy animals or meat animals being finished
for slaughter to graze treated fields within 1
week following treatment. Do not plant broadleaf crops in treated areas until an
adequately sensitive bioassay
shows no detectable clopyralid is present in the soil. Do not apply by airplane
when a temperature inversion exists.
Follow label directions. This product remains active in the soil and may
contaminate water.
Glyphosate (Roundup®, Landmaster™, Rodeo®, or Aquamaster™)
Rate: Use 1 1/2 pints to 2 quarts according to label rates plus 2 quarts of
nonionic surfactant per acre.
Time: Apply when plants are actively growing before flower bud formation.
Remarks: Glyphosate is a nonselective herbicide that kills most plants,
including those that compete with new weed seedlings.
Wait 10 days after treatment, then seed with locally adapted species.
Caution: Follow the label recommendations and precautions, especially for use in
and near water.
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