All plant descriptions and control methods courtesy University of Nevada Cooperative Extension
Scotch thistle is an aggressive plant from Europe and western Asia that has naturalized, often forming dense stands impenetrable by people, wildlife, and livestock. In Nevada, it grows along roadsides, fence lines, ditch banks, open dry areas, and in pastures. It is rarely found in gardens and cultivated fields. Scotch thistle may grow as an annual, biennial, or shortlived perennial. In most of Nevada, it grows as a biennial and produces a large, distinctive rosette in the first year. The second year, Scotch thistle produces many large, branched, winged stalks with smaller leaves, flowers, and seeds. Scotch thistle reproduces only from seed, which is dispersed by water, wind, equipment, animals and humans.
Distinguishing features:
CONTROL MEASURES
Most Effective Strategy:
Scotch thistle is a biennial, so effective control
can be achieved by destroying the plant during the
rosette stage before seed development. 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
Scotch thistle seedlings emerge. A long-term
effort must be continued until the soil seed bank of Scotch thistle seed is
depleted.
Mechanical Control:
Small infestations may be controlled by cutting the taproot
below the soil surface or by hand-pulling the
plant. Tillage kills the plant. Mowing makes the stand more uniform so that
herbicide application is more effective but does
not kill Scotch thistle. Mowing before seed development and dispersal will limit
the amount of seed available for germination.
Mowing is only recommended when used with a follow-up herbicide application.
Cultural Control:
Goats will graze Scotch thistle in the rosette stage, but
sheep and cattle avoid the plant. Establish desirable
vegetation to provide competition to Scotch thistle.
Biological Control:
There are currently no biological control agents available
for Scotch thistle.
Read and follow the label. It is the law!
Chemical Control:
Picloram (Tordon®)
Rate: In the fall, apply Tordon 22K® at 1/2 to 3/4 pint per acre or 1/8 to 3/8
a.i. per acre. In the spring, apply 1/2 to 3/4 pint
Tordon 22K® at 1/8 to 3/8 pound a.e. per acre plus 1.0 pound a.e. per acre
2,4-D.
Time: Apply at the rosette stage, before bolting, in the spring or in the fall
prior to the soil freezing.
Remarks: 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 soil active for a long time. Do not
contaminate water.
Clopyralid (Transline® or Stinger®)
Rate: Apply at 1/3 to 1 pint per acre or 2 to 6 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 after the rosette’s emergence and bolting stage of growth. The lower
rate of 1/4 pint per acre provides
acceptable control only when the plants are 3 to 6 inches tall.
Remarks: Clopyralid, a selective herbicide, effectively kills plants in the
Asteraceae family but does not harm most plant
species that compete with Scotch thistle.
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.
2,4-D amine or ester and/or dicamba
Rate: Use 1 to 2 quarts per acre or 1 to 2 pounds a.e. per acre.
Time: Apply prior to flower bud formation. Repeat herbicide applications may be
necessary during the growing season to
kill newly germinating seeds. When Scotch thistle is present in cereal crops,
apply 2,4-D before the cereal crop
reaches the boot stage.
Remarks: Annual treatments are needed to control Scotch thistle 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 label directions. Do not apply near or over water
unless using a product specifically
formulated for use in water.
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