All plant descriptions and control methods courtesy University of Nevada Cooperative Extension
Canada thistle is an aggressive perennial from Europe that has spread across North America. It occupies croplands, rangelands, waste areas and rights-of-way. Canada thistle adapts to most environments and forms large colonies that are difficult to control due to its creeping roots and long-lived seed that is easily spread by wind, animals, equipment, and as a contaminant of seed and feed. Mechanical control, tilling, plowing, or even mowing creates more plants from the adventitious roots. Infestations of Canada thistle significantly reduce land and crop values.
Distinguishing features:
CONTROL MEASURES
Most Effective Strategy:
Canada thistle is difficult to control, especially in croplands. Seedlings are
easily controlled, but once the deep roots are
established, control is difficult. A combination of mechanical, cultural, and
repeated applications of herbicides can decrease Canada thistle populations.
Once this plant becomes established, eradication is virtually impossible and a
long term management plan is needed.
Mechanical Control:
Frequent cultivation (at least every 21 days during the
growing season) is effective but only when plants are seedlings or when
new plants grow from root pieces. Repeated mowing in alfalfa and other forage
crops controls Canada thistle.
Cultural Control:
Good sanitation practices when handling hay and livestock
products or when transporting equipment help control seed spread.
Intensive grazing by sheep or goats in its early stages of growth may reduce its
spread. Mature Canada thistle is not palatable to livestock due to the
spines on its leaves, so grazing’s effect on established plants is minimal.
Grasses and alfalfa can successfully compete with Canada thistle when good
management practices are used, but alfalfa will not establish in an extensive
infestation.
Biological Control:
There are 3 commercially available biocontrol agents for
Canada thistle. They are the Canada thistle stem weevil (Ceutorhynchus
litura), Canada thistle bud weevil (Larinus planus), and the thistle stem gall
fly (Urophora cardui) that was released in 1977 in Elko County.
Read and follow the label. It is the law!
Chemical Control:
Clopyralid (Transline® or Stinger®)
Rate: Apply at 2/3 to 1 1/3 pints per acre or 4 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 after most basal leaves have formed through flower bud formation.
May also be applied to fall regrowth.
Remarks: Clopyralid, a selective herbicide, effectively kills plants in the
Asteraceae family but does not harm most plant species that compete with
Canada 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 remains active
in the soil and may contaminate water.
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 treatments.
Control from a lower rate may be improved by tank mixing with 1 pound a.e. per
acre 2,4-D.
Time: Apply when most basal leaves have emerged but before bud stage, or apply
to regrowth in the fall. Apply Tordon 22K® at a rate less than
1 1/2 pints per acre (3/8 pounds a.e. per acre) only under favorable conditions
and in combination with 1 pound a.e. per acre of 2,4-D.
Retreatment may be required.
Remarks: Up to 4-years control of Canada thistle 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 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 pounds a.e.
2,4-D per acre.
Time: Apply after most of the rosettes have emerged but before flower bud
formation.
Remarks: Apply only 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.
MCPA
Rate: Use 6 pounds a.i. per acre.
Time: Apply from the bud to early bloom stages of growth and again in the fall.
Remarks: Apply when weeds are young and continue treatment for 2 to 3 years.
Apply in 20 to 100 gallons water carrier per acre.
Caution: Do not plant crops for 3 months following treatment. Use according to
label instructions.
2,4-D amine or ester
Rate: Use 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 for newly germinating
seeds. When Canada 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 Canada 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
the label directions. Do not contaminate water unless using a product
specifically formulated for use in water.
Chlorsulfuron (Telar®)
Rate: Use 0.75 to 2 1/4 ounces per acre of chlorsulfuron (1 to 3 ounce of Telar®
per acre).
Time: Apply during the prebloom to bloom stages of growth or late in the fall.
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: Only apply to noncrop sites. Chlorsulfuron is a selective broadleaf
herbicide that also affects many grasses. Follow the label directions,
especially as they apply to herbicide persistence in the soil and effect on any
competitive vegetation. Do not contaminate water.
Metsulfuron (Escort®)
Rate: Use 1/2 to 2 ounces per acre.
Time: Apply anytime plants are growing well.
Remarks: Suppression (reduced plant vigor and stand) is only achieved with full
coverage. Escort® is not considered effective on Canada thistle, but
can be tank mixed with other products to broaden the spectrum of weed control.
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: Only apply to pasture, rangeland, and noncrop sites. Follow the label
directions. Do not contaminate water.
Imazapyr (Arsenal®)
Rate: Apply 4 to 6 pints per acre.
Time: Use as a postemergence application only.
Remarks: Apply higher rates where dense or well-established infestations occur.
Use a tank mix with Tordon®, Garlon®, Banvel®, Escort®, or Telar®
to increase its effectiveness. Tank mixing with 2,4-D has resulted in reduced
performance by Arsenal®.
Caution: Use on noncropland only.
Triclopyr (Garlon®)
Rate: Use 1 to 8 quarts per acre of Garlon 4® or 2 to 3 gallons of Garlon 3A®.
Time: Apply when plants are actively growing.
Remarks: Can be mixed with other products to increase effectiveness of both
products.
Caution: Follow the label recommendations closely. Avoid water contamination, do
not apply on irrigation ditches. At application rates of 2 quarts
per acre or less, wait 7 days to harvest hay and 14 days to graze after
application. At application rates of 4 quarts per acre, do not harvest
hay until the next growing season.
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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