| PASTURE,
RANGELAND, AND NONCROPLAND
Biennials. Fall is the preferred time
for applying herbicides for biennial thistle control. Fall applications
allow for more time to apply herbicides than in the spring and correspond
to the most effective time for thistle control. Seedlings that emerge
in summer after tillage or previous herbicide applications will not
bolt but remain in the rosette stage. Biennial thistles are most susceptible
to herbicides in the rosette form.
Herbicides should be applied as late
as possible in the fall but prior to a killing frost to allow for maximum
seedling emergence and rosette size. Seedlings that emerge after spraying
will remain vegetative until the following spring and can be treated
then. Long-term eradication of biennial thistles is difficult because
of the large number of seeds each plant can produce.
Biennial thistles can be effectively
controlled with Stinger (clopyralid), Tordon (picloram), or dicamba
(Table 1). Stinger and
Tordon are the most effective of these herbicides and may be applied
in the spring or fall. Tordon and clopyralid (Curtail) are often mixed
with 2,4-D for broad spectrum weed control. Weedmaster (dicamba plus
2,4-D) is an effective treatment and is best applied when the thistles
are in the rosette growth stage. Redeem (triclopyr plus clopyralid)
is labeled for thistle control in non-cropland and CRP. Escort (metsulfuron)
will control biennial thistles in the spring and will eliminate seed
production when applied in the bolting to bud growth stages.
Perennials. Curtail (clopyralid plus
2,4-D), Tordon (picloram), Tordon plus 2,4-D amine, dicamba, Redeem
(triclopyr plus clopyralid), or 2,4-D will suppress or control perennial
thistles (Table 1). Control
is greatest when applied to thistle at the early-bud growth stage (early
summer) or in the fall to plants in the rosette form. These herbicides
applied at low rates may be the most cost-effective method for controlling
dense infestations that require broadcast application. Annual retreatment
will be necessary for several years to obtain long-term control.
2,4-D is used for suppression only
and is most effective when applied in the spring to thistles in the
vegetative growth stage (Table 1).
Tordon at 1 to 2 quarts per acre (0.5 to 1 pound per acre), dicamba
at 3 to 4 quarts per acre (3 to 4 pounds per acre), Stinger (clopyralid)
at 1.3 pints per acre (0.5 pound per acre), or Curtail at 3 quarts per
acre (0.28 plus 1.5 pounds per acre) will provide near complete control
for several years, but are expensive treatments. Redeem should be applied
at 2.5 to 4 pints per acre (0.7 plus 0.25 to 1.1 plus 0.4 pounds per
acre) Redeem herbicide contains 0.75 pounds per gallon of clopyralid
and may be a more cost effective treatment than Curtail which contains
0.4 pounds per gallon clopyralid.
GRAZING RESTRICTIONS
VARY WITH HERBICIDE AND APPLICATION RATE SO READ THE LABEL CAREFULLY
BEFORE USING.
For perennial thistle control in non-cropland, in addition
to herbicides listed above, glyphosate may be applied in the summer
or fall when thistle is at or beyond the bud stage of growth (Table
1). Perennial thistle control is usually greater when glyphosate
is applied in fall rather than spring.
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