September/
October 1998
Fall perennial weed control tips
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Prairie Grains is the
official publication of
the Minnesota
Association of
Wheat Growers,
North Dakota Grain
Growers Association,
South Dakota Wheat,
Inc., and the
Minnesota Barley
Growers Association.
Good active growth of the perennial weed is needed for systemic control. Perennial weeds should have between 6-8 inches of new growth before herbicides are applied.
For best Canada thistle control, spray rosettes of actively growing plants. Target common milkweed in the late-bud to flowering stage.
Banvel, Roundup, Tordon, 2,4-D, Curtail, and Stinger are herbicides that have the greatest activity on Canada thistle. Patch spray common milkweed with Roundup Ultra and ammonium sulfate.
Tankmixes of some herbicides may provide greater control than one herbicide applied alone (except glyphosate).
Highest rates should be used without interfering with next year's cropping pattern.
Situations may occur where spot treatment with herbicides at high rates might be the better perennial weed management option, even if carryover may injure small areas of next year's crop. Small patches are easier and less expensive to treat than entire fields. But bear in mind, research has shown at least 90% stand reduction from Banvel, Roundup, Landmaster BW, or Tordon plus 2,4-D.
Apply herbicides when temperatures are expected to exceed 60-65 F during the day to ensure active translocation.
Timing is critical for fall control. Herbicides should not be applied to perennial weeds that are stressed from drought or frost damage. When frost temperatures below 26-28 degrees occur, wait at least 24 hours to evaluate foliage before spraying. Herbicides cannot translocate if the weed is dead.
Select the correct herbicide that fits the crop rotation sequence. Use the highest rate labeled. Some herbicides have crop rotation restrictions, so pay attention to the label.
Mowing perennial weeds in pastures, roadsides, and CRP earlier in the season will encourage weeds to produce new shoots. This will allow a good setup for chemical treatment later in the fall.
Perennial weed control will be greatest if the field has not been tilled prior to the herbicide application. However, control is enhanced by tillage after a herbicide application.
Tillage following a chemical application is particularly advised if you sprayed at a lower rate or used a less-potent herbicide. Allow two to four weeks time to pass after the herbicide application before you break out the iron, however, to allow the chemical to sink in. n