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Fall Weed Control

Wednesday, September 15, 2004
filed under: Weeds

Sure, this summer’s cool temperatures have you concerned about the development of late-season crops. But what about those poor little weeds? Aren’t you concerned about how the cool temps have been affecting them?

Not too worry. By far, water stress has a greater impact on plant development – including weeds – then modest fluctuations in temperature.

“Plants can withstand periods of extremes, but water is the critical component,” says Richard Zollinger, North Dakota State University extension weed scientist. “You can have abnormally warm or cool temperatures, but if soil moisture is adequate, it won’t have an effect.”

Still, temperatures can influence herbicide treatment weed kill. Ideal temperatures for applying most herbicides are between 65 and 85 F, and weeds may be killed slowly below 60 F. Plants degrade herbicides by metabolism, but plant metabolism slows during cool or cold conditions, which extends the amount of time required to degrade herbicides in plants.

Dicamba, 2,4-D, MCPA, Stinger, Starane, glyphosate (resistant crops) have adequate crop safety and provide similar weed control, but weed death is slowed when cold temperatures follow application.

Conversely, some herbicides such as 2,4-D ester, MCPA ester and dicamba can injure crops if applied above 85 F. Thus, avoid applying volatile herbicides during hot weather, especially near susceptible broadleaf crops and shelterbelts.

Wild oats are actually more sensitive to fenoxaprop (Puma) during cool rather than warm/hot conditions. “Some grass herbicides become ‘hotter’ or more active in cooler weather, so we should have had good activity against wild oats in many areas this season,” says Zollinger.

Conversely, green and yellow foxtail are warm season grasses, and may shutdown under cold conditions, resulting in reduced control. Further, ACCase herbicides such as Poast and Select provide better grass control in warm weather when grasses are actively growing.

The product label is always the best source for treatment performance information under various weather conditions.

Other weed control notes:

Clearfield/Beyond – With the growing season the way it was, less Clearfield™ sunflower was planted than anticipated. But overall herbicide performance and crop tolerance with Beyond was positive, says Vince Ulstad, senior field biologist, BASF, Fargo, N.D.

Fall-applied Spartan – Remember that the full label of Spartan (sulfentrazone) now permits fall application, a treatment option which would allow producers to control the first flush of weeds next spring on fields planted to sunflower. It’s an option particularly beneficial for minimum and no-till sunflower producers, permitting a wider window of application and a greater chance for moisture activation next spring.

Microbial activity breaks Spartan down in the soil, just like anhydrous ammonia. Thus, similar to anhydrous, Spartan should be applied in the fall when the soil temperature is less than 50 degrees, down to about the five-inch soil depth. Soil microbial activity dissipates at this point, minimizing treatment degradation. Apply Spartan right around freeze-up: the treatment will be available when weeds begin to germinate in the spring, and chances are there should be enough snow melt or early spring rain for product activation.

Fall perennial weed control – Fall is the best time of year for controlling many perennial weeds, including Canada thistle, leafy spurge, quackgrass, and common milkweed, since the plants are preparing for winter by storing nutrients in their root systems. This improves herbicide translocation to the roots, and if you kill the roots, you kill the plant.

With most perennial weeds, treatment is generally better when the plants are larger, with full leaves or bloom, since more of the herbicide will translocate down to the roots. Highest rates and appropriate products should be used to avoid interfering with next year's cropping pattern.

Timing is critical for fall control. Apply herbicides when temperatures are expected to exceed 60-65 F during the day to ensure active translocation. Herbicides should not be applied to perennial weeds that are stressed from drought or frost damage, although light frost in the fall can actually enhance perennial weed control efforts. Rather than killing the perennial weed, the frost signals it to store nutrient reserves in its roots. Thus, you can treat perennial weeds with a systemic herbicide, as long as the plants are still growing and have enough healthy vegetative leaf matter to absorb the herbicide.

It usually takes a hard frost to freeze and wilt the leaves of perennial weeds, effectively ending the opportunity for chemical control until the next growing season. 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! – Tracy Sayler

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