Tips for Cool Weather Spraying

In a long spring with periods of prolonged cool then flashes of hot weather, you could be combating advanced stage cool season weeds with outbursts of warm season weeds emerged as well. As seeding and spraying are colliding, here are a few tips to get the best performance out of your herbicides:

  • Spray when weeds are actively growing
  • Spray when the temperature is around maximum (15-25 0C) for the day
  • Apply herbicides when target weeds are most susceptible, timing is more important than application method
  • Use the water volume stated on the label
  • Use the surfactant or adjuvant included or recommended
  • Check the label and abide by rain free period
  • To improve control add at least one other tank mix partner for better results
  • Spray when wind is steady about 3–15 km/hr
  • Do not spray when temperature inversion conditions exist

Some of the herbicides that work well under cool conditions are:

  • Most of Group 4 herbicides
  • Most of the residual herbicides of any group
  • “Fop” group 1 herbicides (e.g. Horizon)
  • Carfentrazone (e.g. CleanStart, Aim)

Some of the herbicides that work well under relatively warmer conditions are:

  • Group 9 herbicide (e.g. Glyphosate)
  • Group 10 herbicides (e.g. Liberty (works better when it’s bright and sunny))
  • “Dim” group 1 herbicides (e.g. Select, Centurion)
  • Non-residual group 2 herbicides
  • Contact herbicides (e.g. Bromoxynil, Sencor, Basagran, Flexstar, Reflex, etc.)

What about morning dew? Dew may increase absorption and weed control by hydrating leaf cuticle, but may reduce weed control if there is too much and spray run-off occurs. If dewpoint is reached, it may be better to spray later in the morning and during day when the dew drying and leaf surface not so wet.

Frost? Do not spray if a frost is expected in the next 48-72 hours, also after frost has occurred, wait for 48-72 hours before resuming spraying.

Submitted by Nasir Shaikh, MAFRD Weeds Specialist

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Caution with Weed Control in Cool Weather

Every year comes with its own challenges. When the challenges are cool/wet conditions and weeds, it’s important to understand weed biology, impact on crops and match the control measures accordingly.

Weed Emergence Timing Concerns:

  • Weeds emerging earlier than the crop are highly competitive.
  • Consider: If planting into fields with weeds already emerged, pre-seed control may be required (e.g. non-selective pre-seed herbicide or tillage)
  • Weeds with delayed emergence or over an extended period can allow escapes.
  • Consider: Selection of herbicides with residual activity or using a tankmix of herbicides with no residual and residual components. There may also be the need to consider multiple applications

Weed Control Management Issues:

  • Cool cloudy/rainy weather following pre-seed or in-crop herbicide applications can reduce control or slowdown the rate of weed death, with extra concern if temperature are below 5 0C.
  • Also large annual weeds, winter annuals and/or perennials control may be reduced
  • Consider: Using the higher recommended herbicide rate for pre-seed treatments to assist in overcoming negative impacts of these cool temperatures. Caution has to be practiced that you follow the label and do not exceed the limits as crop injury may occur. If using tank mixes of two or more herbicides mixes must be applied according to the most restrictive use limitations for either product.

Remember, in-crop, with cool temperatures, there is potential for increased herbicide injury to the crops as the plants have reduced metabolism and may absorb more of the herbicide than normal.

Timing is important! If necessary, hire a custom applicator or plane to achieve the earlier spray timing. Waiting for favorable temperatures to proceed with spraying should be balanced with the size of weeds at time of application. If you have to spray in cool temperatures find out more about your chosen herbicide and the response of crops and weeds under cool conditions, spray when the temperature is at the maximum for the day and avoid application too early or too late in the day.


A carpet of wild oats on May 21, 2014

A carpet of wild oats on May 21, 2014



Also see May 22, 2014 Canola Watch post on weeds at

MAFRD posting on Weed Management in Wet Weather at

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