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After a light frost, spraying could resume when the following conditions are met:
- A minimum of one night with minimum temperatures of 5°C (the minimum for biological activity to occur),
- A minimum of one day of good growing conditions (warm and sunny) have passed,
- Good growing conditions (warm and sunny) are present at the time of spraying,
- No evidence of frost damage (blackening and water soaked appearance) on the crop or the weeds. “Crop” is included here because even a herbicide tolerant canola crop requires that the metabolism of the plant be working at full capacity to enable it to effectively process the herbicide and prevent injury.
Talk to product reps to see how they will support the use of their product following a frost or cool temperatures.
After a heavy frost, check for damaged tissues such as water soaked and darkened leaves that eventually lead to necrosis (dead, dry tissue). If tissue damage is greater than 40 per cent of total leaf area, allow new leaves to grow before making herbicide applications.
Weeds stressed and weakened by frost are not more susceptible to herbicide. In fact, herbicide will likely have lower efficacy on weakened weeds. Because of the above concerns, chemical companies may not be able to guarantee their products’ performance if applied too soon after a frost. And for some products, performance may be reduced if applied at temperatures below (or above) a certain temperature. Check the performance restrictions on a product before using it. Talk to the retailer or check the guide to crop protection. Click your province for your guide: Alberta Saskatchewan Manitoba
No time to wait. Growers who want to do a preseed burnoff and then get seeding may not want to wait 3-4 days for the weeds to recover from a frost. In this situation, growers should recognize that if they go ahead and spray right away, efficacy may be reduced. However, even with the lower efficacy, doing the burnoff and then seeding will likely provide a higher return than seeding without the preseed burn and letting all the weeds compete until an early in-crop application. Consult with your supplier for information on product performance in these conditions to determine the best approach.
In all cases, it helps to know the weed spectrum and weed sizes before deciding the best course of action. Weigh the pros and cons of each option, and set expectations accordingly.