Frost: When can I resume weed control?

After a light frost, spraying could resume when the following conditions are met:

—A minimum of one night, preferably two, with minimum temperatures of 5 C — the minimum for biological activity to occur, and…
—A minimum of one day, preferably two, of good growing conditions (warm and sunny) have passed, and…
—Good growing conditions (warm, not hot, and sunny) are present at the time of spraying, and…
—You see 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.

Always talk to your local product rep 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% 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 production. Click your province for your guide: Alberta Saskatchewan Manitoba.

No time to wait for pre-seed burnoff? 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 may 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.

After emergence, wait for crop recovery before spraying. If weeds seem to recover faster than the crop, which can happen, you may want to wait for an extra day of warm nights and good daytime growing conditions. Canola plants set back by frost are not able to process the herbicide they’re supposed to tolerate. Crop damage can occur. Damage is typical of herbicide damage to weeds: Glyphosate on frost-damaged RR canola that hasn’t recovered can yellow new canola growth and in rare severe cases stunt new growth. Liberty on LL canola that hasn’t recovered will first show up as dried or irregular yellowed spots or patches to leaves, particularly tissues between leaf veins. Purpling of the crop plants may occur with either herbicide or may also occur with cold stress itself. Waiting for two days of warm weather after a frost instead of just one day may benefit the crop, and will likely have little effect on weed management results.