Two common questions after a spring frost

Growers have two common questions after a spring frost:

1. Did the crop survive? (Do I need to reseed?)
2. When can I resume weed control?

These seedlings look OK the morning after the frost, but four days later you can see the extent of the damage — but also that new growth is coming. Photo credit: Justine Cornelsen

Here are our answers:

1. Did the crop survive?

After a frost, it can take a few days to accurately determine how many plants survived, and whether the remaining stand is still uniform. Be patient before making any decisions. Check the whole crop the day after a frost and then again 3-4 days after a frost to assess the situation. Click here for tips on how to check the field.

Note that crop susceptibility to frost can depend on temperatures in the days leading up to the frost event. Canola plants that experienced a few cool days before the frost are acclimatized — or “hardened” — to cool conditions. Canola plants growing through hot days and warm nights are often more susceptible to frost.

With a light frost of 0°C to -2°C, the risk of crop damage is minimal.

With a heavy frost that kills off the cotyledon leaves, it will take a few days before new leaves will start emerging from the growing point. If no growth occurs within this time, the plant is likely dead. Also, if the stem is pinched off or the plant flops over, the plant will likely die. The pinched off or broken stem cannot provide nutrients to the growing point. Important: Dead cotyledons do not mean a dead plant. If the hypocotyl is green and healthy days after the frost, this plant has likely survived and will soon put out a first leaf.

The quickest way to accurately assess recovery is to mark individual plants that could recover and recheck these plants over the following few days.

If many plants have been killed, the question is whether to reseed. Again, it takes a few days to determine the kill rate. If one or two plants per square foot have survived and if that stand is fairly consistent throughout the field, the best choice may be to leave it alone. A thin stand is not optimal for yield but a thin stand may have better yield and quality potential than a reseeded field, particularly in dry soil conditions that wouldn’t encourage rapid emergence. If reseeding into dry soil, don’t chase the moisture below a maximum of 1.5″.

If planning to wait for warmer weather before seeding, note that low temperatures forecast in the short term are not a factor for seed survival and emergence rates, especially in mid May. The forecast for next week when seedlings are emerging is more relevant to seedling survival.

2. When can I resume weed control?

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
–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 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.

Further reading:

Spring frost risk and seeding date
PODCAST: Spring frost risk and seeding date
Spraying in cool conditions