Don’t be in hurry to seed if frost occurs after spraying burnoff


“It froze last night. How long do I have to wait to spray my burnoff?”

The answer? “It depends.”

It depends on the weeds that are being targeted, the size and stage of the weed, the rate of glyphosate being used and the additional products that may be added to the glyphosate.

The first thing to do is determine what weeds are in the field.

Annual weeds take significantly less time to be killed than do winter annuals, and winter annuals take less time than perennials.

For example, seeding after as little as three to four hours is adequate if you are targeting annual weeds such as volunteer canola, cleavers, lamb’s quarter, kochia and wild buckwheat and the weeds are small. These plants are actively growing so the chemical is quickly translocated to the roots. The only exception might be when glyphosate is being used by itself on large (three-plus leaf stage) wild buckwheat plants.

Winter annuals may take a longer waiting period between spraying and seeding. 


Weeds such as stinkweed and shepherd’s purse are easily controlled when in the rosette stage but require a bit more time when flowering.

Large, over-wintered cleavers or winter annuals that have bolted or are flowering may require a day to take in the product. 

However, narrow-leafed hawks beard is the most difficult winter annual. It is easy to control in the fall or early spring, but it begins to regrow early and quickly becomes difficult to manage.

Over-wintered rosettes of narrow-leaved hawk’s-beard can be controlled before seeding with gly-
phosate alone or glyphosate containing herbicides, as long as the rosettes are less than 15 centimetres across and have not bolted.  

Use .18 kg active ingredient per acre if the rosettes are less than eight cm tall or across and .27 kg per acre if rosettes are eight to 15 cm across.  

Wait at least two days before seeding.


Perennial weeds such as dandelion, quackgrass and Canada thistle require a three day wait before seeding, but that should be pushed to four days when dandelions are large. 

Perennial sow thistle will usually not emerge until later in the spring.  You may be able to cheat a little and seed after two days when weather conditions are favourable: night temperatures of 5 C and day temperatures of 15 to 20 C. 

However, it’s important to understand the risk and guarantees that are waived by spraying earlier than recommended.

Generally, the longer you wait before seeding, the better.
 The biggest issue to deal with in early spring is frost. Temperatures will often reach lows of -2 to -4 C and then peak the following day to 12 to 15 C.  

Here are guidelines to consider:

When a hard frost occurs (-3 to -4 C for an extended period of time):

  • For perennials, wait until the warm part of the next day to spray. If it freezes the next night, wait an additional 24 hours prior to seeding.

  • For annuals, wait until the warm part of the same day to spray. Wait until daytime temperatures have reached 10 C for at least two hours before spaying. If frost damage is suspected, wait one to two days to ensure the weeds recover.

  • For both perennial and annual weeds, two-thirds of the leaf area must be green and undamaged to spray.  Wait for new growth if damage is greater.


When light frost occurs (temperatures dip briefly below 0 C overnight for no more than one hour or frost on rooftops):

  • Perennials and annuals: Wait until the warm part of the same day to spray. Minimum daytime temperatures should be at least 10 C.

Thom Weir is an agronomist with Farmer’s Edge. He can be reached by emailing