by Jeanette Gaultier, MAFRD Weed Specialist
Fall is a great time to control perennials such as Canada thistle, perennial sow thistle and dandelion. As temperatures cool, nutrients move from the leaves down into the roots and if a herbicide can be taken up and translocated with those nutrients, it can equal better control. Glyphosate and/or the Group 4s are examples of herbicides that provide effective fall control of perennials.
A frost event though, can kill some weeds or can damage leaf tissue which will reduce herbicide uptake and reduce the level of weed control. Within the next few days after the frost, you need to assess the target weeds in the areas that you want to obtain control – are the weeds still growing? How much leaf tissue has been damaged?
Light frost: A light frost (0 to -3 C) can actually improve weed control by increasing herbicide translocation to the root. However, duration of the frost also plays a role. Check your weeds for frost damage if you plan on a herbicide application after a light frost. Herbicides can only be taken up and translocated by weeds that are healthy and actively growing.
If you do spray – spray in the afternoon when temperatures are warm and sunny, as this will help with herbicide uptake. You're looking for daytime temperatures of ~10 C for at least two hours. Use rates appropriate to the stage and time of year – fall applications of glyphosate are recommended at a higher rate than when controlling weeds pre-harvest.
Hard frost: Depending on the damage, a hard frost (≤ -5 C) can put an end to (effective) post-harvest weed control. However, if the plant leaves are still shiny green with minimal leaf tissue damage (i.e. not blackened/brown or brittle) or if less than 40 per cent of the plant has more serious leaf tissue damage (i.e. blackened/brown or brittle) there may still be a window to make a herbicide application. Wait at least 48 hours before assessing frost damage after a hard frost.
If you do spray – read the 'If you spray' paragraph above, it still applies. You need those daytime temperatures to hit ~10 C for at least two hours. In addition, consider your coverage – higher water volumes may improve uptake in more heavily damaged weeds.
One last thing – look at the forecast for the next week following the application. If daytime temperatures are below 8 C and/or if night-time temperatures are forecasted to continually be below freezing, it may be too late to make the application to get the economic control you are looking for.