Many farmers know the benefits of applying herbicides to perennial weeds in the fall, especially with perennial sow-thistle and dandelion (Figure 1 at top of page). Often, producers will see a reduction in the population of these weeds the next year as well as a delay in their shoot emergence, with both effects providing a competitive advantage for the planted crop. Unfortunately, however, weather around the time of application can be quite variable and can influence a herbicide’s effectiveness.
Let’s go through some “top tips” to make the most of this application window.
1. Choose effective products, rates and tank mixes for common perennial weeds and volunteer crops. Table 1 (below) lists the treatments that public researchers in Ontario have found to be most effective at controlling perennial plants in the fall.
2. Be mindful of air temperature and frost. Ideally air temperatures should be higher than 8 C for a minimum of two hours after application when applying glyphosate. Therefore it’s best to apply during late morning or midday so the target plant is taking up glyphosate during the heat of the day. When it comes to frost, some species are more sensitive than others. For example, milkweed is very sensitive to frost as Figure 2 shows with a plant three days after an evening where the air temperature reached a low of -3 C.
Dandelion and wild carrot were not affected by the same frost event (Figures 3 and 4, at bottom). After a frost event, wait two to three days to evaluate weed growth. If the target plants look fine and air temperatures are above 8 C then resume applications.
3. If you want to fall till, wait a minimum of 72 hours after application to perennial weeds.