By Ontario Ministry of Agriculture Food and Rural Affairs
Sept. 9, 2016 - Many farmers have witnessed the value in applying herbicides in the fall to perennial weeds, especially perennial sow-thistle and dandelion (see Figure 1). Often they will see a reduction in their population the next year as well as a delay in their shoot emergence. This allows the planted crop to have a competitive advantage over those perennial weeds.
Unfortunately weather conditions around the time of application can be quite variable and can influence a herbicide's effectiveness. Let's go through "top tips" to make the most of this application window.
- Choose the most effective products, rate and tank-mixes for the perennial weed that you are targeting. Table 1 outlines what public researchers in Ontario have found to be most effective at controlling perennial plants in the fall.
- Apply when air temperatures are above 8 C for a minimum of two hours after application. This is best accomplished by applying during late morning or mid-day so that the targeted plant is taking up glyphosate during the heat of the day.
- After a frost event, wait two to three days before evaluating weed growth and if the target plants look fine and air temperatures are above 8 C then resume applications. For example, milkweed is very sensitive to frost. Figure 2 shows a milkweed plant three days after an evening where the air temperature reached a low of -3 C. It would not make sense to apply glyphosate on a weed species in that state since its leaves are unlikely to absorb any herbicide. Alternatively, dandelion and wild carrot were not affected by the same frost event (Figure 3 and 4) and one could resume fall applications to those species based on the condition of their leaves.
- Wait a minimum of 72 hours after application to perennial weeds if you want to till the soil. The longer that you can wait after application before making a tillage pass, the more the herbicide will translocate within the plant and do a more effective job controlling the species.
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