Many farmers have witnessed the value of applying herbicides in the fall for the management of winter annual and biennial/perennial weeds (Figure 1). Often, they will see both a reduction in density and a delay in shoot emergence the following spring. This allows a spring planted crop to have a competitive advantage over those undesirable weeds. Unfortunately, weather conditions around the time of application can be quite variable and can influence a herbicide’s effectiveness. Let’s go through five “top tips” to make the most of this application window.
Choose the most effective products, rate, and tank-mixes for the target weeds. Glyphosate is usually the “foundation” of any fall herbicide application. When applied alone, a rate of 1,800 grams of active ingredient per hectare should be used when targeting perennial or biennial weeds. That rate is equivalent to applying Roundup Transorb (glyphosate @ 540 g/L) at 1.34 L/acre. The addition of a tank-mix partner will depend on the target weed species, and is required for glyphosate resistant weeds that overwinter, such as Canada fleabane (Figure 2). Table 1 provides a summary of weed species that can benefit from a tank-mix partner.
When tank-mixing, be aware of any “plant back” or rotational crop restrictions for next spring. Table 2 outlines the plant back intervals required after application (in warm weather months) so that the desired field crop can be safely planted the following spring.
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 herbicide 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, a bolted Canada fleabane plant is very sensitive to frost (Figure 3), whereas a seedling rosette is quite tolerant to frost (Figure 4). It would not make sense to apply glyphosate to a weed species that has severe frost injury since the leaves are unlikely to absorb any herbicide. Fortunately, weeds like dandelion and wild carrot are tolerant to light frost events (figure 5 and 6), and applications to those species could resume if leaf tissue appears green and healthy.
Wait a minimum of 72 hours after application to perennial weeds before tilling 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.