Once harvest is complete, it is still important to consider what is happening in the field. Perennial, biennial and winter annual weeds are still active through the fall due to their growth habits. Harvested fields allow these weeds to grow competition-free and prepare to overwinter.
Fall applications of herbicide can control these weeds while they are still actively growing. Perennials, biennials, and winter annuals will start to translocate sugars to their root systems due changes in environment, including day length and temperature. Systemic herbicides, such as glyphosate, will move with the sugars to the root systems where they kill the food storage component of the plant and prevent it from germinating in the spring. This method of control is important for controlling those hard-to-kill weeds such as Canada thistle. Adding another mode of action can help control broadleaf weeds, but be sure to check if there are any re-cropping restrictions for the following season.
Application timing and weed stage plays an important role in the control of the weed. Ideally, herbicides should be applied from early September to the middle of October while the weeds are still actively growing and moving sugars to root systems. Damage to leaves due to harvest activities or frost can limit the surface area of the plant that will absorb the herbicide; Weeds should have at least 60% of the original plant surface area present. Cool, cloudy weather will limit plant activity and the uptake of herbicides.
In the case of a hard frost, wait two or three days before scouting the field. Weeds should still have significant surface area (60%) green and continue growing. If the temperatures return to about 10°C for a few hours a day and frost is not in the forecast, then go ahead and spray. Light frosts do not affect weeds as much, but be sure to give the weeds a chance to recover so there is efficient uptake of the herbicide. As with hard frosts, make sure daytime temperatures are around 10°C and no frost is in the forecast.
Before deciding to apply a post-harvest herbicide, be sure to check what weeds are present in the field. Remember that annual weeds (volunteer canola, shepherd’s purse, wild oats, etc.) will not survive the winter and it is more effective to control these weeds with a spring burn off.