By Dight Lingenfelter
One of the most common weed management questions we often get relates to perennial weed control in grass pastures and hayfields. Many people want to know when the most optimal time is to control perennial weeds such as Canada thistle, milkweed, hemp dogbane, horsenettle, quackgrass, and others and what herbicides are the most effective. The short answer – late summer and fall with systemic herbicides, but there is more to the story.
Weed Biology and Herbicide Application Timing
With the autumn weather, these plants more actively transport carbohydrates and sugars to underground storage structures such as rhizomes, tubers, and roots to enable them to survive the winter and to provide the necessary energy to begin the next cycle of growth in the spring.
Source : psu.edu