Fall is an important time of year for managing perennial weeds in forage crops, especially pastures. Some weed issues were exacerbated in 2025 due to last year’s drought, and we’ve seen many pastures and hayfields with high levels of hemp dogbane, milkweed, Canada thistle, and others. Perennial species can be particularly hard to control since their root systems and other underground structures allow them to regrow quickly after their above ground structures are killed. Fall is an important window for perennial species control, since actions taken now can reduce the amount of energy they store underground for next year.
Frequent mowing is a type of mechanical weed control that helps deplete the amount of carbohydrates that plants have stored in their root systems. Over time, plants that are mowed frequently will be less able to grow back quickly. Using a systemic herbicide in conjunction with mechanical weed control can further reduce a perennial weed’s ability to bounce back in spring. Systemic herbicides are able to move from where they are sprayed on the plant surface to the underground portion of plants. Fall is one of the most effective times of year for systemic herbicide use on perennial plants as the plants will actively move the herbicide throughout the root system as they fill their carbohydrate storages, but they must be used when plants are still actively growing. Optimal timing will be a few weeks after the last cutting, but before hard frosts begin—in most years, that’s sometime in September for Ohio. Control is more effective if perennial weeds are allowed to regrow to around 6-12 inches tall before applying herbicides….continue reading