At what point in the growth cycle are foliar-active herbicides most effective on perennial plants?

Foliar-active herbicides are most effective on perennial plants during the late and postflowering periods due to the physiological changes that occur in the plant at this time. In late-stage growth, perennial plants are typically investing energy into the formation of seeds and roots, increasing their overall biomass and nutrient transport activities. This can make them more susceptible to herbicide uptake because the herbicides can be effectively translocated throughout the plant when it is actively moving nutrients and resources.

Additionally, during the postflowering period, many perennials begin to store energy and nutrients in their roots, which can be effectively disrupted by the action of foliar herbicides. As these herbicides target actively growing tissues, they can interfere with the processes involved in seed and root development, enhancing their efficacy during this stage of the growth cycle.

While timing can vary depending on the specific herbicide and plant species, the late and postflowering periods generally provide a conducive environment for foliar-active herbicides to achieve their intended effect.