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 because, at this stage, the plants are actively transporting energy and nutrients from the leaves to their root systems to prepare for dormancy. This is also the time when the plants are storing carbohydrates, making them more vulnerable to systemic herbicides that are absorbed through the foliage.

During late-season applications, the herbicides can move efficiently through the plant, targeting and disrupting the growth processes in the roots and other internal structures. This timing helps to ensure that any absorbed herbicide can effectively impact the perennial's ability to regenerate in the following growing season, making it more likely that the herbicide will lead to long-term control.

Other growth stages, such as early growth stages, the mid-growing season, or the pre-flowering stage, may not be as effective due to different physiological processes happening in the plants at those times. Early stages may not have sufficient biomass to effectively absorb herbicides, while during the mid-growing season, plants may be more vigorous and capable of recovering from stress. The pre-flowering stage can also present challenges in effectively delivering herbicides to the root systems, as energy is primarily focused on flowering and seed production.