Herbicides are most effective when weeds are growing rapidly in which conditions?

Herbicides work best when applied under conditions that favor rapid weed growth, which typically occurs in environments that are warm and humid. High humidity levels help plants maintain open stomata, facilitating herbicide absorption and enhancing its effectiveness as the weeds are actively photosynthesizing and growing. Optimum temperatures for growth, generally in the range of 70°F to 85°F (21°C to 29°C) for many weed species, further promote this rapid growth.

When weeds are growing vigorously, they are more susceptible to the action of herbicides, allowing the chemicals to be translocated throughout the plants more effectively. This combination of high humidity and optimum temperatures ensures that weeds are at their most vulnerable, maximizing the impact of the herbicide treatment. Other conditions like low humidity or extreme temperatures can stress plants and reduce the effectiveness of herbicides as either the plants' metabolic processes slow down or they may close their stomata to conserve moisture, inhibiting absorption.