What type of herbicide is best applied when plants are vigorously growing?

The best choice is systemic herbicides, as these are designed to be absorbed by the plant and travel throughout its system when the plant is actively growing. During vigorous growth, plants are more metabolically active, which facilitates the uptake and translocation of the herbicide applied. This allows the herbicide to reach all parts of the plant, including roots and new growth, effectively controlling the targeted vegetation.

Using systemic herbicides during a time of robust growth enhances their effectiveness because the plant's physiological processes—like nutrient and water uptake—are at their peak. This results in a higher likelihood of the herbicide reaching the intended targets within the plant.

Pre-emergent herbicides, on the other hand, are designed to prevent the germination of seeds rather than to act on existing plants and are most effective when applied before weeds emerge. Contact herbicides kill only the parts of the plant they touch and are most effective when applied to actively growing weeds, but lack the systemic action that can eliminate the entire plant. Herbicides intended for dormant plants typically do not perform well because they rely on the plant being active and capable of absorbing the chemical.