What growth condition is necessary for systemic herbicides to be effective?

Systemic herbicides require active growth of the plant to be effective because they work by being absorbed into the plant's system and then distributed throughout its tissues. During periods of active growth, plants are engaged in vital physiological processes such as photosynthesis and transpiration, which aid in the uptake and transportation of the herbicide throughout the plant.

When a plant is actively growing, it is more receptive to the herbicide, allowing it to move through the vascular system efficiently. This movement helps to target various parts of the plant, including roots and leaves, ensuring that the herbicide reaches areas where it can disrupt essential metabolic functions, leading to the plant's demise.

In contrast, if the plant is dormant or experiencing low temperatures, these physiological processes slow down, significantly reducing the effectiveness of systemic herbicides. Similarly, while high moisture in the soil can be beneficial, it does not directly correlate to the plant's growth state, making active growth the critical condition for successful herbicide application.