What is a key requirement for herbicides to effectively kill weeds?

For herbicides to effectively kill weeds, it is essential that they are absorbed by the plant and translocated to sites within the plant where they can exert their toxic effects. This internal movement allows the herbicide to reach vital areas, such as the growing points or other critical parts of the plant, leading to its ultimate death. Successful herbicide action is predicated on the ability of the solution to penetrate the protective outer layers of the weed and enter the vascular system, enabling systemic action.

In contrast, using herbicides only in liquid form is not a requirement since some formulations can be granular or solid. Applying them during peak sunlight hours doesn't ensure efficacy, as factors such as moisture, temperature, and humidity play a more significant role in herbicide absorption and effectiveness. Similarly, while combining herbicides with insecticides might seem beneficial in some scenarios for broader pest control, it is not a key requirement for herbicides to work specifically against weeds. The primary factor impacting their effectiveness remains their ability to be absorbed and distributed throughout the plant.