Which of the following is necessary for a herbicide to effectively kill a plant?

For a herbicide to effectively kill a plant, it is crucial that the herbicide is absorbed by the roots. Many herbicides are designed to disrupt specific physiological processes within the plant, such as photosynthesis or nutrient uptake, which typically occurs after the herbicide penetrates and enters the plant system.

Absorption by the roots allows the active ingredient of the herbicide to move through the plant’s vascular system, reaching critical areas where it can exert its toxic effects. This systemic approach is particularly important for certain types of herbicides that are designed to target and kill the entire plant, rather than just the part that is exposed to the chemical.

The other options may not be necessary for all herbicides. For example, while application timing can influence effectiveness under certain conditions, it is not a universal requirement for herbicide efficacy. Similarly, mixing with water is often a common practice to improve application, but it does not apply to all herbicides; some may be designed to be effective without dilution. The notion that a herbicide must lose potency over time is not relevant; rather, effective herbicides should maintain potency long enough to complete their intended action on the target plant.