What does preemergence control by a herbicide specifically target?

Preemergence control by a herbicide is specifically designed to target weeds at the earliest stage of their life cycle, particularly during the germination phase. This type of herbicide acts on seeds before they have a chance to emerge from the soil. By applying the herbicide at the right time, it creates a chemical barrier that prevents weed seeds from germinating or establishing themselves.

In contrast, other types of herbicides may only affect plants after they have emerged or may act indiscriminately on any contacted vegetation. Thus, options stating that the herbicide kills only when it contacts foliage or only the parts it touches do not align with the notion of preemergence control, which fundamentally aims to stop the weed from ever breaking through the soil. Therefore, the focus of preemergence herbicides is specifically on preventing weed establishment rather than targeting existing plants or only certain portions of them.