When should soil herbicides be applied?

The application of soil herbicides is most effective when the soil is not frozen, as this allows for proper absorption and interaction with the soil particles and target weeds. When the soil is thawed, the herbicides can move into the root zone of weeds more effectively, preventing their growth. Applying these herbicides during any time that the soil is not frozen enables better chemical activity and efficacy against unwanted vegetation.

In contrast, the other options limit the application timing unnecessarily. Applying herbicides only in winter or only during planting season can restrict their use when weed pressures might be high outside of those specific times. Applying when the soil is frozen would hinder the herbicide's effectiveness, as the chemicals would not be able to penetrate the soil or effectively reach the weed seeds or roots. Therefore, the flexibility to apply soil herbicides any time the soil is not frozen provides the best opportunity for optimal treatment of weeds.