When should preemergence herbicides be applied?

Preemergence herbicides are specifically designed to be applied before the germination of target weeds. The precise timing of their application is crucial, as they work by creating a chemical barrier in the soil that prevents weed seeds from successfully germinating and establishing. When applied before crops and weeds emerge, these herbicides can effectively inhibit the growth of unwanted plants without interfering with the crops intended for growth.

Applying preemergence herbicides after crops and weeds have emerged would not be effective, as the herbicide would not stop established plants from growing. Similarly, applying them at the time of planting might not allow the herbicide sufficient time to establish its protective barrier before the weeds begin to germinate. Lastly, applying them after planting but before harvest is typically not recommended, as the efficacy of weed control diminishes and there may be concerns about crop safety and potential residue issues on harvested produce.