When should post-emergent herbicides be applied?

Post-emergent herbicides are designed to target weeds that have already emerged and are actively growing. The rationale behind applying these herbicides during this growth phase is that the plants can more effectively absorb the chemicals, leading to better control of the target species. Herbicides work by being taken up through the leaves and sometimes the roots, so a healthy, growing weed is more susceptible to the action of the herbicide. This enhances the likelihood of the herbicide interrupting normal plant processes, effectively killing the weed.

Applying post-emergent herbicides before weeds emerge would not be effective since those targeted plants are not yet present, and thus, there are no weeds to control. Treating during the dormant season would similarly be ineffective as the weeds are not actively growing and will not be able to absorb the herbicides. Lastly, while weather conditions can affect herbicide application (such as avoiding heavy rain shortly after application), post-emergent herbicides are generally recommended when weeds are actively growing to maximize effectiveness.