The answer to 'when to weed control' is that the best time to apply weed control products is in the spring and fall. In the spring, applying pre-emergent herbicides can help prevent annual weeds like crabgrass from germinating. In the fall, post-emergent herbicides can be used to target perennial weeds that are actively growing before they go dormant for the winter.
It's important to time weed control applications correctly to be effective. Pre-emergent herbicides need to be applied before the weeds germinate, typically 4-6 weeks before the average last frost date in your area. Post-emergent herbicides work best when weeds are actively growing, so the fall is an ideal time when perennial weeds like dandelions and clover are preparing for winter.