When is the best time to apply pre-emergent herbicides to control summer annual weeds?

Applying pre-emergent herbicides in early spring is optimal for controlling summer annual weeds because this timing targets the period just before the weeds begin to germinate. Pre-emergent herbicides work by creating a chemical barrier in the soil that disrupts the germination process of weed seeds, preventing them from establishing.

By applying these herbicides in early spring, typically when soil temperatures reach around 50°F to 55°F, you ensure that the herbicide is active in the soil when the weed seeds begin to sprout. This proactive measure significantly reduces the competition from unwanted summer annual weeds for nutrients, water, and light, ultimately resulting in a healthier and more desirable turf or ornamental landscape.

Applying them at other times, such as late fall or mid-summer, would either miss the crucial pre-germination phase for those summer weeds or would not be effective due to the active growth of the weeds, which occurs after they have already germinated. Late winter applications might coincide with soil thaw, but this timing can be too early for effective control of weeds that germinate later in the spring.