Dry conditions have several effects on herbicide activity. The most important and often overlooked is the role of moisture in “activating” the preemergence barrier in the turfgrass rootzone. Preemergence herbicides will require moisture to migrate into the turf root zone and become available for absorption by newly germinating weed seedlings.
Remember preemergence herbicides require weed seedlings to germinate for them to be controlled. So be sure to follow label directions if using preemergence herbicide and irrigate if called for. Additionally, many preemergence herbicides, such as dintroanilines prodiamine
(Barricade) or pendimethalin (pre-m) can influence turfgrass root growth. Under very dry conditions the affect on rooting might be pronounced.
Finally, dry conditions will reduce desirable turf vigor possibly creating voids that can be filled by invading weeds. Monitor turf density and consider using N fertilizer to increase density as studies have found fertilizing a thin turf alone will reduce crabgrass invasion by 70 percent.
Dry conditions also influence performance of post emergence herbicides that might be used at this time for broadleaf weed control. In general, moisture stress conditions that limit plant growth also reduce the effectiveness of foliar applied herbicides such as 2,4-D. Simple solutions for maintaining effectiveness are to irrigate within 24-48 hours of application, either pre or post application. If irrigating after application be sure product dries on leaf surface before applying water.