Which herbicide has a residual effect that requires rainfall for activation?

The chosen answer reflects the properties of the herbicide Outlook, which contains the active ingredient dimethenamid-p. This specific herbicide is designed to provide a residual effect, meaning it continues to control weeds for an extended period after application. However, for this residual activity to take place, it requires moisture from rainfall or irrigation to activate the herbicide in the soil.

Moisture helps dissolve the herbicide, allowing it to be absorbed by the target weeds. Without sufficient water, its effectiveness diminishes, and it may not work as intended to provide the desired weed control over time.

Other herbicides in the list may have different characteristics regarding their activation and residual effects. This distinguishes Outlook as the one specifically needing rainfall or moisture for proper activation and sustained weed management.