Rainfall can increase herbicide action by which mechanism?

• A. May help to move a pre-emergent herbicide to the roots of germinating seeds
• B. It reduces germination
• C. It deactivates the herbicide
• D. It washes away nutrients

Answer: (A)

Rainfall provides the moisture needed to activate and move a pre-emergent herbicide into the soil zone where weed seeds germinate. A pre-emergent forms a chemical barrier in the topsoil, and water dissolves and transports the active ingredient to the root zone so the young roots or shoots encounter it as they emerge. This delivery increases the likelihood the weed will take up the herbicide right as it is trying to establish, improving control.

Other ideas don’t fit this mechanism. Rainfall doesn’t inherently reduce germination in this context, it doesn’t deactivate the herbicide, and washing away nutrients isn’t the primary way rainfall affects herbicide action.