What activates preemergence herbicides to move into the soil where weed seeds germinate?

• A. Rainfall or irrigation
• B. Sunlight
• C. Temperature alone
• D. Wind

Answer: (A)

Moisture movement is what activates preemergence herbicides. After you apply them to the soil surface, they must be moved into the top layer of soil where weed seeds germinate. Water from rainfall or irrigation dissolves the herbicide and carries it down into that seed zone, activating it so it can kill weeds as they start to germinate. Sunlight doesn’t push the chemical into the soil, and temperature alone doesn’t relocate it, just as wind won’t transport it into the soil. So the practical takeaway is that adequate rainfall or irrigation is needed after application to activate the herbicide.