When are pre-emergent herbicides used compared with post-emergents?

• A. Pre-emergents prevent weed germination.
• B. Post-emergents kill actively growing weeds.
• C. Pre-emergents are only used on woody plants.
• D. Post-emergents prevent weed germination.

Answer: (A)

Pre-emergent herbicides are applied before weed seeds begin to germinate, and they work by forming a chemical barrier in the soil that prevents those seeds from sprouting or from establishing once they begin to push through. This means their job is to stop weeds at the very start, before you can see them above ground. Post-emergent herbicides, on the other hand, are used after weeds have already germinated and are actively growing, with the goal of killing or suppressing those visible weeds. Because pre-emergents must be in place before germination, they won’t affect weeds that have already sprouted, so timing is crucial. They’re often used in lawn and landscape work to reduce annual weed pressures for the coming season, while post-emergents are chosen when you’re dealing with existing weed infestations.