How Pre-Emergent Herbicides Work

We talk a lot about the usefulness of pre-emergent herbicides against weeds on golf courses, sports fields, and home lawns. But what do they actually do? 

As the name implies, pre-emergent herbicides target weeds before they emerge. Pre-emergent herbicides prevent the establishment of weed seeds during germination. For that reason, it’s crucial to apply pre-emergents before germination. The timing will vary by location and weather, but the first week of April is typically a reasonable estimate.

Pre-emergent herbicides are residual, meaning that they continue to work beyond the time of application. However, they degrade over time. The herbicides degrade quickly at first, then more slowly after a while. Because of deterioration, it’s important not to apply pre-emergents too early—the longer between application and germination, the more time the herbicide will have to dissipate. Reapplication helps maintain an adequate level of herbicide in the soil, but a well-timed first application is the best way to achieve success.

Pre-emergent herbicides come in granular forms applied with a broadcast spreader and liquid forms applied with a sprayer. Some fertilizers, both granular and liquid, contain pre-emergent herbicides so that you can apply them at the same time. Regardless of application form, it’s important to water in pre-emergent herbicides or apply them before a rainfall so they will absorb into the soil.

As always, refer to product labels for specific application instructions. And consult your sales representative with any questions you may have about pre-emergent herbicides.