When it comes to pre-emergent herbicides, the last thing you want to have is a breakthrough. But sometimes it happens—why? And more importantly, what can you do to minimize pre-emergent breakthrough?

Pre-emergent “breakthrough” broadly refers to weeds emerging despite the application of a pre-emergent herbicide. The chemical barrier that pre-emergent herbicides are supposed to form near the soil’s surface sometimes can’t develop properly because of certain conditions in the environment. Other times, environmental conditions break down the barrier or make it possible for weeds to bypass.

It’s crucial to water in pre-emergent herbicides because they need water to form the chemical barrier in the soil. Dry conditions make it difficult for the chemistry to disperse into the soil. Even once the chemical barrier has formed, it’s important to maintain moisture in the soil. If the ground becomes extremely dry and cracks, then weeds have a route to bypass the barrier. 

On the other hand, extremely wet conditions can wash out the pre-emergent chemistry. Pay attention to the weather and adjust your irrigation accordingly to give your pre-emergent applications the best chance of success.

Application coverage is also important to the success of pre-emergent herbicides. It may be tempting to conserve product by spot-treating only the areas where you expect heavy weeds, like driveway and sidewalk edges. But doing so generally provides an insufficient barrier in the soil. To minimize breakthrough risk, broadcast your pre-emergent herbicides.

Like cracks in dry ground, aeration makes it possible for weeds to bypass the herbicide barrier in the soil. By disturbing the soil, aeration also disturbs the barrier and weakens its control of weeds. Avoid breakthrough by aerating before pre-emergent application, or wait until fall to aerate.

Another controllable factor that affects pre-emergent breakthrough is mowing height. Mowing the grass too short allows more sunlight to reach the soil, supporting weed growth. Taller turf not only minimizes weed germination but also grows deeper roots itself, which helps it outcompete weeds.

Some conditions that promote weed breakthrough are difficult to control. Sloped areas sometimes see breakthrough because thatch carries the pre-emergent down the slope before it reaches the soil where it was applied. The hydrophobic nature of thatch makes it a hindrance to pre-emergent applications, so dethatching is one way to increase efficacy on sloped areas in particular. 

Making a second pre-emergent application is another option to compensate for problem properties or adverse conditions. Be flexible and willing to adjust your program when necessary to achieve the best results for your customers. Your ATS sales representative is available to help you make adjustments throughout the season.