Crabgrass preventer is a specialized product known as a pre-emergent herbicide, designed to stop weeds before they become visible. Its primary function is to create a thin chemical barrier at the soil surface. This barrier interferes with the cellular processes of weed seedlings as they sprout, preventing the seeds from establishing roots and shoots. Successfully applying this product requires precise execution, and one of the most common questions involves the timing of lawn mowing relative to the application. Getting this sequence right ensures the chemical barrier is uniform and fully effective against crabgrass.
The Mowing Decision: Before or After Application
The most effective strategy is to mow the lawn one or two days before applying a granular crabgrass preventer. Mowing the turfgrass short, typically to a height of about two to three inches, ensures the granular particles have a clear path to the soil surface. This maximizes contact between the herbicide and the ground, allowing for a complete and unbroken chemical layer. If grass blades are too long, they can catch the granules, preventing the formation of a uniform barrier.
Mowing immediately after the application should be avoided for several days to prevent disturbing the newly laid barrier. The action of a mower can physically displace the granular material from the soil surface, compromising the integrity of the chemical layer. This disturbance creates gaps in the barrier, allowing crabgrass seeds to germinate in unprotected areas. Waiting a minimum of two to three days after application ensures the material has settled and is ready to be watered in for activation.
Optimal Timing for Application
The success of a pre-emergent herbicide relies on applying it before crabgrass seeds begin germination. Crabgrass is a summer annual weed, and its seeds only sprout once the soil reaches a specific temperature threshold. The best time to apply the preventer is when the soil temperature at a two-inch depth consistently reaches 50 to 55 degrees Fahrenheit for several consecutive days.
Applying the product too early risks the herbicide breaking down before the seeds are ready to germinate, as most formulations only remain active for a limited time, typically 60 to 120 days. Conversely, an application made too late, when soil temperatures consistently reach 60 to 70 degrees Fahrenheit, means the seeds have already begun to sprout. Once crabgrass seedlings emerge from the soil, the pre-emergent product becomes ineffective, requiring a different type of herbicide. For a practical, visible cue, the blooming of forsythia shrubs often coincides with the correct soil temperature window for application.
Maximizing Efficacy: Post-Application Steps
After spreading the granular crabgrass preventer, the most important step is to water the lawn lightly. This process dissolves the herbicide granules and moves the active ingredients off the grass blades and down into the soil where the chemical barrier must form. A light irrigation of about one-half to one inch of water is required to fully activate the product. This watering must occur within a specific window, usually within 24 to 48 hours of application, to ensure the barrier sets properly.
Once the barrier is activated, avoid any activity that might disrupt the protective layer in the soil. This includes cultural practices such as core aeration or dethatching. These actions physically puncture the soil, which breaks the chemical shield and can bring dormant crabgrass seeds to the surface, allowing them to germinate. Waiting at least two months after application before performing any major soil-disturbing maintenance helps maintain the integrity of the weed prevention treatment.