The successful control of crabgrass hinges entirely on the precise timing of a pre-emergent herbicide application. Crabgrass is a summer annual weed that produces thousands of seeds, which lie dormant until conditions are right for germination. A crabgrass preventer creates a chemical barrier at the soil surface that halts the growth of the weed seedling shortly after it sprouts. For Michigan residents, understanding the biological trigger for germination is the most important factor for a successful application.
Understanding Soil Temperature: The Scientific Trigger
The biological signal that initiates crabgrass germination is not the air temperature, but the temperature of the soil itself. Crabgrass seeds begin to sprout when the soil temperature at a two-inch depth consistently reaches 55 degrees Fahrenheit for several consecutive days. This temperature threshold signals the end of the seed’s winter dormancy.
Applying the pre-emergent product before the soil reaches this consistent temperature is necessary to establish the protective chemical layer. If the application is delayed until after the seeds have already germinated, the pre-emergent herbicide will not be effective. The chemical barrier must be fully activated and in place before the tiny root of the crabgrass seedling pushes through the soil surface.
Practical Timing Guide for Michigan Regions
Translating the 55-degree soil temperature rule into a calendar date requires considering Michigan’s distinct regional climates. The state’s large north-to-south span means the optimal application window shifts significantly between regions. Homeowners can use the Michigan State University Growing Degree Day (GDD) Tracker as an accurate, local resource to pinpoint the ideal timing for their specific zip code.
For the Southern Lower Peninsula (e.g., Detroit and Grand Rapids), the application window typically opens in mid-April. Moving north, the window for the Northern Lower Peninsula (e.g., Traverse City) is usually later, stretching from late April into early May. Residents in the Upper Peninsula face the coldest spring soil temperatures, making their window the latest, generally from early to mid-May. A helpful visual cue across the state is the blooming of the forsythia shrub, as its bright yellow flowers often appear just as the soil approaches the 55-degree mark.
The Correct Application Process
Once the timing is right, the application process must be handled correctly to ensure a uniform chemical barrier. Most crabgrass preventers are granular products that require a broadcast or drop spreader for even distribution across the lawn. It is important to calibrate your spreader according to the product label to ensure you are applying the correct amount of material for your lawn size.
Apply the product to dry grass. To avoid streaks or missed areas, professionals recommend applying half of the material in one direction and the remaining half perpendicular to the first pass. This crosshatch method ensures complete and uniform coverage. The granular material must be watered into the soil immediately after application to activate the chemical barrier.
The product label will specify the amount of water needed, usually about a half-inch of irrigation or rainfall within 24 to 48 hours. This dissolves the granules and moves the active ingredient into the top layer of the soil. Without this activation, the crabgrass preventer will simply sit on the grass blades and fail to form the defense shield against germinating seeds.
Addressing Late Application and Common Mistakes
If the optimal timing window is missed and crabgrass seedlings have already emerged, a standard pre-emergent herbicide will be ineffective, as its job is to prevent germination, not to kill established weeds. Some newer products contain active ingredients that offer a limited amount of early post-emergent control, but this effect is only reliable for very young seedlings. The most common error is applying the preventer too early, which can cause the chemical barrier to degrade before the late-season crabgrass seeds begin to germinate in the summer heat.
Crabgrass preventer chemicals cannot distinguish between weed seeds and desirable turfgrass seeds, stopping both from sprouting. If you plan to overseed or patch bare spots, you must use a specific “seeding safe” crabgrass preventer. Alternatively, delay any seeding until the fall, typically two to four months after the pre-emergent application.