Pre-emergent herbicides are a preventative measure designed to control weeds before they emerge. This type of product works by creating a chemical barrier near the soil line that inhibits the growth of young weed seedlings after they germinate. The seedling absorbs the herbicide as it sprouts, preventing it from developing into a mature, visible weed. Successful weed prevention depends on precise application timing, as the barrier must be in place before the weed seeds begin to stir.
The Science of Soil Temperature
The decision of when to apply pre-emergent herbicides is not based on the calendar or the air temperature, but rather on the consistent temperature of the soil. Weed seeds, especially warm-season annuals like crabgrass, have specific temperature thresholds that trigger germination. Soil temperature changes more slowly and serves as a reliable indicator for biological activity beneath the surface.
For crabgrass, the target temperature range for application is when the soil temperature consistently reaches 50 to 55 degrees Fahrenheit, measured at a depth of two to four inches. This temperature signals the beginning of germination. Applying the herbicide before this threshold ensures the protective barrier is established and active before the weed seeds successfully sprout.
Spring Timing for Crabgrass Control
The spring application window is the most recognized timing for Michigan homeowners, as it targets the highly invasive crabgrass (Digiteria species). Because Michigan spans multiple climate zones, the optimal application date can vary by several weeks from south to north. In Southern Michigan, including areas like Detroit and Ann Arbor, the soil typically hits the 50-55°F range between late March and mid-April. This is when the pre-emergent should be applied.
Moving northward, toward Central and Northern Michigan or the Upper Peninsula, the window for application is often delayed by two to four weeks, extending into late April or early May. A practical way to estimate this timing without a soil thermometer is to observe local environmental cues. The full bloom of the yellow-flowered forsythia shrub is a well-known visual proxy for the soil temperature reaching the proper range.
Pre-emergent products have a finite window of effectiveness, typically offering control for 6 to 12 weeks. Applying the product too early risks the herbicide breaking down before the peak germination period occurs. Conversely, waiting until soil temperatures exceed 70°F means that a significant portion of the crabgrass has already germinated, rendering the application useless for those weeds.
Fall Timing for Cool-Season Weeds
While spring targets warm-season annuals, a beneficial application window exists in the fall to control cool-season weeds and winter annuals. This timing prevents weeds like annual bluegrass (Poa annua), henbit, and chickweed, which germinate as the weather cools. These weeds sprout in the fall, survive the winter, and produce seeds the following spring.
The optimal time for a fall application in Michigan generally occurs when the soil temperature drops back down to the 70°F range, typically from late August through early September. Applying a pre-emergent during this period stops the seeds of these cool-season weeds from establishing themselves before winter. This strategy reduces the population of these weeds that would otherwise emerge and become noticeable in the following spring.
Post-Application Steps for Success
After the pre-emergent herbicide is spread, two steps must be taken to ensure the chemical barrier is properly established. The first step is to activate the product by watering it into the soil. Most granular pre-emergents require about one-quarter to one-half inch of water from irrigation or rainfall to dissolve and move the active ingredient into the top layer of soil where the weed seeds reside. Failure to water the product in promptly can leave the herbicide inactive on the surface, where it can be broken down by sunlight.
The second step is to avoid any activity that might disrupt the newly formed chemical barrier in the soil. Practices like core aeration, aggressive dethatching, or heavy raking can physically break the continuous layer of herbicide. Breaking this barrier creates gaps where weed seeds can successfully germinate and emerge. To maintain the barrier’s integrity, it is advisable to wait a minimum of 6 to 8 weeks after application before performing any soil-disturbing lawn maintenance.