Pre-emergent herbicides are a form of preventative weed control that works by establishing a chemical barrier in the top layer of the soil. When weed seeds begin to sprout, they absorb the herbicide, which prevents the development of the shoot and root, stopping the weed before it emerges above the ground. This proactive approach is effective against annual weeds like crabgrass, which are common problems in Virginia lawns. Correct timing is the most important factor for successful application.
Understanding the Soil Temperature Trigger
The application of pre-emergent herbicide is tied directly to the temperature of the soil, not a specific calendar date. Crabgrass, the primary target of a spring application, begins to germinate when the soil temperature consistently reaches 55°F for several consecutive days. The goal is to apply the herbicide and activate its barrier just before this temperature threshold is met. It is generally recommended to get the product down when soil temperatures are consistently between 50°F and 55°F.
Homeowners should track their local soil temperature rather than relying solely on air temperature, which fluctuates more widely. A simple soil thermometer inserted two to three inches deep provides the most accurate reading for the germination zone. Alternatively, many agricultural extension offices and online resources provide daily soil temperature maps. The blooming of the forsythia shrub is a traditional, though less precise, indicator that soil temperatures are approaching the crabgrass germination point.
Regional Application Zones within Virginia
Virginia’s varied geography, stretching from the coast to the Appalachian Mountains, results in a significant difference in soil warming times. This variation means there is no single application date for the entire state, and the required application window can shift by several weeks depending on the region’s climate.
In the Coastal Plain and Tidewater regions, where the climate is milder, the earliest application window typically falls in late February to early March. Areas in the central Piedmont region warm up slightly later, making the window closer to mid-March. Residents in the Mountain and Valley regions experience the latest soil warming, often delaying the application until late March or early April. These dates are estimates, however, and the 55°F soil temperature remains the final determinant regardless of the calendar.
Second Application and Fall Timing Strategies
Pre-emergent chemicals naturally break down over time due to microbial activity in the soil, and their effectiveness typically lasts for 8 to 12 weeks. To provide season-long control against summer weeds, a second or “split” application is often necessary. This follow-up application should be timed approximately 8 to 10 weeks after the initial spring treatment to maintain the protective barrier throughout the summer months.
A separate application is also recommended in the fall to manage cool-season annual weeds that germinate when temperatures drop. This fall timing, typically in late August or early September, targets weeds such as Poa annua (annual bluegrass), chickweed, and henbit. The indicator for this application is when the soil temperature begins to fall below 70°F. Applying a pre-emergent in the fall prevents these weeds from establishing over the winter and becoming visible problems in the spring.
Application Best Practices and Post-Care
Achieving an effective barrier requires precise application and immediate follow-up care. The herbicide must be distributed uniformly across the lawn, which necessitates using a properly calibrated spreader. Uneven application results in gaps in the barrier, allowing weeds to grow in those unprotected spots.
The product must be “watered in” immediately after application to move the active ingredient off the grass blades and into the soil, where it forms the protective layer. Most products require about a quarter to a half-inch of water from irrigation or rainfall within 24 hours to ensure activation. Without this watering, the herbicide will not bind to the soil particles and will degrade prematurely from sunlight.
A significant conflict exists between applying a pre-emergent and attempting to grow new grass from seed, as the herbicide stops all seeds from germinating. If you plan to overseed your lawn, you must wait a minimum of 10 to 12 weeks after the pre-emergent application. Conversely, if you have just seeded, you must wait until the new grass is fully established and has been mowed several times before applying any pre-emergent.