Updated: Sep 30, 2020
Timing is everything, especially when it comes to the application of pre-emergent herbicides like LESCO's Stonewall to control summer annual weeds. The general guide is to get the application down before the top 2" of soil are 50-55 degrees Fahrenheit for five or more consecutive days.
This factor is very important because 20% of the weed seed will start germinating at that time. The other 80% of weed seed germination, especially crabgrass, goose grass and foxtail happens when the soil temperatures reach a consistent 60-70 degrees Fahrenheit. By applying the product just before the soil is a consistent 50-55 degrees, you're giving it enough time to become activated by water, soak into the soil, and create the chemical shield between the seed and the soil before germination.
Map it Out
Not sure when the soil reaches 50-55 degrees? Start with the USDA's CRN/SCAN Soil Map to see when the ground typically reaches this temperature in your area. This map is fed by a collection of climate monitoring stations across the country, so you can estimate timing for projects nationally, if needed. Once you have an idea of when this occurs, check the project site using a probe thermometer in the days leading up to this window. Rain and sun exposure can influence the soil temperature and impact your timeline.
Look for the Early Indicators
One other less scientific way to tell when it's pre-emergent season is to look for early indicating flowers like forsythia. They're one of the first species to bloom in the spring; typically right around the time soil is reaching the optimum temperature range.
In addition to proper timing, there are a few other guidelines to maximize pre-emergent results:
Clear any winter debris from beds and turf before application to maximize root zone incorporation
Closely follow the pre-emergent manufacturers' instructions
Thoroughly water your turf or beds to activate soluble fertilizers