As of April 24, the average soil temperature reading at the Ag & Extension Service Center is 45 degrees Fahrenheit.
As the lawn starts to green up in spring, there is always an urge to apply crabgrass preventer. But the fluctuating spring weather can mess up the normal lawn care calendar schedule, tricking us into applying the crabgrass preventer either too early or too late in the spring. This can result in poor control of crabgrass during the growing season.
One of the accurate methods to determine the proper timing of crabgrass preventer would be to take an average soil temperature reading for at least three consecutive days at a depth of 4 inches. When the average soil temperature reaches 55 F, crabgrass seeds begin to initiate their germination process. Scheduling the crabgrass preventer during this critical soil temperature forms a barrier in the soil that inhibits the seedling germination process and its emergence. Homeowners can use a cooking thermometer to test their soil temperature.
When selecting crabgrass preventer, select a product that does not have a combination of crabgrass preventer and fertilizer. Applying fertilizer this early in the spring is not necessary. According to Dr. Doug Soldat, University of Wisconsin Turf Grass Extension Specialist, turf grass absorbs very little nitrogen fertilizer in early spring, and there should be plenty of nitrogen in the soil from organic matter and any fertilizer that was applied last fall.
Products containing active ingredients like benefin+trifluralin, dithopyr, pendimethalin, and prodiamine offer excellent crabgrass preventive control. Homeowners who are interested in an organic product can use corn gluten meal, but its effectiveness is only to a maximum of 60 percent control in the first year and its application rate is bulky (20 lbs/1000 sq.ft). However, do not use any of the above products in newly seeded lawns, as they can prevent the germinating lawn seeds, too. Instead use a pre-emergent product with the active ingredient siduron in newly seeded lawns. After even distribution of the preventive product, schedule a light irrigation if there is no forecast of rain showers in next few days after application.