By: Karl Danneberger

An important spring ritual is the application of a pre-emergent herbicide for controlling crabgrass.  The application timing is critical to season-long control.  The ideal situation is to apply a pre-emergent herbicide just prior to crabgrass germination. At this point in time your herbicide will prevent germination and last longer through the growing season. Although there are many factors that influence crabgrass germination, including soil moisture and the density of the turf, there are also plant and temperature keys.

Plant phenological keys are often used to time crabgrass herbicide applications. The Eastern redbud tree is an example of a phenological timing event.  As the redbud tree approaches full bloom, crabgrass germinates. An obvious drawback is redbuds are not found in every landscape. The most common plant used is forsythia. As forsythia bloom, crabgrass begins to germinate. The problem with timing based on flowering is the wide variation in air temperature that can occur. For example, in some years forsythias may bloom multiple times based on the spring weather.

Soil temperatures are often utilized for determining crabgrass germination. When soil temperatures fall within the range of 52 to 58 F or 55 F at the 4-in. depth, crabgrass will begin germinating. Purchasing a soil temperature thermometer is a good investment if predicting crabgrass germination is important to you. Soils will warm more quickly under a thin turf than under a dense turf.