Pre-emergent chemicals are tools used by both homeowner and commercial landscaper alike. Pre-emergent herbicides kill the weed seeds as they are trying to germinate in the soil. Just as the seeds are putting out their first root (known as a radicle) the pre-emergent chemical enters the germinating seed and kills it.
There are several types of commercially used pre-emergents. In general , they all work the same way with some differences in how well they work in different types of soil, application amounts and how to apply them. But in the case of ALL pre-emergent herbicides timing is everything. Different weeds germinate at different times of year based on the soil temperatures. For example, common crabgrass seeds germinate when soil temperatures reach 59 degrees.
Most of us do not have a soil thermometer laying around so we have general times of year that we apply pre-emergent based on historical soil temperatures and what types of weeds seeds germinate during that time. For example, to control crabgrass in home lawns or commercial lawns in Southern California, a pre-emergent herbicide should be applied in early to mid-February. This application will also control many other weeds that germinate during this time dependent upon the type of pre-emergent used. Another popular time of year to apply pre-emergents is during mid-October when the soil temperatures reach the optimum temperature for many of the weeds that ail us during the winter months.