Pre-Emergent Crabgrass Spray

Soil Temperatures

• Crabgrass seeds germinate when soil temperatures are between 53 to 58 degrees Fahrenheit for several days. Gardeners must spray their lawns with pre-emergent herbicide before soil temperatures favor crabgrass germination. To check your soil temperature, make a hole in the first 6 inches of top soil with a screwdriver. Press a soil thermometer inside of the hole to obtain the soil temperature. Check the soil temperature in the morning and late afternoon and average the two temperatures to get an accurate reading.

Pre-Emergent Herbicide

• Several pre-emergent herbicide types are on the market. Typically, the pre-emergent herbicide label will indicate if it is safe to use in your grass. Use pre-emergent herbicides that contain bensulide, benefin, pendimethalin and trifluralin, recommends the University of California. Some products contain both pre-emergent herbicide and fertilizer for gardeners to use. These products are referred to as "weed and feed." Weed and feed products can be tricky to use, because the soil may be warm enough for a pre-emergent herbicide application but the grass may not be out of dormancy for fertilizer.

Application

• Apply pre-emergent herbicide over the entire lawn. An even coverage is important, because crabgrass seeds can easily spread to almost any area of the lawn. Water the lawn after applying the pre-emergent herbicide. You can also apply the pre-emergent herbicide in the morning on a day where afternoon showers are expected. The chemicals in pre-emergent herbicide cannot activate without water, according to Grounds Maintenance Magazine. If fact, pre-emergent herbicide left on the soil will not be as effective to prevent crabgrass seeds from germinating.

Second Application

• Depending on where you live, you may need to reapply pre-emergent herbicide. Pre-emergent herbicide is only effective for four to five months, according to the University of California. Crabgrass will die out after the first frost, but if you live an area of the country that experiences a long growing season, you should reapply pre-emergent herbicide so that crabgrass doesn't pop up. Reapply the pre-emergent herbicide as you did the first application.