Answer: Pre-emergent herbicides only work if they are applied to your lawn before the weed's growth period. But if applied too early, weather will dilute the herbicide and the weed will grow unencumbered.
According to garden experts from many regions of the U.S., the two dates to remember are March 15 and September 15. Those are the two dates of the year around which pre-emergent fertilizers should be applied so that they activate before seasonal weeds make an appearance.
The summer weed date is March 15th, because that is usually when average soil temperatures reach above 50 degrees. Major summer weeds like crabgrass or clover will only emerge once the soil is consistently over this temperature.
There are a number of brands available. Scott's and Vigoro are readily available, but there are other manufacturers, as well. Your garden center will probably have pre-emergent fertilizers on sale at the right time to apply them to your lawn.
Best wishes with your lawn!
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