Contrary to popular belief, pre-emergent weed killers don't destroy weeds and their seeds. They simply stop them from growing. Some seeds are known to last fifty years, so if the herbicide isn't applied each year, the weeds will grow. The big question for pre-emergent weed killers is when to apply them. 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 theyear around which pre-emergent fertilizers should be applied so that they activate before seasonal weeds make an appearance.
There are two application dates because there are two types of weeds,winter weeds and summer weeds. This is especially true in regions that don't completely freeze over in the winter. 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. In warmer areas, the date will probably be earlier. The September 15th date is to catch weeds as they set new seed. This fall application will limit any late fall growth and hopefully begin weeding work for next year.
If you use a pre-emergent herbicide, keeping these two dates in mind can help you get a head start in weed control.
Linda Smalley, South Office Source: Alex Russel, Columnist, All About Lawns