Pre-emergents (weed preventers) are applied to the landscape to prevent seeds from germinating.
The key is the timing. Pre-emergent herbicides MUST be applied BEFORE summer weeds (crabgrass, etc) seed germinate.
In general, to get good summer weed control in Atlanta you should apply a pre-emergent on March 1.
For winter weeds (annual bluegrass, henbit or chickweed) the apply date is September 15.
If you want specifics: Weed scientists use soil temperatures to determine the trigger point for their application of pre-emergence herbicide.
Crabgrass seeds germinate when spring soil temperatures tend upwards from 55 degrees F.
Annual bluegrass starts germinating in late summer or fall as soil temperatures fall below 70°F.
The University of Georgia College of Agricultural and Environmental Sciences oversees a network of 50 weather stations across Georgia. Click here to chart your own soil temperatures.
Pre-emergents for lawns. Some prevent both grassy weeds and broadleaf weeds:
Pre-emergents for Ornamental Beds (not lawns)
Pre-emergents for Ornamental Beds and Vegetables
PRE- and POST-emergent combo products for lawns
REMEMBER: Read all herbicide labels thoroughly to be sure the product can be used on your particular turfgrass and that it controls the weed(s) you are trying to manage.