When to apply pre-emergent herbicide to prevent crabgrass and other lawn weeds is a question often asked. Proper timing is indeed critical, as applying pre-emergent either too early or too late is likely to result in a weed problem.
A common practice is to apply pre-emergent with a single application prior to germination of crabgrass and other lawn weeds. This is an effective means of applying pre-emergent, but minimizes margin of error in regard to the timing itself. For if application is made too early, the efficacy of the pre-emergent may diminish while weed pressure still exist. On the other hand, if application is made too late, there will be nothing in place to prevent weeds.
Though a single application isn’t what I recommend, if I were to apply pre-emergent with a single application in the Atlanta area, I would target March 1 – March 15 as the window of time for it to be made.
There is a superior means of achieving weed control, in my opinion. To lengthen duration of crabgrass control with pre-emergent herbicide application, and to ensure that treatment is made on time, the pre-emergent dosage can be “split”. In splitting the application, half of the dosage is applied well before risk of germination, and the other half is scheduled to occur when the efficacy of the first is expected to be weakened. The result is both insurance against early germination, and an extended window of control.
The timing of the first application should be February – early March, with the second occurring 6-8 weeks later.
Remember, the objective isn’t to put out more herbicide overall, but rather to split the intended dosage in half to gain a longer window of control with the same amount of product.
There are environmental benefits to using the split application method, as a lengthened window of control may result in less use of “post-emergent” herbicides.
Make sure to read the entire label of any pre-emergent used, and use common sense.