In my years of working with people to establish quality turf and control many of its problems, I have come across many misconceptions people have about lawn establishment and control.
Misconception number 5: The only time to apply weed control is in the spring.
Spring is a good time to control most weeds, but doing it ONLY at this time will not control all weeds especially if it is applied too early when the temperature is too cold. Different weeds emerge at different times of the season, so weed control needs to be done at different times as well.
Weed killer can be applied during the summer, but it can burn the lawn if it is too hot or mixed incorrectly.
Fall is an excellent time to apply weed killer and the weeds do not have a chance to reestablish before the next spring.