Weeds in new lawns can be sprayed once the lawn has been mowed two or three times. Weeds in established lawns should be treated when they are actively growing. In most cases, this is in the spring and early fall. In the summer, after a wet spell, is also a good time to treat as weeds can easily be killed at this time. Trying to kill weeds when it is too cold (below 60 degrees) or too hot (above 88 degrees) can lead to poor results.
New herbicides like Earl May’s Super Brush and Weed Killer are much more effective at cooler temperatures. For the most difficult-to-control weeds, apply weed killer twice, 10–14 days apart, mixing Turbo Spreader Sticker with the weed killer. A spreader sticker makes the spray solution cover better and stick to the leaves of the weeds. When done in October, weeds will not store energy for winter, causing them to die by the following spring. They may not look dead in the fall after you have sprayed them, but they are dying on the inside.
Before resorting to weed killers, you can reduce the amount of weeds in your lawn by mowing higher in the late spring and summer. This lets the lawn shade the soil which reduces germination of weed seeds. Simply fertilizing a lawn will make it more vigorous, also helping to choke out weeds. Once weeds begin to grow, kill them before they seed for fewer weeds in your lawn next year.
Invest in Crabgrass Preventer BEFORE it strikes. Some products should be put on a lawn before there are even signs of the problems they treat. Crabgrass is much easier to prevent than it is to control.