The first and best method of weed control begins with correct lawn management. This requires knowing the lawn grass you have, mowing it at the correct height, watering it on an as needed basis and fertilizing it correctly. Each lawn grass type has different requirements. For example, the same lower mowing height that keeps bermudagrass thick and healthy will quickly result in St. Augustinegrass thinning and possibly dying. And, the same fertilization program used to keep bermudagrass green and growing will result in a centipedegrass lawn declining or dying. Bermudagrass requires much more fertilizer as compared to centipedegrass.
Correct lawn management encourages a dense, healthy lawn, which is more competitive with weeds.
Use this UF/IFAS Extension link for a publication on management of the various lawn grasses. https://edis.ifas.ufl.edu/topic_lawn_grasses
Sometimes we try to grow a lawn in an area that is not well suited for lawn grass. Examples include areas where there is too much shade and/or tree competition and in areas that stay too wet, causing the lawn roots to decay.
Sometimes a lawn declines and thins simply as a result of its age. In this case, the best solution may be to redo or to renovate an older, declining lawn VS applying herbicides.
Use this UF/IFAS Extension link for publications on lawn establishment and renovation. https://hort.ifas.ufl.edu/yourfloridalawn/renovation_establishment.shtml
Regardless of cause, weeds move in where the lawn thins.
When weeds do show up in a lawn, timing of herbicide applications is of utmost importance in preventing/controlling them.
If you intend to use a preemergence herbicide to help prevent summer annual lawn weeds, you’ll need to apply it during February to early March when day temperatures reach 65°to 70°F for 4 to 5 consecutive days. This will be just before summer annual weeds emerge.
Not every lawn needs an application of preemergence herbicide. If your lawn has no history of summer annual weeds, there’s probably no need to apply a preemergence herbicide to prevent non-existent seedlings from emerging.
Preemergence herbicides should only be used on lawns that have been established for at least a year.
Some preemergence herbicide active ingredients to look for include the following. The herbicide’s active ingredient(s) will be listed on the label of the product. Pendimethalin is one of the more common lawn preemergence herbicide active ingredients that the homeowner is likely to find. Some others may include oryzalin, benefin, bensulide, isoxabin, dithiopyr, prodiamine and atrazine. There are others. Check with local garden supply stores.
Always follow all label directions and precautions when using any pesticide, including herbicides!
More info on lawn weeds and their control is available using this link. https://edis.ifas.ufl.edu/topic_lawn_weeds