In a healthy lawn, because of the thick stand of turfgrass, weeds will naturally be minimized without the use of herbicides. It's in an unhealthy lawn where weeds become a major problem.
If you have a thick, green lawn, then weeds will show up from year to year. Instead of putting down a selective herbicide over the entire lawn, it is better to spot treat the occasional weed with a small hand sprayer. Not only is this less expensive, but it is better for the entire yard. Herbicides, even selective herbicides, put stress on the plants. Avoiding unnecessary stress on the lawn is better in the long run.
Spot treat whenever possible. Early spring or fall treatments are usually best. Younger weeds are more easily controlled than mature weeds.
Use the following precautions when applying herbicides:
Do not apply during hot days because the herbicide may injure the lawn, even herbicides rated ok for particular turfgrasses.
Do not apply during the rain. Broad-leaved herbicides require 24 hours of dry weather to ensure good uptake by the plant.
Delay mowing or irrigation for several days after application.
Do not spray under windy conditions.
Do not use 2,4-D or dicamba products on newly seeded lawns.