See: Weed photo ID
Occasional weeds in a healthy looking lawn are to be expected. For the most part you can ignore a few nuisance weeds that popup during the growing season. However, next season you'll probably see more weeds throughout the lawn. At this point you might want to spot treat them to prevent their spread.
A lawn full of weeds signals more problems than just weeds. Most weed problems are related to the top soil's health and condition, and the vitality of the turf grass.
Soil problems usually fall into several categories:
Lack of soil nutrients
Thick thatch layer
Inadequate water, or too much moisture
Too much shade for the turf grass to develop
Weed seeds exist in every soil by the thousands per square yard. The reason you don't see thousands of weeds is because they must have the right conditions to sprout. Often all the weed seeds need is a little moisture and sunlight! Sunlight is the key. A thin lawn allows plenty of light to reach the soil. A thick lawn blocks that light.
When you have soil problems, you reduce the vitality of your lawn's health. It slows the vigorous growth normally associated with a healthy turf grass. The lawn begins thinning out and when this happens, more sunlight reaches the soil and creates the right environment for weeds to take root.
Lawns extremely weed infested need something more than to just kill the weeds.
Begin planning soil improvements. This means regular fertilization, dethatching, regular aeration, adding top-dressing occasionally and overseeding your lawn at the appropriate time. For most cool season grasses, fall is the best time for all of these projects.
Identify and correct the underlying cause before trying to treat the symptoms by applying weed killers across the board.
Once the cause has been addressed then begin a plan of attack. If lawn weeds are extensive, it may be best to wait until early fall when new grass seed can be applied. Killing off all the weeds in mid summer could leave some large holes in the yard. If crabgrass is one of the lawn weeds, then a pre emergent weed control should be applied in the spring, but new grass seed should be applied in early fall. This will allow plenty of time for the new grass seed to develop.
Post emergent weed controls can handle most broadleaf lawn weeds. However, these weeds must be actively growing for this treatment to be effective. Too early in the spring, even though the weeds may look green, they may not have yet begun to actively grow. During the hot summer months, some weeds experience heat or drought stress which forces them into a hibernation state and weed controls will be ineffective. Some weeds as they mature develop tough exterior layers that prevent weed controls from being effective.
When applying spot treatments, you don't need to douse the weed with the herbicide. All you need is just a little bit to get the job done. There's a perception that if a little bit is good, a whole lot is better. Not true. Too much herbicide, even herbicides safe for lawns can cause damage to healthy turf grasses.
CAUTION: Read and follow pesticide label directions carefully, before applying. While most herbicides are safe if used as directed, some do have harmful chemicals that can cause problems, injury, and even long-term consequences to pets and humans if directions are not followed.
Did you know that the warning label on a pesticide is a binding legal agreement between you and the manufacturer? You are legally bound by the contents of that label and failure to follow those directions absolves the manufacturer of any liability should something unforeseen happen.
So read the label before you open the package.