Kill Visible Dandelions
The best way to attack dandelions is to kill the whole plant, taproot and all, and then keep new weeds from establishing themselves in your lawn. To take care of your dandelions, apply a weed-and-feed product, such as Scotts® Turf Builder® Weed & Feed in the spring. If that doesn't get them, follow up in 2-4 weeks with an application of Ortho® Weed-B-Gon MAX® Weed Killer For Lawns. Just be sure to follow the instructions on the label.
Don't hand pull them, as they will grow right back unless the tap root (often 2-3 feet deep) is completely removed.
Choke Em Out
A thick lawn is the best method for preventing dandelions and other broadleaf weeds in the lawn. Mow your lawn at a high setting on your mower, and follow a regular feeding program to achieve a lawn that is thick enough to keep weeds like dandelions from establishing in the first place.
Dandelions are a broadleaf perennial that can grow in any soil and are most numerous in full sunlight. In the early spring, new sprouts will emerge from the taproot, which can be 2 to 3 feet deep in the soil. They grow yellow flowers that mature and turn into white puffballs that contain seeds that spread with the wind to other lawns. Even though they disappear in the fall, the taproot survives deep in the soil to start the cycle again in the spring. To keep dandelions out of your yard, this root has to be killed.