The dandelion is a familiar perennial weed whose leaves grow flat on the ground atop a long perennial taproot. This familiar lawn weed is often enjoyed by children but disliked by adults, especially when its bright yellow flowers go to seed. It grows best in sunny, thin lawns, and can tolerate a rather dense, hard soil but not a lot of foot traffic.
Dandelions are actually controlled easily with the use of 2,4-D, a common broad-leaf herbicide. However, renewed interest in the environment has more people looking for non-chemical control measures.
The least you should do to control dandelions is mow frequently and collect clippings when they begin to go to seed. Reducing your weed seed crop will not guarantee an end to more dandelions, though. Seeds can blow in from elsewhere and remain viable for years in the soil, waiting for a chance to grow. And because they're perennial weeds, dandelions will keep coming back, larger and stronger each year unless they're removed or killed.
Dandelions are at their weakest right after they bloom and food reserves in their roots are at their lowest. They bloom in both spring and fall when days are less than 12 hours long. Spring bloom is clearly the heaviest and best time to remove them manually.
When you have only a small number of dandelions, dig them out after the ground has been softened by rainfall or thorough watering. Use a weeding fork, a dandelion digger, or try one of the "weed-poppers" available today. Try to get 4" to 6" inches of root so the remaining portion doesn't have enough energy to sprout new buds and leaves. If a plant does re-sprout from a bit of the root, dig it out again.
The most effective herbicide for killing dandelions is 2,4-D. You'll probably find it mixed with other broad- leaved weed-killers such as MCPP (Mecoprop) and dicamba. Other herbicides may be used by lawn chemical applicators. The single best time to apply these products is in September when the plants are transporting carbohydrates from the leaves to the roots for winter storage. The herbicide is moved internally to all parts of the plant, disrupting its growth pattern. You may not see much response in the fall immediately after application, but it's unlikely the dandelions will sprout again the following spring.
If you choose to use an herbicide in spring, the best time to apply it is usually in May. Temperatures should be in the 60s or 70s, wind should be calm, and there should be no rain forecast for at least 24 hours; preferably 48 hours. It is much easier to damage non-target plants in spring than fall, because tender young leaves are much more sensitive to spray drift than tougher, older leaves. Hold your sparayer close to the ground, and spray only those parts of the lawn that need it. As always, be sure to READ THE LABEL CAREFULLY AND FOLLOW ITS DIRECTIONS EACH AND EVERY TIME YOU USE THE HERBICIDE.