Did you know that dandelion greens are considered a delicacy? They are a slightly more bitter than spinach, but with a little olive oil and balsamic vinegar, you have a salad. Of course, you have to find greens which have not been sprayed this spring by well-meaning, but misinformed chemical applicators (or homeowners). It is a misconception that a pre-emergent herbicide will control dandelions. The dandelion is a perennial which is best controlled in the fall when they more actively translocate herbicide to the plants root system as they store energy for the winter. We do have some success killing the flower of the plant which helps keep the seed from spreading. If you have a great number of dandelions, the problem is a lack of turf density. The thicker your turf, the more difficult it is for any weeds to become established in your lawn (think overseeding).
We make a concerted effort in the fall to hunt dandelions down to reduce the population. As we see flowering plants during our fertilizer applications, we will spray those.
Pre-emergents are used for the control of grassy weeds such as crabgrass, foxtail, goose grass, and barnyard grass (grassy weeds). There is an even longer list of broadleaf weeds which they control. The mode of action is directed at the shoots and roots of plants as they emerge.