Late October to early November is the most effective time to control broadleaf weeds in lawns. Dandelions usually produce a flush of new plants in late September, and the winter annual weeds henbit and chickweed should have germinated in October. These young plants are small and easily controlled with herbicides such as 2,4-D or combination products (Trimec, Weed-B-Gon, Weed-Out) that contain 2,4-D, MCPP and Dicamba. Even established dandelions are more easily controlled now than in the spring because they are actively moving materials from the top portion of the plant to the roots in the fall. Herbicides will translocate to the roots as well and will kill the plant from the roots up.
Choose a day that is 50 degrees or higher. The better the weed is growing, the more weed killer will be moved from the leaves to the roots. Cold temperatures will slow or stop this process.
Weed Free Zone (also sold under the name of Speed Zone) contains the three active ingredients mentioned above, plus carfentrazone. It will give a quicker response than the other products mentioned especially as temperatures approach 50 degrees. (Ward Upham)