The best time of year for herbicide treatments of dandelions and other perennial lawn weeds is in fall when trees and shrubs are not actively growing and less susceptible to herbicide drift. In addition perennial weeds will translocate more of an herbicide to their root system in the fall as they actively move energy below ground for winter survival. This greatly increases the chance that an herbicide will kill the entire weed. Something else to consider: dandelion leaves can make a cheap and tasty addition to any garden salad. So instead of poisoning them, consider eating them instead. That is sweet revenge!