The best weed control is a healthy lawn. However, some weeds are invasive and compete well with lawns. Weed control should be practiced only on established lawns. Young seedlings are often injured by weed control agents, even when the control agent is for broadleaf weeds.
Weed control requires knowledge about the weed itself.
This includes proper identification and knowledge of the life cycle of the weed. The idea is to control the weed at its weakest point in the life cycle. The four terms you should be aware of as Master Gardeners are selectivity , tolerance , pre-emergence and post emergence . Pre-emerge agents kill seeds as they germinate, while post emergent weed control agents control weeds already established. Pre-emergence weed control agents are used in the fall and spring at lower elevations and in the spring at high elevation sites.