Use pre-emergent herbicides only on established lawns, because they will affect most germinating seeds, including desirable ones. Although pre-emergence herbicides are used primarily to control annual grasses, they also help control annual broadleaf weeds, minimizing the need for control of those weeds later in the season. If the pre-emergence herbicide is applied at the wrong time, it may not remain effective until the seeds of the problem weed sprout. Annuals live for one year. There are summer annuals and winter annuals.
Summer annuals germinate in the spring and winter annuals germinate in the fall.
Pre-emergence herbicides for summer annuals must be applied in the spring, which is when they germinate. Examples of summer annuals include crabgrass, knotweed, and purslane. However, we have a lot of quackgrass, and the two are commonly mistaken for each other and are similar in appearance. The second type of annual is the winter annual. Winter annuals germinate in the fall, and if you apply pre-emergence herbicides for winter annuals in spring, the herbicide will probably be broken down by soil microbes before the seeds germinate, and the application will be wasted.