A Pre Emergent lawn care product eliminates weeds at the earliest stage of growth — before you even see them. Several key factors are important to consider if you want to use this type of weed killer effectively. Pre-emergent herbicides are designed for application before the targeted weed germinates, and are an effective preventative method for controlling weeds. Crabgrass preventer is a good example. Pre-Emergent Herbicide Usage, a guide that discusses the active ingredients in pre-emergent herbicides and how they work.