Pre-emergent is a type of herbicide used to prevent the growth of weeds before they emerge from the soil. It works by creating a barrier that inhibits seed germination, making it effective for controlling annual weeds like crabgrass and foxtail. Typically applied to lawns and gardens in early spring or fall, pre-emergent needs to be watered in to activate its effectiveness. It is important to follow application guidelines to avoid harming desirable plants, as it can also affect the germination of certain seeds, including those of flowers and vegetables.