Preemergent weedkillers work by preventing seeds from sprouting or germinating. Preemergents are used in lawns and flower beds where desired plants are actively growing to prevent crabgrass and broadleaf weeds from sprouting. To be effective, they must be applied at the right time. Preemergents work by simulating chemicals that certain plants produce to prevent other plants around them from germinating to reduce competition.
Postemergent weedkillers work by killing plants that are already actively growing. Postemergent weedkillers can be selective or nonselective, meaning they kill only certain plants or kill every plant they contact. A typical postemergent is easily absorbed by the plant through the leaf surface and works by destroying the plant's ability to process nutrients, eventually killing the plant. Others work by confusing the plant's growth processes leading to deformity and death.
Selective weedkillers kill only certain plants. They may affect only monocots, which are plants with grasslike leaves, as well as all grasses. On the other hand, they may be designed to kill broadleaf plants, known as dicots.
Nonselective weedkillers kill all plants they have contact with and are used where no vegetation growth is desired.