Manual weed removal may not be practical for large lawns and gardens or for areas overgrown with many weeds. In these cases, you may choose to use herbicides. When you apply them properly, herbicides are very effective at eliminating weeds. Herbicides are available in two main categories:
Systemic herbicides enter the plant through the roots and leaves and move throughout the inside of the plant.
Contact herbicides kill from the outside in. They attack the exposed parts of the plant, killing the weed by reducing its ability to feed itself through photosynthesis.
Within these two categories, herbicides may also be selective or nonselective:
Selective herbicides, when you apply them as directed by the manufacturer, kill only certain plants. A good example of a selective herbicide is a lawn weed killer designed specifically for the removal of broadleaf plants. These products will remove the weeds without killing the established lawn in which the weeds grow. Young, freshly sewn grass would still be susceptible to the herbicide however, since it would not have had an opportunity to fully establish itself.
Nonselective herbicides kill plants without discretion. They will kill all plants they come into contact with. You can use these products, for example, when preparing an area for planting or when attempting to establish a new lawn. Through their use, all living vegetation — including problem plants — can be removed from an area, giving the gardener a clean slate with which to work.
Finally, herbicides are either pre-emergent or post-emergent.
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-emergents establish a chemical barrier that will not kill established plants, but will prevent weeds from successfully growing. The protective barrier breaks down in six to eight weeks. Use of a pre-emergent, therefore, requires proper timing to be effective – apply them very early in the season. Be aware that pre-emergents can harm some desirable ornamental plants and turf grasses. As always, read and follow the manufacturer's instructions.
Post-emergent herbicides are designed to attack weeds that are already established and growing. All of the contact weed killers are post-emergents. Apply post-emergents later in the growing season, after weeds are established but before they have gone to seed.
The timing of pre- and post-emergent herbicide application is critical. Applying them too later or too early is basically a waste of time and the herbicide.