Herbicides come in several formulations, made with chemicals to kill different types of weeds. Most common herbicides sold at garden centers contain chemicals safe for use around humans and animals, but they may be harmful to plants you don't intend to kill. It's important to read label instructions and determine which type of weed the herbicide will kill and how before using it.
Nonselective, post-emergent herbicides most often contain glyphosphate, a chemical known by the brand name Round-Up. Such weed killers are designed to kill anything that uses photosynthesis while the plant actively is growing. These herbicides will not harm dormant weeds, such as bermudagrass that has turned brown over the winter. They also will not kill weed seeds. Use care when applying nonselectives because wind and water may carry the spray to leaves and stems of ornamental plants.
Available in liquid or granular form, selective pre-emergents prevent weed seeds from germinating in the soil. They tend to provide control for up to three months. Products are available to control grassy weeds only, while others are designed to kill broadleaf weeds. Products sold as "weed-and-feeds" that feature a "crabgrass preventer" contain some type of pre-emergent. Weeds already growing in lawns or beds are not affected. Often you can mix two products together and apply the mixture to an empty bed before planting shrubs or trees. Usually, pre-emergent weed killers must be watered in after they are applied. Do not apply pre-emergent herbicides to lawns in late summer if you plan to over-seed your lawn, as they will sterilize the grass seed.
Selective post-emergent herbicides kill weeds already growing in lawns and beds. Some, such as 2-4-D control broadleaf weeds without damaging lawns. Others kill only grassy weeds without harming ornamental shrubs and perennials. Common brands of such chemicals are Fusilade and Vantage. Apply these to groundcover beds to kill off bermudagrass and crabgrasses.