Weeds are not just plants out of place, but they are thieves that steal water and nutrients from others. Their competitive growth strangles or crowds out other plants. Herbicides are poisons chosen for their ability to kill weeds. Because all herbicides kill plants, choose and apply types of herbicide carefully so that they kill only the weeds and not the plants whose home soil they have invaded.
All herbicides are toxins that poison growing plants. Contact herbicides kill from the outside in; they dehydrate leaves so they cannot produce chlorophyll for the plant to use for processing nutrients. If the plant cannot produce new leaves faster than the old ones shrivel, then the plant dies. Systemic herbicides are taken from the soil into the plant's circulatory system where it is carried around the plant, killing cells as it goes.
Herbicides may be either selective or nonselective. Selective toxins kill only certain plants, work under certain conditions or work only at certain concentrations. Nonselective herbicides kill all plants. No herbicide is absolutely selective and none are effective under all circumstances.
Apply preemergent lawn herbicides in early spring; they inhibit germination of seeds and are most commonly applied to prevent crabgrass, which is an annual summer weed. Ohio State University Online reports that they often contain benefin, bensulide or DCPA. Post-emergent herbicides, also called "broadleaf weed control" herbicides, are applied in spring when dandelions, plantains, Creeping Charley and dozens of other perennial weeds start to grow. Broadleaf herbicides generally contain 2,4-D in combination with some other chemicals.
Glyphosate is a nonspecific herbicide used when an area must be cleared of all vegetation. Herbicides like 2,4-D and triclopyr are used to clear broadleaf herbaceous plants. Triclopyr and fosamine kill woody plants like honeysuckle and other wild shrubs. Fosamine and glyphosate degrade rapidly in contact with soil; 2,4-D persists for a week to 10 days and triclopyr has a half-life of a month. The Wisconsin Department of Natural Resources cites 2,4-D as safe for aquatic use.
To avoid chemical use, homeowners may try "natural" herbicides like corn gluten for crabgrass control, and boiling water, vinegar or alcohol for spot nonselective control. When using any natural herbicide, consider that they may have consequences not noted on their labels. Corn gluten adds nitrogen, vinegar is an acid, and alcohol is a persistent toxin.
When using herbicides, always follow label directions exactly. Under-application may not achieve a good result, and overuse may have unintended effects, such as plant elimination. All labels list species for which the herbicide is intended. Because there are so many chemical combinations, never use an herbicide for an unlisted species.