Herbicides are best used as a barrier to weeds, as opposed to an attempt to kill weeds. Post-emergent herbicides take the form of sprays that attach to the surface of the weed. Mowing the yard can impact the effectiveness of applying an herbicide. Successful use of the herbicide requires proper application. Always read the instructions on your choice of herbicides before applying the chemical to the yard.
Preemergent herbicides take the form of sprays or granules. The chemical prohibits the germination of the seeds, which helps to reduce and kill the weeds over a longer period of time. The best time to apply a preemergent herbicide, such as a "weed and feed," is early in the season before the weeds begin to flower. April to early June is the ideal time to apply a weed-and-feed fertilizer or plain herbicide granules. Spray-on varieties need to sit on the surface, so apply spray-on varieties when the grass will remain dry for a few days. Granules need to absorb into the ground, so spread granules right before a rainstorm or before you water.
Before applying preemergent or postemergent herbicides, mow the yard. Mowing the yard after the fact spreads the chemicals out over a broad area, which could damage plants and shrubs. The chemicals may also attach to the cut grass, which, if bagged or raked, is removed from the yard and therefore reduces the amount of chemicals on the lawn itself. Cut the grass to a height of 2 to 3 inches, and mow frequently. Apply the herbicide immediately after mowing to be fully effective. Remember to water granules immediately, or allow sprays to sit for a few days.
Postemergent herbicides target existing weeds. This type of herbicide is ideal for sudden sprouts. Spray the weed before it has a chance to flower and spread. One spray is often not enough. A good habit with postemergent herbicides is to mow the yard weekly, and follow up the mowing by wandering around the yard while spraying the visible weeds. Always choose a selective herbicide that targets weeds and not grass. For patches of weeds that are excessively stubborn, use a nonselective weed killer that targets everything. You will have a bare patch, but it is easier to grow grass in a bare patch than attempting to grow it amidst a jungle of weeds.
Perfectly green lawns are not naturally occurring. The ideal method of getting a truly green yard is to develop patterns. Apply nitrogen-rich fertilizer four to five times per year. Use a weed-and-feed variety in the early spring and standard fertilizers throughout the summer. End the season with a fertilizer for fall. Thick grass chokes off weeds, so throw out seed in the spring and fall when you fertilize. Use selective herbicides every time you mow, after you mow, to target the weeds when they appear. The best practice, once the yard is green and virtually weed-free, is to pull up weed sprouts — roots and all — by hand. The process is time-consuming but much more effective.