Weed Killer and New Grass

Weeds in new grass may be ugly, but applying weed killer too soon can cause serious damage. If you don't want dead grass, use preventive measures to control weeds before planting, and control them by hand during the early weeks of growth. You can use some chemicals sooner than others, but always follow label instructions.

There are two major types of weed killers, and both are dangerous to new grass. Pre-emergent weed killers, often used for crabgrass, eradicate weeds just after sprouting. Postemergent weed killers destroy annual and perennial weeds already visible above the soil surface.

Pre-emergents remain in the soil as long as one month. If you want to use them before planting seed, follow the label instructions on how long you must wait before planting your grass. Only siduron, used for crabgrass, is safe at planting time, according to Ohio State University. You shouldn't use other pre-emergent weed killers, such as pendimethalin, until a minimum or three to four months after planting seeds.

You should wait until you mow seedling grass a minimum of three to four times before applying most standard postemergent weed killers, such as 2, 4 D. Products differ, so follow the label directions. For sod plantings, wait at least a month to six weeks before using weed killers, Turfgrass Science recommends.

You don't have to spread weed killer over the entire turf to control weeds. Use spot treatments if your lawn doesn't have a lot of weeds. Postemergents are the most suitable for spot treatments.

Work the soil thoroughly and dig out weeds before you plant to minimize problems. If you add topsoil or a cover of straw, make sure it doesn't contain weed seeds. Check seed labels to make sure you get the lowest possible percentage of weed seeds.

Use the sun's heat to kill weeds before you plant by covering the soil with dark sheets of plastic for two weeks.

Keep weeds under control by hand pulling until it's safe to use weed killer. Wait at least two weeks after planting so your shoes don't damage the tender shoots, recommends. Then irrigate before you pull so the weeds are easier to remove.

Proper lawn care and maintenance are essential for discouraging weeds. For example, a lack of nitrogen, compacted soil and poor drainage all encourage weeds. Provide frequent shallow watering to new grass, and use deeper, less-frequent irrigation on established turf. Also avoid mowing grass too short, which weakens it. You shouldn't expect perfection, according to the University of California Integrated Pest Management Program, but you can have attractive grass without herbicides.