Pre-emergent Herbicides

By Diane Schlindwein

July 9, 2010 4 min read

To avoid whacking weeds in your yard, it's best to get rid of them before they take root and take over your lawn. Experts say using a pre-emergent herbicide treatment at the right time is the best way to keep your lawn lush, lovely and weed-free.

Weeds certainly have earned their bad reputation. They can steal nutrients, light and water from your grass. Some weeds cause allergic reactions in both people and pets. And if you are at all interested in aesthetics, you probably don't like the way weeds ruin the look of your lawn. Of course, weeds love to grow in gardens, too.

As it is in any battle, it is best to know your enemy when fighting weeds. The primary targets of pre-emergent herbicides are annual grasses, such as crab grass, but they also go after some small-seeded annual broadleaves, such as dandelions, and other major lawn weeds.

Remember that pre-emergents have to be in place and activated before weeds become a problem. "Pre-emergent herbicides are applied before seeds germinate," says Bruce Augustin, chief agronomist for Scotts Miracle-Gro. "This applies to crab grass in the lawn and various weeds in the garden."

Using pre-emergent herbicides requires homeowners to remember back to the previous growing season. If you had weeds in your lawn or garden last year, chances are you'll need to treat them -- or eventually pull them out -- this year.

Jack Robertson, who has been running his large lawn care business in Illinois for 34 years, says homeowners should use pre-emergents only as they are needed. "In Illinois, we do it strictly in the spring, but in warmer places -- especially in the South -- you might use them in the fall and in the spring."

Generally, fall applications are made in September or October, and spring applications are made in March or April. "Typical (spring) timing is April for most of the North," Augustin says. "Timing is driven by soil temperature warming up to the optimum for weed seed germination."

"We use granules, but both liquids and granules work equally well," Robertson says. "We use a pre-emergent crab grass control that has fertilizer in it."

You can either request that your lawn service provider apply the pre-emergent treatment or do it yourself. Of course, you always should read label directions when using any lawn treatment product. Remember that if you apply a pre-emergent herbicide, you shouldn't reseed turf grass for several months.

If you miss that all-important window of opportunity and weeds have sprouted, Augustin says you can still get rid of them. "On lawns with crab grass (by summer or early fall), it is too late to apply (Scotts) Turf Builder with Halts," he says. "It is possible to use Ortho Weed B Gon with crab grass control to kill existing weeds. In the summer (or whenever weeds have come up), garden weeds can still be removed by hand. Then garden weed preventer can be applied to prevent weeds for the rest of the summer."

Robertson warns against going overboard with weedkillers. In fact, homeowners should use them only as needed. "It's rather expensive to use pre-emergent herbicides," he concludes. "So that's why you want to do it at the best time."

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