Gardening | Preemergent for Turf and Gardens

Dealing with weeds is a year-round struggle, but the greatest frustration in the effort seems to come in winter when the bright green color of healthy growing weeds is set against the thatch color of dormant winter turf. Weeds show up like neon lights shining so all passersby will notice them.

Many of the winter annual weeds we see today would have been controlled by preemergent herbicide applied last year in September and December. The seeds that develop into winter weeds have been lying dormant since last spring and before.

Winter weeds like Poa annua (annual bluegrass) actually germinate in late fall and winter; they grow slowly in early winter, then rapidly in late winter. They are damaging to turf during the spring green up because they compete with grass for sunlight, moisture and nutrients. When they die back they leave bare spots where summer weeds like crabgrass will sprout. The winter and summer weed seeds remain in those spots so the cycle will repeat.

Preemergent does not kill weeds that have already germinated. Instead it impedes root growth so weeds can’t develop. Proper application of preemergent forms an herbicide barrier that thwarts weed development. When germinating seeds grow through a zone treated with preemergent, emerging roots and shoots absorb the herbicide. As a result they die back, thereby preventing the seeds from growing. Any breaks or holes in the barrier allow seeds in those spots to germinate.

March and June preemergent applications control summer weeds like crabgrass, goosegrass and some broadleaf weeds. Yes, you see winter weeds now; however, you need to apply a chemical barrier against upcoming summer weeds before the soil warms up and those seeds start to germinate.

Proper timing is vital in the application of preemergent. Different weed seeds germinate at different times. Preemergent herbicides are typically effective for six to 12 weeks. Consequently, for optimum control of target weeds it is critical to follow the application directions for the product you choose. Don’t try to second guess the weed scientists.

Weed seeds collect in garden soil as well as in turf grass. Weeds are always waiting to germinate. As with turf grass, they lie dormant in garden soil until conditions are right. Spring planting, like any other soil disturbance, exposes buried seeds. When you plant take care that preemergent does not go into a planting hole. Fill planting holes, as necessary with untreated soil. That allows new plants to better establish their roots. Mulch, and then follow instructions on your preemergent’s label.

Cornmeal, an organic by-product of the corn milling industry, may be used in vegetable gardens. The amino acids in it disrupt root development during the germination process. Avoid damage to young seedlings by waiting until they are three to five inches high before you apply the meal around them. Cornmeal is high in nitrogen; consequently, it also fertilizes growing crops.

Additional guidelines when using preemergent:

▪ Granular preemergent should be watered in with ½ inch or more of rain or irrigation to establish a chemical barrier.

▪ Preemergent chemicals should be used only on well-established lawns.

▪ Preemergent products are labeled for lawn or garden use. Be sure to select the right one for your intended use.

▪ Avoid ‘weed ‘n feed’ products. Preemergent and fertilizer are generally applied on different time schedules.

Read all package information before you use a preemergent. Make sure the product is safe for your type of grass. Follow warnings about proximity to drainage, ponds and waterways. Pay attention to any restrictions on use near trees. Follow application instructions.

The application of preemergent is ongoing endeavor, just as weed seeds are a continuing problem. When applying preemergent keep in mind that you are working about a season or two ahead of when target weeds would become visible in your yard.

Preemergent, sometimes called crab grass killer, is usually sold in granular form. It is available under various trade names. Clemson has prepared excellent charts listing pre and post emergent weed control products. They detail trade names, active herbicidal ingredients and target weeds. Google hgic 2310 Managing Weeds in Warm Season Lawns to view the charts and for additional information.

This story was originally published February 21, 2016 at 12:00 AM with the headline "Gardening | Preemergent for Turf and Gardens."