Watch nature for timing on use of pre-emergents

Fewer weeds grow where turf is dense and competitive. The green part of the lawn shows weeds where turf is thin and open.

My daughter goes to college next year.

To the chagrin of our Clemson friends, we toured the University of South Carolina last weekend. Students, staff and faculty did a wonderful job presenting campus life in Columbia. Meanwhile, my wife and I were having dorm-life flashbacks of community bathrooms, cramped lofts and midnight packages of Ramen noodles.

Students were taking advantage of the unusually warm weather. Forsythia shrubs had taken notice of the temperatures as well. They were in full bloom. While this might seem like nothing more than a visual treat, horticulturists see this as a sign.

Phenology models predict the emergence of insects and weeds by calculating the amount of accumulated heat. Degree-day models are a scientific means of measuring developmental heat, but we don’t need to understand any of these fancy words or methods to be phenology scientists. Nature does it for us.

Trees and shrubs responded to degree days just like insects and weeds. A lot of research has identified the correlation between certain flowers and the emergence of specific pests.

In 1989, Don Orton published “Coincide,” a compendium of environmental cues that signal pest development.

Among turf professionals, forsythia is commonly associated with the first application of pre-emergent herbicides. Forsythia is a multistem shrub with bright yellow flowers. It’s not as common in the Lowcountry as it is farther north, but the cue is still valid.

Weeds begin growing months before you notice them. Summer annual weeds such as crabgrass typically begin germinating in late February. Pre-emergents have to applied before germination takes place. They do not prevent weeds from germinating, but rather kill them as they germinate.

Pre-emergent granules bind to soil particles. When soil temperatures average 55 degrees at a 4-inch depth, weed seeds germinate and contact the pre-emergent-laced soil particles. If pre-emergents are applied after seeds have germinated, there will be no effect.

Perennial weeds, such as dollarweed and Florida betony, are not controlled by pre-emergents because they spread vegetatively and will have to be sprayed with post-emergent herbicides.

Valentine’s Day is our target date for the first pre-emergent application of summer weed control. However, we’ve had an unusually warm winter as evidenced by Frisbee-throwing college students and blooming forsythia. The latter indicates that summer annual weeds will be germinating sooner than usual. As a result, pre-emergents need to be applied now.

Do you need to apply pre-emergent herbicides? No, you don’t. In fact, the best approach to weed control is maintaining a vigorous, thick turf that smothers and outcompetes weeds.

In some situations, emphasis on proper watering, fertilization and thatch control can do as much for weed control as herbicides. The less herbicide you put down saves you money, time and reduces environmental risk.

In fact, accidental over-application can damage turf by stunting root development. Yellowing turf results where clubbed roots can’t take up nutrients and water. This is often seen where the spreader is turned.

Evaluate your weed problems before deciding on pre-emergents.

If your lawn is thin due to environmental stress or previous damage, this is an open invitation for fast-germinating weed seeds. Pre-emergents can suppress weed growth until turf can recover. Once the competitive edge is restored in turf’s favor, pre-emergents can be reduced or discontinued.

If you decide to apply pre-emergent herbicide in February, avoid weed-and-feed products that combine herbicide and fertilizer.

Regardless of how contagious spring fever becomes, wait until the end of April to fertilizer with nitrogen when turf is fully out of dormancy and hungry to grow.

Most pre-emergents, like Halts, Barricade, and Crabgrass Preventer, remain active for six to 10 weeks. If Valentine’s Day passes you by, it’s not too late. Some products such as Dimension can control germinated weed seeds to a certain extent.

If your turf is exceptionally thin, a second application can be made at the end of April to extend weed control through the summer. In most cases, the second application is not as critical as the first because turf is actively growing and outcompeting weeds.

Weed and feed, however, can be used for the second application because that’s when it’s time to fertilize.

Tony Bertauski is a horticulture instructor at Trident Technical College. To give feedback, e-mail him at tony.