Control crabgrass with fall-applied pre-emergents

The signs of spring are indicators to most turfgrass managers that a ruthless competitor is lying in wait--crabgrass. Many choose to battle this pest with pre-emergence herbicides--chemicals you apply before crabgrass seed germinates. Pre-emergence herbicides come in liquid and granular formulations and form a chemical barrier on the soil surface. Crabgrass seedlings that attempt to breach the barrier do not survive.

Herbicide activity Some active ingredients inherently are longer-lived than others. Thus, some herbicides provide season-long crabgrass control with a single application whereas others require two applications. For those herbicides requiring two applications, label instructions typically divide the total recommended rate of active ingredient between the initial application made near the crabgrass germination date and a subsequent application 6 to 8 weeks later. Extended protection from crabgrass encroachment is important because not all crabgrass seeds germinate at the same time. Some germinate in spring, and others later in the growing season.

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The chemical barrier of pre-emergence herbicides is not permanent. Microorganisms in thatch and on the soil surface degrade chemicals over time. Sunlight also breaks down herbicides in the process of photo-degradation. Thus, pre-emergents have a limited lifespan, and factors that influence microbial activity and light penetration also affect herbicide longevity.

Turf-management practices influence herbicide longevity by altering the factors I just mentioned. For example, thatch is a microbial paradise. Therefore, thatchy turf leads to more rapid breakdown of herbicides. Environmental conditions also affect the duration of herbicide activity because warm temperatures and precipitation favor microbial activity. Thus, herbicide degradation is relatively slow during cool, dry conditions and more rapid during warm, wet weather. Other cultural factors--low mowing height, for example--that increase light penetration and photodegradation also can increase herbicide breakdown.

In some situations, disturbance of the soil surface after a spring herbicide application may provide an avenue for crabgrass encroachment. For example, intensive vertical mowing might break the chemical barrier and encourage crabgrass emergence. Research has shown, however, that routine aeriation has little, if any, effect on pre-emergence herbicide activity.

Herbicide timing To maximize the usefulness of pre-emergence herbicides, many turfgrass managers attempt to apply the chemical as close to the date of initial crabgrass germination as possible. Many turf managers use calendar dates to guide their application time. Others observe flowering times of selected ornamentals, such as forsythia or redbud, to serve as a guide for crabgrass activity and herbicide application (see "Using plants to time pest control," April 1996). You also can use soil temperature to estimate crabgrass-germination time, and some researchers have developed models for this purpose.

However, because spring is such a busy time for landscape managers, many are eager to broaden their pre-emergence-herbicide application window to free up time for other spring activities. Further, questions exist concerning the necessity of applying pre-emergence herbicides in synchrony with crabgrass germination. Therefore, great interest exists in determining how early you can apply a pre-emergent and still effectively control crabgrass.

The minimal potential for herbicide degradation during cold weather enticed some turf managers to apply pre-emergents in late fall for crabgrass control the following season. Early fall often is a busy time with turf renovation, fertilization, cultivation, broadleaf-weed control and leaf removal. Late fall, however, can be a relatively slow time, and pre-emergence herbicide application at this time may ease the spring rush.

Research summary Researchers at various universities have conducted several studies to test the effectiveness of fall-applied pre-emergence crabgrass herbicides during the following year. The table above summarizes results from studies in Indiana, Iowa, Kansas and Maryland. In all these studies, researchers applied the herbicides between October 15 and November 15 and then evaluated crabgrass control in August of the following summer.

Crabgrass control greater than 90 percent is excellent in any situation. In these studies, bensulide, dithiopyr and the effectiveness of fall-applied crabgrass pre-emergents during the following year. The table on page C 17 summarizes results from studies in Indiana, Iowa, Kansas and Maryland. In all these studies, researchers applied the herbicides between October 15 and November 15 and then evaluated crabgrass control in August of the following summer. Crabgrass control greater than 90 percent is excellent in any situation. In these studies, bensulide, dithiopyr and prodiamine all yielded this level of control in at least one study location. Rates in these studies were within label requirements (except one study that used 2x rates of dithiopyr). These studies evaluated only a single fall application. It may be possible to extend crabgrass control with split applications in late fall and late spring.

The most important conclusion from these studies is that a single application of certain pre-emergence herbicides in late fall can provide season-long crabgrass control the following summer. Thus, the application window for some chemicals is much wider than turf managers thought and matching the application time to crabgrass germination may not be critical, at least with herbicides that exhibit extended longevity.

A fall application schedule may not be feasible for all turf-management operations, however. Many lawn-care operators serve clients on January-to-January contracts. You gain no advantage by providing a customer with season-long crabgrass protection for the following year unless you have a firm commitment they'll remain a customer.

However, operations that don't work on calendar-year contracts may be well-suited to fall pre-emergence herbicide applications. For example, a golf-course operation or a park in the central or northern United States may benefit by applying pre-emergence herbicides in the fall to free up time for other activities in the spring.

You need to consider the influence of fall-applied pre-emergents on other operations as well, particularly overseeding projects. An autumn pre-emergence-herbicide application may not be appropriate on turf seeded the same season. Check the herbicide label for the earliest date you can apply the product after seeding. In addition, do not apply a pre-emergence herbicide in the fall if you'll be overseeding the same site in the spring. The herbicide will prevent germination of the desirable grasses just as readily as it prevents crabgrass emergence.

Evaluate your situation and consider potential benefits of a fall pre-emergence-herbicide application. The spring rush to intercept emerging crabgrass may not be necessary.

Dr. Jack Fry is associate professor of turfgrass science at Kansas State University--Manhattan, Kan.

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