What's New: Fall-applied pre-emergents for broadleaf weeds

Though most grounds managers don't think of fall as the time to apply pre-emergents, doing so results in two main benefits. One benefit is controlling winter weeds, especially winter annuals. The other is to get an early start on the control of summer weeds, freeing up time in spring when so many other activities demand your time.

Most pre-emergence herbicides do not control weeds after they germinate, so you must make applications before germination. If weeds already are present at the time of application, you must control them by hand weeding or with post-emergence herbicides. A pre-emergence herbicide then can maintain the area free of weeds for an extended period.

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Microorganisms break down herbicides in the soil. Because microbial breakdown occurs faster in warm soils, herbicides you apply during cooler temperatures in the fall exhibit longer residual weed control than when you apply them in spring or summer.

Certain pre-emergence herbicides also control newly emerged weeds. This early post-emergence activity provides an additional benefit over compounds that have only pre-emergence activity. Herbicides with both pre-emergence and post-emergence activity include atrazine, oxyfluorfen and dichlobenil.

Let's look at the fall-application uses of pre-emergence herbicides in turf and landscape beds.

Winter weed control One reason to apply pre-emergents in fall is for winter weed control. Winter annuals are weeds that begin to germinate as temperatures begin to cool--early September in most parts of the United States. Familiarize yourself with germination times in your particular region.

Germination continues through the fall and early winter. Little germination occurs in the dead of winter--December, January and February--when temperatures are close to or below freezing. A second flush of germination for winter annuals occurs in March and April when temperatures begin to warm. Winter annuals flower in spring and then die off when hot, dry weather arrives. Some perennial weeds, such as dandelion and buckhorn plantain, also germinate in fall or early spring. Understanding when a weed germinates is an important factor when determining application times for pre-emergence herbicides.

Common examples of winter annuals include common chickweed, henbit, vetch, speedwells and bittercress. Horseweed is a plant that most people do not notice until spring, but mostly germinates in fall.

Early control of summer weeds The other reason for applying pre-emergence herbicides in the fall is for early-season control of summer annual weeds. This takes advantage of the longer soil activity of fall-applied pre-emergence herbicides in cool conditions. Apply pre-emergence herbicides any time during fall or winter, as long as the ground is not frozen and no snow cover is present. A pre-emergent you've applied in September, October or November will control weeds well into spring and early summer, depending on the application rate and weather conditions. An advantage of fall applications is that frequent rain or snow helps activate pre-emergence herbicides. Higher rainfall and soil temperatures decrease the longevity of pre-emergence herbicides. Therefore, a given herbicide probably lasts longer in more northern parts of the country.

A fall application generally will not control weeds through the end of the following summer, so you'll need to make a second application in late spring or early summer. However, applying a pre-emergence herbicide in the fall "buys" you time in the spring when you're busiest.

If your primary goal is winter weed control, apply the pre-emergent in late August or early September. If you are making an application for early-season control of summer weeds, put the material down later in the fall--September through November.

Fall applications for broadleaf control in lawns Turf managers generally control broadleaf weeds in turf through the use of post-emergence herbicides, especially combinations of 2,4-D and related chemicals such as dicamba, triclopyr, MCPP, clopyralid or 2,4-DP. Combinations of these herbicides effectively control a wide range of weeds.

Using these post-emergence herbicides has some disadvantages, however. * Because you apply these products after germination, a lawn may exhibit brown patches as the emerged weeds die. This "brown out" effect does not result when the use of pre-emergents has prevented extensive weed invasion in the first place.

* If you apply 2,4-D and related products late in the weed's growth cycle, the weed may already have produced viable seed, causing subsequent infestations.

* Because 2,4-D and related herbicides generally have short residual activity, additional applications may be necessary to control broadleaf weeds that germinate after application.

* Growth-regulator herbicides can injure broadleaf ornamentals so you must use caution around landscape beds.

Most of the pre-emergence herbicides registered for turf will not injure ornamentals. In fact, many turf pre-emergents also are registered for use in ornamentals. This is an important benefit to lawn-service professionals, who can avoid having to buy different herbicides for ornamental beds and turf.

Several pre-emergence herbicides control annual grassy weeds--such as large crabgrass, goosegrass and annual bluegrass--in turf. These products, often called "crabgrass preventers," include the dinitro-aniline herbicide products (benefin, benefin + trifluralin, prodiamine, pendimethalin and oryzalin) along with bensulide, oxadiazon, dithiopyr and DCPA. These products also will control certain broadleaf weeds from seed. For example, DCPA, dithiopyr and the dinitroaniline herbicides control common chickweed if you apply them before weed emergence. The crabgrass preventers also control henbit.

Unfortunately, these products do not control all broadleaf weeds from seed, so the main benefit of applying these herbicides in fall is for control of annual bluegrass and early season crabgrass control. Control of weeds such as dandelion, buckhorn plantain, wild mustard and vetch is poor or erratic with most of the crabgrass preventers. Therefore, consider any broadleaf control from these herbicides a bonus.

You can apply certain pre-emergence herbicides, such as simazine, atrazine and metribuzin to some dormant warm-season turfgrasses. These products control a range of annual broadleaf weeds including common chickweed and henbit. However, they are injurious to cool-season grasses. Check the label to determine time of application and registered turfgrass species.

Isoxaben (DowElanco's Gallery) is a pre-emergence herbicide that is safe on many cool- and warm-season turfgrasses. It controls a wide range of winter-annual broadleaf weeds, including common chickweed, henbit, bittercress and sowthistle. It also provides pre-emergent control of perennial broadleaf weeds such as dandelion, buckhorn plantain and white clover as they germinate. Make a post-emergence application of a 2,4-D combination product to control existing dandelions and other perennial broadleaf weeds. You then can apply isoxaben for residual control.

Isoxaben provides poor control of annual grasses so think of it essentially as a broadleaf herbicide. In residual studies that my graduate student, Rakesh Chandran, and I conducted, isoxaben showed long soil residual from fall applications. It provided about 6 months of broadleaf control in general and longer control of certain broadleaf weeds such as buckhorn plantain.

Fall applications in ornamental beds For most ornamental species, no selective post-emergence herbicide for broadleaf weed control exists. I often receive phone calls in March requesting a selective post-emergence herbicide for chickweed control in herbaceous- and woody-ornamental beds. The best way to control winter annual broadleaf weeds in ornamental beds is with a pre-emergence herbicide in late summer or early fall. You can make applications later in the fall, but you'll have to control already-emerged winter annuals by hand-weeding or other means.

* In annuals and herbaceous perennials, consider products such as prodiamine, DCPA, napropamide, pendimethalin, oryzalin, trifluralin or DowElanco's XL (benefin + oryzalin) for fall applications. Some of these products are available in both granular and sprayable forms. I prefer granular applications because sprayable formulations of certain of these products can stunt growth or decrease flower production in annual flowers. Check herbicide labels to see which species you can treat with a given herbicide. Apply pre-emergents in annuals after transplanting. Do not apply these herbicides to seeded annuals, as they will inhibit germination. Treat bulb plantings before bulb emergence.

Isoxaben is a component of the combination product Snapshot TG (isoxaben + trifluralin), which you can use on a variety of woody ornamentals. Isoxaben also is safe on certain herbaceous perennials, such as Liriope, hosta and daylily, but will injure others such as Ajuga or Veronica. Read the label for specific tolerances.

* Woody ornamentals can tolerate some herbicides that you cannot use in herbaceous plants. For example, oxadiazon and combination products containing oxyfluorfen (Scotts' Rout and Ornamental Herbicide II) will burn the foliage of plants that emerge through the soil, such as tulips and many perennials. However, you can apply these products to a range of woody ornamentals.

Rout, Ornamental Herbicide II and Snapshot control a range of annual weeds because they contain both a broadleaf and a grass herbicide. If you use oxadiazon, consider applying another herbicide with it to improve control of common chickweed. Oxadiazon and the combination products listed above will not control established perennial broadleaf weeds.

Dichlobenil is registered for use on certain well-established woody ornamentals. Dichlobenil controls a range of winter annuals and certain perennial broadleaf weeds. Do not allow dichlobenil to reach turf areas, as it will injure fescue, bluegrass and other turfgrasses. Due to its volatility, apply dichlobenil only in cool temperatures between late fall and early spring.

Weeds germinate year-round in lawns and landscape beds, so a weed-control program must address both summer and winter weeds. Fall applications can be part of a program to control winter weeds as well as to get a start on controlling summer weeds. Spring is a hectic time for grounds managers. By applying pre-emergence herbicides in the fall, you can start off the spring with few weed problems instead of playing catch-up due to fall or early spring weed germination.

Dr. Jeffrey Derr is an extension weed specialist at Virginia Polytechnic Institute and State University--Virginia Beach, Va.

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