Control landscape weeds pre-emergently
Although we have not completely escaped the effects of winter, you are probably thinking about what you will be doing when the busy season hits. To ease the crunch during the peak season, you need to stay ahead in your landscaping operation. This includes implementing a control program for summer weeds. For many weed species that infest landscape beds, pre-emergence control is often the best option. The saying "an ounce of prevention is worth a pound of cure" fits here, because few easy cures exist for emerged weeds, especially broadleaf species, in landscape ornamentals.
* Advantages of pre-emergence control. Pre-emergence control has several advantages over post-emergence control: *Pre-emergents control weeds during germination, so no unsightly dead foliage appears after the weeds die. * Weeds you pre-emergently control do not have the opportunity to produce seed. If you apply post-emergence herbicides late in the growth stage of a weed, it may already have produced viable seed. * Pre-emergence herbicides can control weeds for several months. Most post-emergence herbicides do not possess residual activity, so they do not control weeds germinating soon after application. Thus, you would require more frequent herbicide applications if you relied solely on post-emergence control. *In general, pre-emergence herbicides pose less risk of crop damage than post-emergence herbicides. This is primarily because non-selective herbicides are the most common choice for post-emergence control of broadleaf weeds. Shie lded sprays or wiper applications may be necessary to improve safety of non-selective products around ornamentals. * Pre-emergence herbicides often are available in granular form, which may be more acceptable to the public than spraying. However, many pre-emergence herbicides are reasonably safe for over-the-top applications. *Hand weeding is time-consuming and expensive, so methods that prevent weeds can reduce the cost of a weed-control program. Also, it can be difficult finding employees who are enthusiastic about pulling weeds when it is 90F outside. * Disadvantages to pre-emergence control. Some disadvantages to pre-emergence control exist, however. * You must know what weeds to expect at each location, because you must apply the product before the weeds are emerged and identifiable. *Because you don't know exactly where weeds will emerge, you must apply pre-emergence herbicides with broadcast applications. By contrast, you can spot-apply post-emergence herbicides, which requires less overall chemical use. This distinction is highlighted by the use of pre-emergence- + post-emergence-herbicide combinations applied as a tank mix. This is an excellent strategy-one-stop shopping, if you will. The post-emergence herbicide controls emerged weeds, while the pre-emergent provides residual control, extending the weed-control period for several months. The problem is that you have to apply the mixture over the entire area-you cannot spot-apply such a mixture or you'll leave some of the landscape without any pre-emergent. * Another limitation to pre-emergence control is that most pre-emergence herbicides do not control perennial weeds. A few exceptions exist, but it is a good concept to keep in mind when developing a weed-management strategy. You have several choices when developing a pre-emergence program, using either chemical or cultural control, or both. Cultural control includes use of mulches, fabrics or black plastic. In most landscapes, you can benefit from a combination of both cultural and chemical controls.
Mulches Mulches are useful in landscapes for moisture conservation, appearance and the added benefit of weed control. You can use an organic source such as pine bark, hardwood bark, pine straw or similar materials, or an inorganic mulch such as lava rock or white marble chips. Organic and inorganic mulches both have pros and cons. One advantage to rock mulches is that they provide somewhat better annual-weed control than organic mulches. This is because organic mulches break down and become good growing media for weeds. However, you can till organic mulches into the soil when renovating a landscape, benefiting the soil. Rock mulches generally remain in place during a heavy rain, while organic mulches can wash away, especially on sloping ground. Rock mulches can be a nuisance, however, especially if a lawn mower picks one up and throws it. Rock mulches can sink into the soil beneath, allowing weed seed to germinate in the mulch layer. Consider using a landscape fabric underneath rock mulch to act as a soil separator.
Use a 2- to 4-inch mulch depth. Deeper layers provide greater weed suppression but can lead to plant decline. Deep layers of mulch also can lead to wet, poorly aerated soil underneath. In addition, piling mulch up against the trunk of a tree can cause rotting of the bark if the trunk stays moist. When remulching an existing landscape, use a shallow layer so as not to build up the mulch layer above a 4-inch depth.
Organic and inorganic mulches control many annual weeds but not most perennial weeds. In fact, mulch may actually favor perennial weeds. Yellow nutsedge, a perennial with rhizomes and underground tubers, is a troublesome weed in landscapes and occurs in most areas of the United States. In one of my trials a few years ago, I observed greater populations of yellow nutsedge in mulched plots than in bare soil. Several factors may have caused this, including greater soil-moisture levels under the mulch, favoring growth of this species. Also, competition from other plants was less due to the control of annual weeds by the mulch. A similar phenomenon can occur with pre-emergence herbicides, where effective annual-weed control favors growth of perennials.
Fabrics and films As I stated earlier, mulches provide only limited weed control. One way to improve their effectiveness is to place solid black plastic (polyethylene) under the mulch. Black plastic provides excellent annual-weed control, along with suppressing perennial species.
Black plastic has its limitations, however. The surface does not hold mulch well, especially on a slope. Heavy rains can wash off the mulch, exposing the plastic and detracting from the appearance of a landscape. Also, this allows sunlight to reach the plastic. In time, the material will break down, allowing weeds to emerge. Another disadvantage to black plastic is the lack of porosity, limiting water movement and gas exchange.
Landscape fabrics allow water to move through the material to reach plant roots below the fabric and allow for exchange of gases such as oxygen and carbon dioxide. Therefore, landscape fabrics are better choices for long-term weed control in woody ornamental beds. The addition of a landscape fabric under mulch improves weed control over mulch alone, though generally not as great an improvement as that with black plastic. Some fabrics are now on the market that have a chemical added for improved weed control. Biobarrier II (manufactured by ReeMay), which contains trifluralin, the same active ingredient as the herbicide Treflan, is one example of a chemical/fabric combination. This improves control over landscape fabrics applied alone.
Chemical controls For site preparation, fumigants are an option for controlling weeds. Fumigants such as methyl bromide provide excellent weed control but require special handling due to high toxicity. Methyl bromide use is being gradually phased out because it is a listed ozone depleter. Dazomet (BASF's Basamid) is another fumigant available for pre-plant use in ornamental plantings.
Pre-emergence herbicides are another tool to improve weed control over mulch alone. Because most pre-emergence herbicides do not control existing weeds, application timing is important to ensure that the herbicide is in place before weed germination. Late winter or early spring applications control summer-annual weeds such as large crabgrass and prostrate spurge. Your choice of chemical depends on the ornamentals present at the site and the major weed species you expect. Check product labels, where you'll find specific lists of tolerant crops and susceptible weed species.
You have several choices for annual-grass control in ornamental beds (see table, page XX for a more detailed listing of the following compounds). Compounds such as trifluralin, prodiamine, oryzalin, pendimethalin, dithiopyr, metolachlor and napropamide are labeled for a wide range of herbaceous and woody ornamentals. These products will control most annual grasses, such as crabgrass and foxtail, but will only control certain annual broadleaf weeds. Most are available in granular and sprayable forms, but I recommend granular formulations, especially in herbaceous ornamentals, due to their somewhat greater crop safety. I have observed suppression of flowering with certain sprayable grass herbicides, while generally no effect occurred on flowering with granular formulations. For new plantings, apply the herbicide after you've planted and irrigated the ornamentals.
In woody ornamentals, a greater variety of pre-emergence herbicides is available. Established trees and shrubs can tolerate certain chemicals that would injure herbaceous species. Products that contain isoxaben, oxadiazon, oxyfluorfen or simazine will control broadleaf weeds better than the grass herbicides. Isoxaben is available alone or in combination with trifluralin. Similarly, oxadiazon is sold alone but also is available in combination with napropamide. You can apply oxyfluorfen to conifers and dormant deciduous trees as a sprayable product, but it is also available in combination with grass herbicides. One example is oxyfluorfen plus oryzalin. These combination products, using a grass and a broadleaf herbicide, control a wide range of annual weeds. A limitation to these products is their lack of perennial weed control, but this is common to most pre-emergents
Dichlobenil and pronamide control annual and some perennial weeds such as quackgrass in certain established woody ornamentals. Keep these products away from turf areas, because both can damage most turfgrasses. You typically apply these two chemicals during cold weather to decrease vapor losses.
Landscapers frequently ask me what they should do first-mulching or pre-emergence-herbicide application. In research I conducted with Dr. Larry Kuhns of The Pennsylvania State University, we learned that most pre-emergence herbicides work better if you apply them under the mulch as opposed to applications after mulching. Oxadiazon was the major exception, providing greater control when applied above the mulch.
Develop a spring weed-management program for each of the sites you maintain. Choose among the available cultural and chemical options based on the specific ornamental and weed species at that location. Apply pre-emergence herbicides before weed germination and lightly irrigate to activate the chemical. With your pre-emergence program in place, you'll have one less problem with which to deal when your busy time arrives.
Jeffrey Derr is associate professor of weed science at Virginia Polytechnic Institute and State University (Virginia Beach, Va.).
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