Maximize pre-emergence control in cool-season turf
The weeds that are most commonly the target of pre-emergence controls in cool-season turf are smooth crabgrass and goosegrass (also commonly referred to as silver crabgrass). Effective control strategies involving cultural practices and herbicide use can differ for these two species. However, certain aspects are effective for both, as well as many other weeds.
Culture first For best control of both species, it is important to have sound cultural practices in place to ensure that the turf is as dense and competitive as it can be. Because both goosegrass and crabgrass are summer annuals, they germinate in spring, grow during the summer and then flower and set seed in late summer and early fall. Generally, smooth crabgrass germinates when the soil temperature is consistently above 50F for several days and soil moisture is adequate. Goosegrass typically starts to germinate about 3 weeks after smooth crabgrass. In either case, it is important that the turf in which these weeds are germinating be as dense as possible during the germination period. Being summer annuals, these grasses are tropical species and require high light intensity for good growth, especially in the seedling stage. Once germination takes place, the seedlings must be able to access direct sunlight to continue their development. A dense, cool-season grass canopy is quite capable of causing establishment difficult ies for crabgrass and goosegrass. Proper fertility (particularly nitrogen) to encourage foliar growth and a slightly higher cutting height during the time these weeds are germinating can prove beneficial. Such tactics also help pre-emergence herbicides that you've applied to be more effective.
The amount of crabgrass or goosegrass pressure that exists in any given year is primarily a function of how much viable seed was produced the preceding year. Smooth crabgrass and goosegrass tend to produce the bulk of their seed in late August and throughout September. One effective measure that you can use is to lower the height of cut during this period and be sure to collect clippings. You actually can harvest much of the viable weed seed with this method and then dispose of it with the clippings. Lowering the mowing height at this time of year is usually not stressful to cool-season turfgrasses and, as a fringe benefit, it will encourage basal tillering, leading to a denser stand in the spring.
All management practices to increase turf vigor and competitiveness should be in place in the fall. For example, if goosegrass is a chronic problem, a high likelihood exists that the soil is compacted in those areas where goosegrass successfully competes with the desired turf. Core cultivation in the fall timed with aggressive overseeding can alleviate compaction and provide higher turfgrass plant density to improve competitiveness the following spring. Be sure to practice fall fertilization to increase density but not with rates so high that you might compromise the turf's winter survival. Once growth has slowed in late fall, withhold additional fertilization to allow the turf to harden off. Dormant fertilization with nitrogen sources that do not become available until spring also is a good practice to give the turf a good, early start. In locations where snow mold is chronic, be sure to apply appropriate fungicides to minimize losses and reduce overall turf weakness in the spring when you want the grass t o be highly competitive. In areas where winter losses from other maladies-ice damage, desiccation and direct low-temperature kill-occur frequently, it is imperative to take the necessary precautions to minimize such losses to improve control of summer annual grasses.
Any cultural practice that increases the competitiveness of the existing turf will reduce the capability of crabgrass and goosegrass and enhance the level of control that pre-emergence herbicides provide. In research trials that evaluate pre-emergence herbicides, results commonly show that control plots that receive only fertilizer show a reduction in crabgrass of up to 15 percent compared to control plots that receive no additional fertilizer.
Pre-emergence herbicides Industry and universities have been evaluating pre-emergence herbicides for crabgrass control since the late 1950s. Many chemicals have come and gone, but several early products (DCPA in 1959 and benefin in 1962, for example) still remain useful. One factor remains constant for successful control using the pre-emergence approach-proper application timing. When you compare all the reasons for pre-emergence failure, poor timing is the primary culprit. Successful pre-emergence control depends on the fact that, regardless of the chemical, water (either irrigation or rainfall) is necessary for activation. Therefore, you must apply the chemical far enough before the onset of germination that activation of the herbicide can occur.
Many indicators are available to assist in successful application timing. The most reliable method is to monitor soil temperature. Measure soil temperature as near to the surface as possible because that is where last year's weeds deposited their crop of seed. You must note the temperature (in early morning) before the area receives any direct solar radiation. When the soil temperature is above 50F for 3 consecutive days, it's time to make your pre-emergent application. Some practitioners use the bloom of forsythia as a phenotypic indicator and that usually works well. However, the proper indicator is petal fall, not first bloom. If you apply pre-emergence herbicides when the forsythia first blooms, you run the risk that the herbicide's residual will not remain high enough to be effective throughout the full germination period. For smooth crabgrass, most of the germination is complete by mid-July, but goosegrass will continue to germinate through the entire summer, which puts a lot of pressure on pre-emergence herbicides.
Provided your pre-emergence application is timely, several herbicides can be effective. Of course, one way to be sure that the material is on the ground in a timely fashion is to apply it in the fall. Researchers first tested fall applications of pre-emergence herbicides for control the following year at the Pennsylvania State University in 1967 using DCPA with an application date of Dec. 7. In this study, late-season applications provided adequate pre-emergence control the following season. Since the mid-1980s, a considerable amount of research has further documented that late-fall applications can be successful, particularly when using longer residual materials such as prodiamine (Novartis' Barricade) and dithiopyr (Rohm and Haas' Dimension). The level of efficacy for fall applications approaches that of spring applications but is not usually quite as good (although it often reaches commercially acceptable levels).
Fall applications can help reduce the workload in the spring and eliminate some timing problems. However, they can create difficulties, particularly if you've lost turf due to winter problems and you must perform a renovation in the spring. Research into the problems associated with renovation and overseeding in the spring following fall-applied pre-emergence herbicides has found that overseeding with perennial ryegrass usually can be successful. However, overseeding with creeping bentgrass in these situations has not succeeded, and overseeding with Kentucky bluegrass has met with only moderate success. Fortunately, winter losses of Kentucky-bluegrass turf are infrequent in most locations.
In the near term, it appears that the most consistently successful and risk-free approach to the use of pre-emergence herbicides is to apply them with the appropriate timing in the spring of the year. If you are concerned that the initial application might not last through the entire germination period, you can use split applications per label recommendations (usually the second application is half the rate of the first). Another way to address the timing issue is by using dithiopyr, which has relatively good early post-emergence activity (at least until the crabgrass starts to tiller). Nevertheless, the strength of dithiopyr is as a pre-emergence material.
The turf species you're treating may determine your choice of pre-emergent. Economics also might play a major role in your choice. Certainly, some chemicals are more expensive than others, and those that are available in combination with fertilizer carriers may represent inexpensive but effective options. Whatever the factors are that play a role in your choice of product, successful weed management still relies on using solid agronomic principles in your management program. When the agronomic side of your program is in good shape, the control of any pre-emergent will be better.
Dr. Thomas L. Watschke is professor of turfgrass science at the Pennsylvania State University (University Park, Pa.).
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