Effective greens topdressing depends on approach
Putting greens constitute a small percentage of the golf course, but they demand major attention from both the golf-course superintendent and the golfer. Whether appropriate or not, many golfers consider putting greens to be the most important measure of a golf course's reputation. Because of this attitude, you may need to occasionally remind golfers that the green speeds achieved during professional and local tournaments are typically faster than for day-to-day play. As a result of such comparisons among courses, golf-course superintendents face intense pressure to provide faster and truer putting surfaces. They use several management practices to gain faster greens: topdressing, shorter mowing, verticutting, rolling, grooming and keeping the putting surface dry. While each of these practices is important, we'll focus on topdressing here. The importance of material choices:
Topdressing helps accomplish several objectives:
* Smoothing the putting surface
* Controlling thatch
* Modifying the surface soil
* Aid in renovating and overseeding
* Protecting in winter.
Providing a true putting surface and controlling the thatch are important for all greens because a trend currently exists for faster putting surfaces. Although many courses establish new greens with high sand-content mixes, thousands of other courses have built on native soils.
For native-soil greens, your first objective should be to use topdressing soil that matches the original soil--if the green is performing well. You may prefer to use a mix that is sandier than the native soil. It is difficult to locate consistent and adequate quantities of topdressing soil that match the native soil. For that reason, most superintendents use sand for topdressing on native-soil greens. Nearly all greens built today are created with sand (or a mix of sands) with a small fraction of organic matter--most commonly peat. Sand is the product-of-choice for topdressing these greens.
An effective sand-topdressing program has three main requirements:
* Selecting the proper topdressing material
* Determining the appropriate rate of application
* Adjusting the frequency of topdressing to site conditions.
The first step is locating a sand-source that meets the particle-size range you desire. Other important factors to investigate include mineralogy, particle shape, cost, consistency and long-term availability. Most agronomists suggest a minimum of 60 percent in the medium sand (0.25 to 0.5mm) range with not more than 20 percent fine sand (0.1 to 0.25mm). If possible, you should avoid coarse sand above 0.75mm. Sand coarser than this is difficult to work into the turf and can interfere with putting and dull mower blades. This is especially important when topdressing some of the new bentgrasses, which have high turf-blade density.
Most companies that provide sand for topdressing know they must have a consistent source of quality product. Still, you should check the sand carefully upon delivery to be sure it matches your specifications. Keeping a sample of the original sand enables you to compare the particle size you ordered with that delivered. You may want to monitor your sand even more closely. If so, consider purchasing a set of screens you can use to test sand being delivered. The sieves and a scale will cost a few hundred dollars, but will be worth every penny. Don't forget that sand must be dry for effective testing with the screens.
Quality sand should consist of minerals that are hard and will tolerate traffic. Sands high in silica are best, but other minerals are acceptable. An agronomist can identify the mineralogy of a sand. Avoid sands high in limestone content, if possible. In some regions, this may not be feasible.
How much is too much? You must consider steps two and three in a successful sand-topdressing program together. For many years, superintendents considered the standard to be applying 2 to 4 cubic feet of sand per 1,000 square feet every 3 to 4 weeks. However, when you consider variations in turf species, traffic, soil conditions, shade and climates, it becomes obvious that each course must make adjustments to fit local and seasonal situations.
An important example of a necessary adjustment applies to greens that are planted with the new, aggressive creeping bentgrasses that tolerate very short mowing. The density of some of these grasses is so high that you cannot sufficiently manipulate sand into the turf. Some superintendents find it best to use 1 to 1.5 cubic feet of sand per 1,000 square feet on a weekly basis when grass is actively growing. Applying 1 cubic foot is such a light rate (only 0.006 inch) it is impossible to measure this depth of application. For that reason, application-rate calculation and equipment calibration must be volume-based (cubic feet).
Base any adjustments you make in your topdressing program on careful evaluation of site conditions. Large greens with limited traffic will probably have greater thatch accumulation, necessitating more frequent topdressing--or at least higher rates. Greens in shaded areas may need less aggressive topdressing because the grass does not grow as quickly. For cool-season grasses, the most rapid growth occurs in spring and fall. During summer, the green experiences more traffic, and growth may be slower. It makes sense that you should topdress more frequently in spring and fall (at 2- to 3-week intervals, depending on rate of application) than during the summer (perhaps 3- to 5-week intervals). Doing so will please your golfers. If some areas have serious wear, avoid topdressing those areas. Sand can be very abrasive and injure weak turf when it experiences traffic.
The bottom line is that you should adjust topdressing to match the rate at which thatch accumulates for individual greens. The profile should be a uniform mix of sand and plant material containing no layers. A common reason why layers develop is that you miss one or more topdressings. Thus, you must allot adequate time to implement your topdressing program.
You also create layers when you apply too much material at one time. This may occur because of inadequate calibration of equipment, variation in the moisture content of the sand or poor planning. When topdressing to fill the holes after aerating, it is difficult to prevent developing a layer of sand. Mowing after aerating--but before topdressing--will remove some of the turf that tends to trap sand on the surface. Using dry sand makes it easier to work the granules into the holes.
Changing your sand A question frequently raised is, "Can I change the topdressing sand?" This often comes up when a "cheap" source of sand was used in the original mix, but it is too coarse for topdressing on greens with either the new, dense bentgrasses or even with older cultivars. The typical answer is, "That depends." For example, it may be possible to have the sand company remove anything above 0.75mm. If this component is no more than 10 to 15 percent of the sand, the overall change in sand is small and will present no problem. Do not switch from a predominantly coarse sand mix to ones that are primarily fine blends. Doing so can create a layered situation causing a perched water table, interrupting proper drainage. Always have the soil-testing lab check both the original sand and proposed sand before making a change.
One of the problems commonly observed on greens with a high sand mix is the tendency for the first few inches of root zone to accumulate high concentrations of organic matter from dead roots and other buried plant material. A good topdressing program--in conjunction with other management practices--will help control this layer, resulting in improved water movement, oxygen status and rooting.
If you are thinking about adding amendments with the sand to change properties of the topdressing material, you have several choices. However, check for research to support the claims of companies who provide these materials.
You should place as much emphasis on selecting topdressing material and implementing the greens-topdressing program as you do on selecting the soil mix and constructing and establishing new greens. In both cases, you create the soil for the future. Once in place, you can't easily remove it. The old adage, "Do it right the first time," is appropriate for topdressing.
Dr. Paul Rieke is professor of turfgrass science at Michigan State University (East Lansing, Mich.).
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