There are many different sand topdressing programs used by superintendents. Budgets, experience, labor and equipment availability, and play schedule are just some of the factors affecting topdressing programs.
Our research hopes to provide answers to many of the common topdressing questions and help present a better understanding of the topdressing requirements for high quality putting greens. Sand topdressing decreases thatch, increases surface firmness and smoothness, and decreases grain. Insufficient sand topdressing may result in excessive organic matter accumulation in the upper soil profile. Excessive organic matter, or thatch, results in greater pest damage, shallow rooting, poor soil air exchange, and may cause the turf to be more prone to scalping.
The most effective surface organic matter management programs for putting greens normally combine hollow tine aerification with regular sand topdressing. In addition to seasonal heavy applications to back-fill aerification holes, sand should be applied frequently enough to match seasonal shoot growth and to prevent an organic layer from forming. Typically, topdressing every one to three weeks during the growing season is done using light applications of sand to dilute organic matter and smooth wear areas. If applications are spaced too far apart, layering occurs and little thatch reduction actually takes place.
Although topdressing application frequency has increased, the overall annual amount seems to be declining. Ultra-light topdressing on a weekly basis is not always enough to keep up with organic matter accumulation. It is important to remember that for effective thatch management, it is necessary to match the rate of organic matter production with appropriate amounts of topdressing material. Some greens may require more topdressing than others due to differences in growing environments, fertility programs, traffic, compaction, etc. Cool-season turfgrass organic matter production is highest during periods of cool temperatures (32-55 F) and in areas with poor air circulation and high moisture (Carrow, 2003).
A long-term successful topdressing program normally includes the use of a material with a particle size distribution that matches the underlying rootzone. On many closely mowed newer putting greens, coarse topdressing sand particles may be easily picked up by greens mowers. Finer sand is easier to work into the turf canopy, especially at lower mowing heights, and with new high shoot density bentgrass cultivars, and a desire to have less impact on play, are all persuading turf managers to switch to finer sands. The long-term implications of this practice are not well understood.
Within reason, topdressing particles slightly coarser than an existing rootzone will not adversely affect long-term soil physical characteristics. Conversely, topdressing sand that is dramatically finer than an existing rootzone may have serious negative consequences on soil physical properties. A decline in saturated hydraulic conductivity or percolation results from the loss of macropore space when a finer sand is used for topdressing. Additionally, a distinct layer forms at the surface resulting in a perched water table. This layer restricts drainage and air movement, resulting in a softer, wetter surface more prone to scalping.
Sand topdressing is still an essential cultural practice to maintain the highest quality putting green. Remember, one size does NOT fit all. It is important to critically evaluate your topdressing program. If you are applying topdressing more frequently, ask yourself if you are actually meeting the critical annual amount necessary to minimize thatch and excessive organic matter and ensure firm, smooth surfaces? Analysis of topdressing material prior to sand selection can prevent potential long-term layering issues and detrimental effects to soil physical characteristics caused when a sand finer than an existing rootzone is used for topdressing. Mechanical damage can easily occur if plants are stressed and topdressing should be delayed until the turf is actively growing and healthy. Appropriate topdressing rates and frequency with sands matching the existing rootzone are a few steps in the right direction towards maintaining high quality putting greens.
Current Research
Research is ongoing in the third year at Purdue University evaluating the effectiveness of various putting green sand topdressing programs. Our research objectives are to develop specific topdressing requirements for low (‘Penncross’), medium (‘L-93′), and high (‘A-4′) shoot density bentgrass cultivars maintained at two (2.3 and 4 lbs N/1000ft 2 /yr) different annual nitrogen levels. There are large differences between the shoot density of cultivars and their ability to maintain density during the summer months which will affect topdressing needs and strategies as well. Our research also evaluates topdressing frequency. We are monitoring the long-term changes in rootzone physical properties of a sand-based putting green rootzone topdressed with two sand sizes. Additionally, performance characteristics such as appearance, volumetric soil water content, surface hardness, dollar spot incidence, and moss encroachment are being documented.
Observations to date
• Tiller density results were as expected, ‘A-4′ had the highest number of tillers per unit area throughout the growing season and ‘Penncross’ had the lowest number of tillers.
• Surface hardness was lower in ‘A–4′ and ‘L–93′ compared to ‘Penncross’.
• Non-aerified plots yielded the firmest surface while topdressing frequently with fine sand had the softest surface.
• Volumetric water content measurements were highest in plots receiving fine topdressing sand, which correlates with surface hardness results.
• Turf quality was highest in non-aerified plots although they had significantly more dollar spot during disease outbreaks. Plots frequently topdressed with fine sand produced the lowest quality, likely because of higher percentages of moss.
• ‘L-93′ at the high annual N rate had the best turf quality among all cultivars while ‘Penncross’ at the low annual N rate had the lowest quality, primarily because of more moss and increased dollar spot. Among cultivars across all cultivation and N levels ‘A–4′ had the most dollar spot.
• Particle size distribution was evaluated and numerous plots receiving frequent fine topdressing no longer meet USGA rootzone specifications.
For more information, see the research summary at http://www.agry.purdue.edu/turf/report/2006/07.pdf
See and hear the latest results at the Midwest Regional Turf Field Day on July 17.