The Root of the Problem
The use of dense creeping bentgrass (Agrostis palustris) cultivars to achieve desired greens speeds often leads to excessive organic matter buildup in the surface of newly established putting greens, complicating your job as superintendent. These cultivars produce stolons with very short internodes, which can lead to a dense, tight surface and the greens speeds that are demanded by players. While this seems great to them, it can cause management problems for you. The excess plant material produced by these grasses is slow to break down by microorganisms, which leads to thatch accumulation. Often, superintendents supply increased amounts of nitrogen to these new greens in order to push them towards maturity at a faster pace. Applying extra nitrogen means that the grass will grow even faster, and the production of extra plant material contributes to the accumulation of even more organic matter.
Accumulation of excessive organic matter near the putting surface causes problems by negatively impacting water and oxygen infiltration into the rootzone, increasing water-holding capacity and causing secondary problems such as disease activity. Summer stress decline of creeping bentgrass in warm environments is typically directly related to organic matter accumulation. Decreased water infiltration means that the water you apply under these conditions may be getting hung up in the thatch material and not infiltrating to the rootzone to be used by plant roots. The water will remain in the thatch area causing soggy conditions, prime for disease activity.
It seems that the most severe problem associated with summer stress decline is the one caused by the lack of oxygen diffusion into the rootzone. When soil temperatures are high, root respiration increases. This increase in the amount of oxygen used by the plant lowers the amount of oxygen in the rootzone. Soil microbial respiration also increases with high temperatures, meaning that even more oxygen is being depleted. Therefore, at high temperatures it is important for oxygen to be able to diffuse into the rootzone. If oxygen does not reach the rootzone in sufficient amounts, the roots begin to die; and if root dieback is excessive, microorganisms cannot break down the fresh organic matter rapidly enough to prevent accumulation. Dead roots become gel-like and can cause macropore sealing, which, in turn, makes it more difficult for the remaining roots to obtain enough water to allow for transpirational cooling. This macropore sealing also leads to even less oxygen diffusion, more root death, and a “death spiral.” An extended period of hot and humid weather could cause initial yellowing and possible death of the turf as a result of this cycle that began with the lack of oxygen in the rootzone.
Cultivation practices to remove organic matter from the surface of putting greens is an essential part of managing summer stress. However, superintendents sometimes have a hard time convincing their members that the benefits of cultivation outweigh the short-term drawbacks. Using practices to remove high amounts of organic matter while causing as little damage to the turf as possible could satisfy both parties.
Using a Hydroject or solid-tine aerification to punch holes in the surface and provide temporary channels for an oxygen diffusion into the rootzone can be a quick and effective short-term fix, especially for summertime emergencies. However, these practices do not remove any of the organic matter contributing to the problem. The only way to be rid of this nuisance is to mechanically remove it using core aerification or aggressive verticutting. Short-term problems associated with organic matter removal include the labor and the obvious disruption of play. However, in the long run these are only minor problems when compared to the possible problems associated with excessive organic matter accumulation.
In April 2003 the University of Arkansas initiated a project to test the effectiveness of several cultivation methods on:
The removal of organic matter,
Limiting organic matter accumulation following cultivation through sand incorporation into cultivation channels,
Minimizing recovery time, and
Minimizing negative impacts on ball roll distance and trueness.
Treatments were applied in the spring and fall to a “Penn-G2” creeping bentgrass putting green constructed to United States Golf Association (USGA) specifications. There were three verticutting treatments of varying blade width, and six core aerification treatments with various combinations of tine diameter, tine length and tine spacing, as well as an untreated control (see Table 1 above).
|Treatment ID||Cultivation unit||Spacing (in)||Tine diameter (in)||Tine length (in)||% sand incorporated|
|2||Toro greens aerator||2.5 × 2.1||.5||4.5||229|
|3||Toro greens aerator||2.5 × 2.1||.5||3.5||270|
|4||Toro greens aerator||1.6 × 1.2||.25||4.5||199|
|5||Toro greens aerator||1.6 × 1.2||.25||3.5||151|
|6||Toro greens aerator||1.6 × 1.2||.5||4.5||120|
|7||Toro greens aerator||1.6 × 1.2||.5||3.5||154|
Researchers evaluated organic matter removal by collecting the debris removed for each treatment. After the material was collected, the amount of organic matter in the material was determined using standard USGA procedures. Change in organic matter content over time was determined by sampling 7 days before and 1, 30, 60 and 90 days following cultivation with a cup cutter plug and using the same USGA procedures for determining organic matter content. Recovery and aesthetic quality ratings were taken 1, 4, 7, 14, 21 and 28 days after cultivation and continued once every two weeks throughout the growing season on a scale of 1 to 9, 1 being dead turf and 9 being ideal dark green, dense, uniform turf. Digital images were taken and used to help analyze recovery as well. One issue with aggressive verticutting is the difficulty of brushing sand back into the grooves, which could lead to high organic matter re-accumulation rates. Therefore, researchers evaluated sand incorporation by comparing the volume of material removed to the volume of sand that could be reasonably worked back into each plot area.
Significantly higher amounts of material were removed by the verticutting treatments with an average of about 6,200 grams of material for these treatments (see Figures 1 and 2 on page G6). The most organic removal of the core aerification treatments was about 1,500 grams. Of the material removed, an average of 72 grams was organic matter for the verticutting treatments. For the core aerification treatments, the highest amount of organic matter removed was about 25 grams by Treatment 6. Although the verticutting treatments removed significantly more organic matter, the percentage of the total material removed that was organic matter was smaller. Only 72 grams out of 6,200 is about 1.2 percent organic matter, whereas about 21 grams out of 800 was organic matter for Treatment 7, a percentage of 2.6 percent. This means that although the verticutting treatments are removing more material, they are disrupting more of the rootzone to do it.
Recovery and quality ratings showed that core aerification caused much less damage to the turf than the verticutting treatments (see Figures 3 and 4 on page G8). After only one week, the closely spaced, small-diameter aerification treatments showed no difference in quality from the control. However, the verticutting treatments took nearly five weeks to reach the same quality as the control. The closely spaced tine treatments using a commercially available adapter were rated significantly higher for recovery five days after cultivation than were the standard treatments. Five days after cultivation, Treatment 4 had an average rating of 7.5 and Treatment 6 had an average rating of 6.0, whereas the Treatment 2 had an average rating of 4.75 (see Figure 3 on page G8). Verticutting treatments were evaluated lower for recovery with regard to digital image analysis as well (see Figure 5 below).
Sand was incorporated back into the plot area at a rate of about 2.5 times the amount removed for the standard aerification treatments. About 1.5 times the amount removed was incorporated back into the plot area for the closely spaced treatments. Only 0.66 times the amount removed was reasonably incorporated into the plot area for the verticutting treatments (see Table 1 on page G5).
The aggressive verticutter may be applicable when there is a need to remove a high amount of organic matter in a short amount of time, but under normal circumstances, these treatments caused a great deal of surface disruption. It would be possible to make a couple of passes with one of the closely spaced aerification treatments and remove as much organic matter, and theoretically cause less damage.
Summer stress decline of creeping bentgrass is directly related to organic matter buildup and a decline of oxygen diffusion. It is obvious that this is a problem that demands your attention — even if it means irritating the members for a week or two by cultivating in the spring and fall. They would probably agree that a few holes in the turf are a better option than dead greens in August. To remove as much organic matter as possible while minimizing damage, use a block setup with closely spaced tines. Although not as much organic matter is removed in comparison to aggressive verticutting, surface disruption is much less severe. A couple of passes with this setup will probably remove as much organic matter and still allow for faster turf recovery.
With the increasing use of dense creeping bentgrass cultivars, organic matter removal is becoming more and more important despite how disruptive this process can be. Finding practices that remove high amounts of organic matter but do not cause a great deal of surface disruption will help superintendents maintain healthy greens while keeping most members satisfied as well.
Josh Landreth is a turfgrass science research specialist and Douglas Karcher, Ph. D., is assistant professor of turfgrass science, both at the University of Arkansas (Fayetteville, Ark.). Karcher served as Landreth's advisor for this turfgrass project.
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