How long can you mow

A traditional method of grouping turfgrass species has been according to their recommended height of cut. However, as new cultivars of almost every species enter the marketplace, it now has become virtually impossible to make general recommendations concerning mowing height. Cultivars within species can now be separated into groups with regard to mowing height, which was unheard of not too many years ago.

Breeders have developed and released creeping bentgrasses specifically for their capacity to tolerate close mowing; Kentucky bluegrasses for their ability to persist under closely mowed golf-course-fairway conditions; and dwarf tall fescues tolerant of closer mowing than KY-31, the ancestor of contemporary cultivars. Cultivar development within other species has and is undergoing similar change.

What does this mean to turfgrass managers as they make adjustments to their cultural programs? Do we really have the same need to manipulate cutting height in response to environment conditions as we did in the past? The answers to these questions may not be as simple as we would like. It is, therefore, important to review the classical responses that turfgrass plants have to height of cut.

Mowing height affects turfgrass health Every turfgrass species (and often cultivars within species) has a range of cutting heights within which it remains most competitive. This competitiveness is due to the turf's ability to occupy available space, access water and nutrients, intercept light, resist disease and insects and maintain vigor. Lower heights of cut within the tolerance range minimize apical dominance, so the plant increases its basal-tillering rate. Increased basal tillering results in increased density, which improves the turfgrass plant's ability to occupy space.

However, with lowered height of cut, the amount of available leaf-blade surface for photosynthesis is less, and the root mass usually decreases. These negative responses can limit the plant's ability to tolerate mechanical and environmental stresses such as wear, heat and drought. Reduced root mass also limits the available pool of nutrients that the grass plant can exploit.

The reduction in photosynthetic area also creates problems for the plant with regard to carbohydrate metabolism. Because the available leaf area for photosynthesis is lower, the plant must rely on stored food reserves for some of its regrowth. As the plant depletes stored food reserves, growth, vigor and competitiveness decrease. Consequently, turf managers must increase maintenance intensity to provide an "assist" to the plant: more frequent irrigation, some foliar nutrition and more emphasis on disease management. When height of cut becomes so low that the maintenance intensity no longer can sustain the resulting aesthetic value and improved playability, the only wise course of action is to raise the height of cut.

Even a small increase in height of cut can provide significant benefits. Considering the total number of leaf blades in a given area, even a millimeter more of length for each blade can significantly increase total leaf area and the photosynthetic capacity of the sward. By increasing photosynthesis, the plant can produce more carbohydrates, allowing it to more effectively accommodate growth needs without utilizing stored food reserves.

More leaf-blade tissue also results in denser shading of the soil, thus reducing heating of the root zone. Reduced soil heating reduces root sloughing as well as the severity of root diseases. Increasing the amount of leaf-blade tissue in the canopy also improves resistance to the mechanical stresses of mowing and traffic.

You'll often notice the first evidence of problems related to low mowing height on putting greens during the cleanup pass on the perimeter. This area of stressed, closely mowed turfhas a much more difficult time maintaining high quality in contrast to the adjacent higher cut turf in the collar (which often consists of the same species or mix). I cannot think of a more "clear-cut" example of the benefits of a higher height of cut. Such an example can be useful to superintendents when they are asked to explain the benefits of a higher height of cut.

The new generation of turfgrasses As I stated earlier, breeders have placed considerable emphasis on the development and release of new cultivars that tolerate a lower range of mowing heights. Turfgrass breeders have been able to accomplish these advances with a variety of methods. Skillful selection of plants that have evolved over decades of exposure to close mowing has resulted in varieties with enhanced close-mowing tolerance. These ecotypes tend to have: *The capacity to maintain a deeper root system than less-adapted types *The capacity to produce basal tillers at reduced heights, allowing for sustained competitiveness *More horizontal leaf-blade growth (exceptions exist) *Less leaf area removed during each mowing (due to horizontal leaf-blade habit), which lessens the usual negative impact on photosynthesis.

Additionally, new technology (the identification and manipulation of genes for close-mowing tolerance) has provided the genetic-engineering tool for the development of more turfgrasses that tolerate close mowing.

Because the range of recommended heights of cut also has an upper limit, it is important to understand that phenomenon as well. Generally, when you mow turf at a height taller than its recommended range, its overall density decreases. Apical dominance becomes more of a factor as the stimulus for basal tillering lessens. Without vigorous basal tillering, the turf becomes less competitive, weeds more easily encroach and chemical control may become necessary. Additionally, reduced density may compromise the playability of the turf to the extent that it is unacceptable. Therefore, advice touting the benefits of higher-cut turf only applies up to a point.

Turf cut above its tolerance range also can result in puffiness, something you often see with creeping bentgrass or bermudagrass cut higher than 0.625 inch. This puffy condition creates mowing problems (increased scalping) and also reduces playability, particularly on golf-course fairways.

New cultivars have clouded the issue of mowing height in most species. However, I'm sure that as we move into the next century, research will allow us to better understand the cultural needs of this new group of turfgrasses. It would be useful for each cultivar to have its own ranking of mowing-height tolerances.

Perhaps the new cultivars can reduce the necessary maintenance intensity of those practices we traditionally think of as being critical. Currently, our knowledge base is limited with regard to the imposition of close mowing and the resulting morphological and physiological responses of the new generation of cultivars. Do they maintain their photosynthetic capacity? Do they continue to produce adequate basal tillers? Do they continue to produce adequate roots? Do they have good resistance to mechanical and environmental stresses? Do they tolerate disease and insect attacks well enough? The answers to these questions are slowly becoming known as a result of field evaluations at universities and in the private sector. For example, the National Turfgrass Evaluation Program (NTEP) continues to provide significant performance-evaluation data. However, we critically need more basic research to provide a better understanding of the responses these new cultivars have to cultural inputs and practices, including close mowing.

Dr. Thomas L. Watschke is professor of turfgrass science at the Pennsylvania State University (University Park, Pa.).

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