The adaptation of Kentucky bluegrass to high and low nitrogen
New NTEP trials make it easier to identify Kentucky bluegrasses tailored to specific fertility regimes.
Hundreds of Kentucky bluegrass cultivars have come and gone from the market during the past 50 years. Each had some useful or attractive characteristic that is passed from generation to generation, such as color, texture, disease resistance, shade tolerance, insect resistance or some other potentially useful quality. Kentucky bluegrasses also vary in their adaptation to environmental conditions as well as varying levels of maintenance intensity. As a result, cultivar adaptation is highly regional - a variety that does well in one area may not do well in another.
Categorizing Kentucky bluegrasses Various systems for dividing and describing Kentucky bluegrass cultivars exist. The most widely used divides them into two categories: common and improved.
Common types. The common types, or "public varieties" as they are known in the turfgrass seed industry, are usually older cultivars or selections from older cultivars that have been in use for many decades. Common cultivars generally display an upright growth habit and a relatively high susceptibility to leaf-spot disease when under intense management. Their positive attributes include early spring greenup and relatively good tolerance of environmental stress.
The older common varieties are well-adapted to conditions of lower maintenance intensity. Under such management regimes, the turf receives no supplemental irrigation and is allowed to go naturally into summer dormancy. It then recovers when temperatures cool and rainfall increases during the late summer.
"Low maintenance" generally means limited use of fertilizer as well. Most common types were selected prior to World War II (or selected from grasses used in that time) when turf managers and homeowners used little, if any, nitrogen (N) fertilizer on lawns. Lawn irrigation also was rare at that time. Kentucky bluegrass was popular because it was able to avoid drought through dormancy and recover quickly in the late summer and fall when conditions became more favorable.
The seed of common cultivars is often less expensive because it matures early and can generally be produced without irrigation.
Improved types. The improved types are newer releases selected or developed in the past few decades. The first of these releases was Merion, selected primarily for its tolerance to leaf spot. Since then, breeders have developed many other improved cultivars of Kentucky bluegrass. Today, a wide variety of improved types are available.
As a group, the improved cultivars display more prostrate habit, a slower growth rate and improved tolerance to several grass diseases. Breeders mostly selected these cultivars under high moisture and higher N fertility regimes. Many are quite slow to resume growth after winter dormancy. At my own home, I have common bluegrass in the back yard and improved types in the front. I generally mow the back yard two to three times in the spring before I mow the front yard for the first time.
The improved cultivars of Kentucky bluegrass are clearly the best choice when the turf is irrigated or where natural rainfall is sufficient to prevent summer dormancy. Under intense management, the common cultivars are disease-prone and usually do not perform as well as improved types.
However, under less intense management or when you know you'll allow the turf to undergo summer dormancy, the common cultivars are likely to give more satisfactory results. Thus, parks, school grounds, cemeteries, grassed areas along airport runways, low-maintenance home lawns and similar areas may be better suited to the common types. Table 1 (above) contains a list of cultivars the Iowa State University Extension Service recommends for low-maintenance situations. This list includes common cultivars as well as improved types known to perform well under low-maintenance conditions. Check with your local cooperative extension service for cultivars recommended for your region.
Evaluating cultivars Cultivars respond in different ways to different environmental conditions. This makes evaluation of new cultivars over a wide geographic range a necessity for providing useful variety information. This is the function of the National Turfgrass Evaluation Program (NTEP; Beltsville, Md.), an organization responsible for coordination of turfgrass cultivar evaluation across the United States. NTEP coordinates the collection of cultivars from seed companies and then distributes them to universities and other research entities. Each year they compile the data from these tests and distribute it through written reports and on their web site at http://www.ntep.org.
The increased emphasis in recent years on environmental issues has spurred additional interest in low-intensity management regimes and turfgrasses tolerant of such management. For example, reduced N fertilization for turf is frequently recommended for sites where concern exists about elevated nutrient levels in surface and ground ater.
Standard fertility programs for Kentucky bluegrass lawns range from 3 to 5 pounds of N per 1,000 square feet per year. While researchers have collected considerable data on low-maintenance conditions, including no irrigation and low fertility, more detailed data on cultivar responses to varying levels of N is more limited. Fortunately the NTEP web site does contain some useful information on the effect of reduced N programs on Kentucky bluegrass (Table 2, page 26, and 3, above). The information in these tables includes averages of data collected from research sites around the United States. At some evaluations sites, the cultivars were maintained at very low N levels (0 to 1 pound per 1,000 square feet). At others, intermediate levels (1 to 2 pounds of N) were used, and some sites used still higher rates. As the tables show, NTEP calculated mean quality ratings across all N levels.
It is difficult to draw many conclusions from just 2 years of information. However, some interesting things are apparent in the data. First, clear differences exist in cultivar response to each of the N levels. Cultivars such as Eagleton and Baronie are likely to provide better results under very low-N conditions than Baruzo or Lipoa. Notice, too, that cultivar rankings were fairly consistent under different N regimes. Those that performed well with low N also did well at higher rates.
Cultivar response was fairly consistent in 1998 and 1999. However, some interesting differences show up. Kenblue and South Dakota ranked fairly low in 1998 but ranked near the top in 1999. These are both common varieties that do well in drier conditions. While information is not available on rainfall at all testing sites, it is likely that 1999 was drier at many of the locations.
NTEP is in the process of establishing a new Kentucky bluegrass trial that will include about 150 cultivars at many sites around the United States. Data from this trial will begin to appear on the NTEP web site in 2002 and will be another useful source of information on this widely used species.
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