Which Kentucky bluegrass cultivars perform better with less water?

Twenty-eight Kentucky bluegrass cultivars and two hybrid bluegrasses were tested for their ability to retain visual quality under reduced irrigation.

Dale Bremer, Ph.D.; Steve Keeley, Ph.D.; Jack Fry, Ph.D.; and Jason Lewis, Ph.D.

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Well-watered plots are shown at the beginning of the
dry-down study on June 4, 2007 .
Photos by Jason Lewis

Field research at Kansas State University indicates that water requirements may differ significantly among cultivars of Kentucky bluegrass (Poa pratensis L.), depending upon desired turfgrass quality. Given the certainty of periodic drought, limited water availability and increasing irrigation costs, having choices of Kentucky bluegrass cultivars that may maintain better quality with less water is an attractive option. Ideally it would be helpful to select a turfgrass that can perform well with less water.

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Kentucky bluegrass plots with obvious drought stress are shown at two months into the study, Aug. 4, 2007. Plots were sheltered from precipitation by the rainout shelter (upper left in each photo), which automatically moved on tracks to cover the plots during rainfall.

A helpful concept when discussing Kentucky bluegrasses is their classification into phenotypic groups. Individual cultivars of Kentucky bluegrass are classified into phenotypic groups based on common growth and stress performance characteristics gathered from field trials (1). Previous research has indicated that such groupings may be useful in predicting drought tolerance. Because cultivar turnover is rapid in the turfgrass industry, determining the relative irrigation requirements of phenotypic groups may enable researchers to predict irrigation requirements of cultivars not included in any particular study.

Using a rainout shelter, we compared seasonal irrigation amounts among 28 Kentucky bluegrass cultivars for two growing seasons. By shielding plots from rainfall, water could be withheld until wilt symptoms were evident. Our objectives were to identify Kentucky bluegrass cultivars and phenotypic groups that maintain better visual quality with less irrigation, using wiltbased irrigation. We hypothesized that if visual quality were good at the beginning of the season, we would maintain minimally acceptable quality in Kentucky bluegrass (for example, for a moderately maintained lawn or golf course rough with in-ground sprinklers) by irrigating when at least 50% of a given cultivar showed signs of wilt. Two hybrid bluegrasses (Poa arachnifera Torr. × P. pratensis) were also included in the study.

Methods

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This study was conducted at the Rocky Ford Turfgrass Research Center near Manhattan, Kan. Data were collected for 105 days in 2007 (June 19- Oct. 1) and 108 days in 2009 (June 22-Oct. 7). Turfgrasses included 28 Kentucky bluegrass cultivars and two hybrid bluegrasses (Table 1). Commercially available cultivars of Kentucky bluegrass were selected to include representatives from major Kentucky bluegrass phenotypic groups (in the results section, only groups with three or more cultivars were used when comparing groups). Also, because visual quality was of interest, cultivars were selected based on performance in National Turfgrass Evaluation Program (NTEP) trials.

The plots were maintained and well watered until the study began each year. Thereafter, water was withheld until 50% or more of a plot displayed drought stress. One inch (2.54 centimeters) of water was then applied by hand to the individual plots. Turfgrass quality and drought stress symptoms were evaluated daily. This process continued until the end of the study, after which all plots were re-watered and allowed to recover. Plots were mowed weekly at 3 inches (7.62 centimeters).

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Turfgrass quality evaluations, based on color, density and uniformity of the canopies, were made using a visual rating scale of 1 to 9, where 1 = brown turf, 6 = minimally acceptable turf or a home lawn or golf course rough and 9 = optimal turf. Drought stress was defined as the turf displaying wilting, failure of the canopy to remain upright after foot traffic and a general darkening color of the turf. Because drought stress sometimes changed rapidly from day to day, particularly under high temperatures, it was not unusual for irrigation to be applied when more than 50% of a plot (for example, up to 70% or 80%) displayed drought stress.

Results

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Total water applied and days to wilt between irrigation cycles

Water applications, averaged over the approximately 3.5-month period in each year of the study, ranged widely from 9.17 inches (23.3 centimeters) [mean = 0.086 inch (2.2 millimeters)/day)] in Bedazzled to 17.67 inches (44.9 centimeters) [0.165 inch (4.2 millimeters)/day)] in Kenblue (Figure 1). In Bedazzled, Apollo, Cabernet and Unique, 9.84 inches (25.0 centimeters) [0.090 inch (2.3 millimeters)/day] or less of water was applied, which was significantly less than that applied to Kenblue, Blue Knight, Wellington, Moonlight, Baron, Diva, Midnight II, Touchdown, Shamrock and Blue Velvet. In the latter 10 cultivars, 13.81 inches (35.1 centimeters) [0.129 inch (3.3 millimeters)/day) or more of water was applied. However, there were no statistical differences among the 15 cultivars that received the least amount of water (Figure 1, Bedazzled through Skye).

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Days to wilt between irrigations, which was roughly inverse to the amount of water applied (r = −0.91), ranged from 6.4 days in Kenblue to 13.1 days in Cabernet, a difference of nearly one week (Figure 2). Days to wilt was greater in Cabernet, Bedazzled, Unique and Apollo (11.9 to 13.1 days) than in the 18 bluegrasses with the least days to wilt (6.4 to 9.0 days; Kenblue through Park in Figure 2). These intervals provide the practitioner with an estimate of irrigation frequency required to maintain the various Kentucky bluegrasses at a performance level similar to this study, at least in the transition zone of the United States. In addition to requiring less frequent irrigation, cultivars with more days to wilt have a greater likelihood of receiving rainfall between irrigations; this could result in further water conservation and reduced irrigation costs.

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Notably, all cultivars in the phenotypic group Mid-Atlantic (Cabernet, Eagleton and Preakness) and four of five in the Compact America group (Apollo, Bedazzled, Kingfisher and Unique) were among the 15 cultivars that received the least amount of water (Table 1, Figure 1). When averaged over all cultivars within each phenotypic group, 10.75 inches (27.3 centimeters) of water was applied to Compact America types and 11 inches (27.7 centimeters) to Mid-Atlantic types [both about 0.102 inch (2.6 millimeters)/ day), which was less than the Common, Compact and Compact Midnight groups (Figure 3). The Common types received more water (15.79 inches [40.1 centimeters]; 0.149 inch [3.8 millimeters]/day]) than all other groups except Compact. Days to wilt was also greater in Mid-Atlantic and Compact America than in all other groups (Figure 4), indicating cultivars in Mid-Atlantic and Compact America could generally go longer without irrigation.

Visual quality

With the exception of the Common types in 2007, the visual quality of all bluegrasses was acceptable (>6) at the beginning of the study in each year (see the top photo, Page 76). In all bluegrasses and in both years, however, visual quality declined to below what was considered minimally acceptable (bottom photo, Page 76). This indicates waiting until 50% wilt to apply irrigation was insufficient to maintain acceptable visual quality in Kentucky bluegrass, at least for homeowners or superintendents who desire a moderate standard of quality in the stressful climate of the transition zone. Perhaps visual quality could have been maintained at acceptable levels by applying water when only 25% of the plot exhibited symptoms of drought stress; further research is required. Our method may be appropriate, however, for the typical homeowner with no in-ground sprinklers or superintendents with low-maintenance roughs on their golf courses, or where the primary concern is water conservation and some dormancy is acceptable. Visual quality in all bluegrasses generally remained >4, and recovery was rapid in the fall after resuming irrigation (data not shown).

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Although visual quality declined to <6 in all cultivars, the time required to do so ranged widely from 8.1 days in Kenblue to 44.8 days in Blue Velvet (data not shown for all cultivars; for greater detail, see [2,3]). The decline was slower in Blue Velvet, Award, Midnight, Cabernet, Unique and NuDestiny (36 to 44.8 days) than in Park, Baron, Wellington and Kenblue (8.1 to 14.2 days). Thus, four of five cultivars in the Compact Midnight group maintained quality longer than all cultivars in the Common group (Table 1). As a group, the Compact Midnight types maintained quality >6 for longer than the Common as well as the BVMG types, but also received more water than the Compact America and Mid-Atlantic groups (Figure 3).

Relationships between water applied and visual quality

Ideally, cultivars or groups that require the least water would also have the highest visual quality. Those relationships are illustrated in the scatter biplot in Figure 5, in which cultivars with the most favorable characteristics appear in the lower right section. In general, irrigation applications were greater in bluegrasses with poorer quality (Figure 5, upper left section). This pattern probably resulted from improved cultivars with morphological properties that both enhanced turf quality and reduced evapotranspiration (water use). Such improved properties include compact or dwarfed growth habits, horizontal leaf orientation and greater shoot density.

All 15 bluegrasses with the lowest water applications were also ranked among those with the highest visual quality (Figure 5; there were no statistical differences among cultivars with average visual quality >5.5). The amount of water applied to these 15 cultivars with superior turf quality was also below the mean water applied to all 30 bluegrasses (13 inches [32.8 centimeters]). Similarly, visual quality in 12 of the 15 bluegrasses that received the least water was greater than the mean of all 30 bluegrasses (5.78), although all 15 were statistically similar.

In contrast to the 15 top performers, six cultivars were ranked within the group that received the most water and had the lowest visual quality (Figure 5). Those six cultivars — Kenblue, Wellington, Midnight II, Baron, Diva and Shamrock — had neither the high visual quality nor low water requirement traits we were screening for in this study.

Conclusions

Cultivar selection in Kentucky bluegrass had significant impacts on water requirements and visual quality ratings. Among cultivars, differences in seasonal water applications were as great as 8.5 inches (21.6 centimeters), and differences in days to 50% wilt between irrigations were as great as 6.7 days (that is, nearly one week). Based on statistical range tests, only 15 of the 30 cultivars were in the group that both received the least water and had the greatest visual quality. Results indicated that, under conditions similar to those in our study, Kentucky bluegrass in the Compact America and Mid-Atlantic phenotypic groups can be selected for their lower irrigation requirements without sacrificing visual quality, and types from those two groups may represent the best selections for breeding efforts to achieve such goals. More detailed results from this study can be found elsewhere (2,3).

Funding

This research was funded by United States Golf Association (USGA), Turfgrass Producers International (TPI) and the Kansas Turfgrass Foundation.

Acknowledgments

The technical assistance of Tony Goldsby was greatly appreciated.This article was published in the Spring 2013 (Vol. 26, No. 1) issue of Sports Turf Manager.

Literature cited

  1. Bonos, S.A., W.A. Meyer and J.A. Murphy. 2000. Classification of Kentucky bluegrass genotypes grown as spaced-plants. HortScience 35:910-913.
  2. Bremer, D., J. Lewis, S. Keeley and J. Fry. 2012. Effects of wilt-based irrigation on visual quality and seasonal water applications on 30 bluegrasses in the transition zone. USGA Turfgrass and Environmental Research Online (TERO) 11(6):1-12.
  3. Lewis, J.D., D.J. Bremer, S.J. Keeley and J.D. Fry. 2012. Wilt-based irrigation in Kentucky bluegrass: Effects on visual quality and irrigation amounts among cultivars. Crop Science 52:1881-1890.

DaleBremer, Steve Keeley and Jack Fry are professors in the department of horticulture, forestry and recreation resources, Kansas State University, Manhattan, Kan.; and Jason D. Lewis was an assistant professor in the department of horticulture and crop science, California Polytechnic State University, San Luis Obispo, Calif.