Performance in the Transition Zone of Two Hybrid Bluegrasses Compared with Kentucky Bluegrass and Tall Fescue

Dale J. Bremer, Kemin Su, Steven J. Keeley, and Jack D. Fry, 2021 Throckmorton Hall, Department of Horticulture, Forestry & Recreation Resources, Kansas State University, Manhattan, KS 66506

Corresponding author: Dale J. Bremer. bremer@ksu.edu

Abstract

Hybrid bluegrasses (HBG) are crosses between Kentucky bluegrass (Poa pratensis L.) (KBG) and Texas bluegrass (Poa arachnifera Torr.), which may exhibit better heat tolerance and drought resistance than other cool-season turfgrasses. Two HBG cultivars (‘Thermal Blue’ [HBG1] and ‘Dura Blue’ [HBG2]), one KBG (‘Apollo’), and one tall fescue (Festuca arundinacea Schreb.; ‘Dynasty’ [TF]) were evaluated for two years in northeastern Kansas for establishment rates after seeding, visual quality and growth characteristics, and drought resistance. Irrigation treatments included 60% and 100% evapotranspiration (ET) replacement and a control receiving only natural precipitation. Tall fescue reached full cover 37, 52, and >73 days faster than HBG1, KBG, and HBG2, respectively. In both years, average quality over the growing season ranked: TF > KBG > HBG1 > HBG2; an infestation of bluegrass billbugs (Sphenophorus parvulus Gyllenhal) in 2003 reduced quality among bluegrasses but not TF. Canopy density was lower in HBG2 and higher in TF among treatments. Clipping biomass of TF was 42 to 73% higher than that of the bluegrasses. Vertical growth rates were highest in HBG1 and TF and lowest in KBG. Drought generally reduced quality among bluegrasses, but effects on TF were negligible. Results indicate that TF is better adapted than HBG where soils are deep in the transition zone. Further research is needed using new cultivars of HBG and in areas with different soils.

Introduction

Kentucky bluegrass is a cool-season turfgrass species that is commonly used in lawns and golf courses in the United States. (2,5). Tall fescue, another cool-season species, is also popular for use in lawns and is sometimes used in golf course roughs. In some areas of the United States, these grasses are subjected to frequent drought, which results in heat and drought stress symptoms, and irrigation is required to maintain acceptable quality. Kentucky bluegrass commonly goes dormant during periods of high temperature and drought (5). Tall fescue has good drought avoidance because of its relatively deep rooting system, but some turfgrass managers prefer the finer texture and recuperative capacity that KBG offers.

New hybrid bluegrasses (HBG) cultivars, which are genetic crosses between Kentucky bluegrass (KBG) and native Texas bluegrass, have the appearance of KBG but may be able to withstand higher temperatures and extended drought without going dormant (1,15). In warm climates such as the southern United States, HBG may stay green all year long (11). Furthermore, HBG may use less water than other cool-season species while maintaining their green color (16). This is especially important given the increasing competition for water and the rising costs of irrigation (3,13).

Despite the potential for using HBG in lawns and golf courses, there are little scientific data available about their performance relative to KBG and TF under the stresses of different climates or cultural practices. One HBG (Thermal Blue) maintained a higher quality than TF (Dynasty) and KBG (Apollo) under high temperatures in a growth chamber (15). In a field experiment in the upper Midwest, the mean quality of two HBG (Thermal Blue and Dura Blue) was similar to two KBG (Apollo and ‘Unique’) and two TF cultivars (‘Masterpiece’ and ‘Rembrandt’) when mowed at 2.5, 5, and 7.5 cm heights and fertilized with N at either 48 or 144 kg/ha/year (14).

A greenhouse study revealed significant variability in drought resistance among thirty cultivars of HBG and their genetic parents (two KBG [‘C-74’ and ‘Midnight’] and one Texas bluegrass accession [‘186’]). In that study, drought resistance in HBG was not always greater than in KBG (1). In a field experiment in Colorado, a HBG (‘Reveille’) maintained turfgrass quality longer than KBG (‘Bensun’ [A-34]) did during a prolonged soil dry-down (16). Multiple-year stand longevity in the latter study was poorer in HBG than in KBG, however, probably because of limited freezing stress tolerance in HBG. Koeritz et al. (7) determined that freezing stress tolerances were poorer in two HBG (Thermal Blue and Dura Blue) than in TF. In consequence, the use of HBG may be limited in cooler climates.

In this two-year study, two HBG were compared with KBG and TF in the stressful climate of the transition zone in the USA. The transition zone is a difficult region in which to maintain turf quality because of hot summers and cold winters (5). The objectives of this field study were to evaluate KBG (Apollo), TF (Dynasty), and two cultivars of HBG (Thermal Blue and Dura Blue) for: (i) canopy establishment rates after fall seeding; (ii) visual quality and growth characteristics of canopies; and (iii) drought resistances under different irrigation regimes and deficits.

Study Site and Experimental Design

The study was conducted from September 2002 to October 2004 at the Rocky Ford Turfgrass Research Center near Manhattan, Kansas (39.12°N, 96.35°W). The soil at the site was Chase silt loam (fine, montmorillonitic, mesic, Aquic, Argiudolls).

Nine whole plots (4 × 2 m each), separated by 1-m alleys, and four subplots (2 × 1 m each) per whole plot, for a total of 36 subplots, were established for this study in a split-plot design. Two irrigation treatments and a control, replicated three times each, were applied to whole plots arranged in a Latin square design. Four species or cultivars of turfgrasses were established in the subplots. One species/cultivar of each turfgrass was planted once within each whole plot; species/cultivar was randomly assigned to subplots within each whole plot. Therefore, each irrigation-by-species/cultivar treatment combination was replicated three times in the entire study.

Subplots were seeded on 24 September 2002 (day of year [DOY] 267) with four species or cultivars of turfgrasses that included tall fescue (TF [Dynasty]), Kentucky bluegrass (KBG [Apollo]), and two hybrid bluegrasses (HBG1 [Thermal Blue] and HBG2 [Dura Blue], both from The Scotts Co.). Seeding rates were 244 and 98 kg/ha for TF and KBG, respectively, and 146 kg/ha for HBG1 and HBG2.

Irrigation treatments included the replacement of 100 and 60% of the water lost from plants and soil via evapotranspiration (ET), and control plots received only natural precipitation. Water was applied twice weekly through a fan spray nozzle attached to a hose; a meter (Model 03N31, GPI, Inc., Wichita, KS) was attached to ensure proper application rate. To determine irrigation requirements, evapotranspiration (ET) was calculated by using the Penman-Monteith equation (4), and climatological data were obtained at a weather station located at Rocky Ford Turfgrass Research Center.

In 2003, irrigation treatments were not applied because of a moderate billbug infestation that affected a number of plots of KBG, HBG1, and HBG2 despite insecticide applications. Plots in 2003 were irrigated every three to four days, providing 40 mm of water per week in the absence of rain, to minimize stress related to billbug damage. Therefore, irrigation treatments were applied for only one year (2004) of the study.

Canopy Evaluations and Measurements and Statistical Tests

Turfgrass canopy establishment was evaluated visually after seeding, from 17 December 2002 (DOY 351) to 20 July 2003 (DOY 201). Establishment was estimated as percentage of the ground surface covered by turfgrass canopies within each plot (0 to 100%).

All plots were evaluated biweekly for visual turfgrass quality from 13 June (DOY 164) to 21 October (DOY 294) in 2003 and from 29 April (DOY 120) to 9 August (DOY 222) in 2004. Turfgrass quality was rated on a scale from 1 (dead, brown turf) to 9 (optimum uniformity, density, and color) and 6 was considered acceptable quality for a home lawn; all evaluations in each year were conducted by the same person. Turfgrass density and color were also evaluated visually in each plot independent of, albeit less frequently than, quality during the same periods in 2003 and 2004.

Clippings were collected every 3 weeks, or 4 times from 5 August (DOY 217) to 10 October (DOY 283) in 2003, with a walk-behind rotary mower equipped with a modified collection bag that allowed for complete capture of clippings from each plot. Clipped biomass was determined gravimetrically after samples had been dried in a forced-air oven for 48 h at 65°C.

Vertical growth rates were measured biweekly to monthly from 14 July (DOY 195) to 21 October (DOY 294) in 2003 and biweekly from 14 June (DOY 166) to 12 August (DOY 225) in 2004. Daily vertical growth rates were calculated as the increase in canopy heights between consecutive mowing events, divided by the number of days between mowing. Canopy heights were measured immediately after mowing at two randomly selected locations within each plot; each location was marked with a flag that remained until the next mowing date. Before the next mowing, canopy heights were measured a second time at the same (marked) locations. Canopy heights were measured by placing a circular piece of lightweight cardboard over the canopy and centered on the flag; the cardboard was rigid enough to hold its shape but lightweight enough to minimize the bending of the canopy by its weight. The height of the canopy was then measured at 4 perpendicular spots around the circumference of the cardboard. The flags were then removed before mowing and then reinserted at new, random locations for the next measurements.

Tests of differences in canopy establishment, visual quality, canopy color and density, clipping biomass, vertical growth rates, and irrigation effects among treatments were conducted with the mixed linear model procedure of SAS (P < 0.05; SAS Institute Inc., Cary, NC).

Plot Maintenance

All plots were mowed with a walk-behind rotary mower at a height of 2.5 inches once or twice weekly as needed to prevent removing more than 1/3 of the canopy height. All plots were fertilized with urea N at approximately 225 kg/ha/year in 2003 and 2004, in split applications in September, November, May, and July. Because bluegrass billbug had infested some bluegrass plots at the research center in previous years, plots were treated in 2003 with 0.36 kg a.i./ha of imidacloprid (1-(6-chloro-3-pyridylmethyl)-N-nitroimidazolidin-2-ylideneamine, 1-[(6-chloro-3-pyridinyl)methyl]-N-nitro-2-imidazolidinimine) on 7 June (DOY 158) and 0.09 kg a.i./ha of bifenthrin (2-methyl-1,1-biphenyl-3-y1)-methyl-3-(2-chloro-3,3,3-trifluoro-1-propenyl)-2,2-dimethyl cyclopropanecarboxylate) on 1 August (DOY 213), and in 2004 with 0.44 kg a.i./ha of imidacloprid on 19 April (DOY 110), 0.125 kg a.i./ha of bifenthrin on 27 May (DOY 148), and 1.69 kg a.i./ha of halofenozide (Benzoic acid, 4-chloro-, 2-benzoyl-2-(1,1-dimethylethyl)hydrazide) on 9 July (DOY 191). Preemergence herbicide applications included 0.56 kg a.i./ha of dithiopyr (/S,S/′-dimethyl 2-difluoromethyl-4-isobutyl-6-trifluoromethylpyridine-3,5-dicarbothioate) on 17 May 2003 (DOY 137) and 27 May 2004 (DOY 148). Broadleaf pests were treated as needed, and no fungicides were applied during the study.

Turfgrass Establishment, Quality, and Growth

Establishment was most rapid in TF, which was at 90% cover by 17 December 2002 (DOY 351) and reached 100% by 7 May 2003 (DOY 127) (Fig. 1). Both KBG and HBG1 established more slowly than TF and were similar to each other; HBG1 reached 99% cover by 13 June (DOY 164) and KBG by 28 June (DOY 179). The HBG2 was slowest to establish, reaching 96% cover by 19 July (DOY 200). Percentage cover for HBG2 was lower than for TF, KBG, and HBG1 during the entire period of establishment evaluations. Establishment in TF was 99% complete 37, 52, and more than 73 days faster than in HBG1, KBG, and HBG2, respectively.

Fig. 1. Visual estimates of percentage ground cover during plot establishment; plots were seeded on 24 September 2002 (day of year 267). Species/cultivars include tall fescue (TF), Kentucky bluegrass (KBG), and two hybrid bluegrasses (HBG1 and HBG2). Symbols plus (+) below the data indicate significant differences (P < 0.05) between one and the other three species/cultivars, diamond-plus (diamond +) indicate significant differences among all species/cultivars except two, and the letter (x) indicates significant differences among all species/cultivars on a given day.

Visual quality was generally lower in the bluegrasses (i.e., KBG, HBG1, and HBG2) than in TF during 2003 and 2004 (Figs. 2 and 3). Mean quality, when averaged over the season, ranked TF>KBG>HBG1>HBG2 both years; all differences in mean quality among species or cultivars during the season were significant in both years, except between KBG and HBG1 in 2003 (data not shown). The ranking of visual quality among species was, in part, a function of differences in canopy density and color. For example, canopy density was highest in TF and generally similar between KBG and HBG1 in both years, but a lighter color in HBG1 resulted in its lower quality rating in a number of instances (data not shown). In 2003, quality in HBG2 was consistently lower among plots early in the growing season because of its slower establishment (Fig. 1). Slower establishment in HBG2 resulted in a lower canopy density than in TF, KBG, and HBG1 up to 11 July 2003 (DOY 192). Canopy density also was consistently lowest in HBG2 during 2004. Higher seeding rates may be required for HBG2 to obtain an adequate stand.

Fig. 2. Visual quality ratings of tall fescue (TF), Kentucky bluegrass (KBG), and two hybrid bluegrasses (HBG1 and HBG2) during 2003. Ratings were on a scale from 1 (dead, brown turf) to 9 (optimum uniformity, density, and color). Dashed, horizontal line at 6 indicates acceptable quality for a home lawn. Means followed with the same letter within each respective day of year (vertical) are not significantly different (P < 0.05).

Fig. 3. Visual quality ratings of tall fescue (TF), Kentucky bluegrass (KBG), and two hybrid bluegrasses (HBG1 and HBG2) during 2004. Ratings were on a scale from 1 (dead, brown turf) to 9 (optimum uniformity, density, and color). Dashed, horizontal line at 6 indicates acceptable quality for a home lawn. Means followed with the same letter within each respective day of year (vertical) are not significantly different (P < 0.05).

In 2003, billbug damage in the bluegrasses seemed greatest from about 9 to 29 July (DOY 190 to 210), but the quality of the bluegrasses generally improved (Fig. 2) by keeping plots well watered. Reinert et al. (12) reported that HBG were susceptible to billbug, although a wide range of variability existed among HBG cultivars, and that even individual plants within a single cultivar may be tolerant or susceptible. Spatial variability in billbug damage in our bluegrass plots was high, and some plots were not damaged. But KBG, HBG1, and HBG2 all seemed to be equally susceptible. The decline of KBG late in 2003 was caused by a rust disease that also affected, to a lesser extent, HBG1 and HBG2.

Aboveground biomass production, as measured from clippings, was greater in TF than in any of the bluegrasses in 2003 (Fig. 4), reflecting the higher quality in TF (Fig. 2). Clipping biomass was statistically similar among the three bluegrasses on all measurement dates except for 5 August (DOY 217), when clippings were lower in HBG2 than in TF and HBG1. Averaged over the 2003 season, clipping biomass was higher in TF than in all bluegrasses and also was higher in HBG1 than in HBG2. Clipping weights were 42% higher in TF than in HBG1, and were 69 and 73% higher in TF than in KBG and HBG2, respectively.

Fig. 4. Biomass from clippings (dry weight) of tall fescue (TF), Kentucky bluegrass (KBG), and two hybrid bluegrasses (HBG1 and HBG2) during 2003. Means followed with the same letter within each respective day of year (vertical) are not significantly different (P < 0.05).

Vertical growth rates of HBG1 were similar to TF in 2003 and 2004, and were higher than KBG on three of four measurement dates in 2003 and on the last two dates in 2004 (Figs. 5 and 6). Averaged across the season, vertical growth rates of HBG1 were higher than those of KBG in 2003 and higher than those of KBG and HBG2 in 2004 (data not shown). It is clear that HBG1 shows little promise of reduced mowing frequency, compared with conventional cool-season species. The vertical growth rate of HBG2, by contrast, was more similar to KBG in both years. Averaged across the season, the vertical growth rate of HBG2 was lower than that of TF in 2003 and 2004 (albeit not significantly in 2004). Thus, there is potential for reduced mowing frequencies with HBG2 (and perhaps future cultivars of HBG), compared with TF.

Fig. 5. Vertical growth rates of the canopies in tall fescue (TF), Kentucky bluegrass (KBG), and two hybrid bluegrasses (HBG1 and HBG2) on four dates during 2003. Means followed with the same letter within each respective day of year (vertical) are not significantly different (P < 0.05).

Fig. 6. Vertical growth rates of the canopies in tall fescue (TF), Kentucky bluegrass (KBG), and two hybrid bluegrasses (HBG1 and HBG2) on five dates during 2004. Means followed with the same letter within each respective day of year (vertical) are not significantly different (P < 0.05).

Effects of Water Deficit

In 2004, frequent, above-average rainfall from May through August minimized the impact of water-deficit treatments; precipitation between June and August 2004 was 468 mm, which was 148 mm above normal. Drier (albeit cooler) weather during a 52-day period in September and October of 2004 allowed for a stronger comparison among species and cultivars under drought conditions; precipitation was 26 mm during September, which was 67 mm below normal, and only 7 mm occurred in early October before the study ended (14 October; DOY 288).

Visual quality ratings were similarly high among species and cultivars in well-watered plots at the end of the dry period in September and October 2004 (Fig. 7). Visual quality in the 60% ET treatment was significantly lower in HBG2 than in KBG, and quality in HBG1 and HBG2 was below the rating of 6 determined as acceptable for home lawns. In the control, which received only natural precipitation during this period (i.e., no irrigation), the quality of all bluegrasses was below 6 and ratings of KBG and HBG1 were significantly lower than those of TF. Visual quality in TF declined only from 8 to about 6.5 to 7 in the reduced-irrigation and control plots, indicating that TF was not substantially affected by drought in this study.

Fig. 7. Comparisons of visual quality of tall fescue (TF), Kentucky bluegrass (KBG), and two hybrid bluegrasses (HBG1 and HBG2) within each irrigation treatment after a 52-d dry period (24 Aug to 14 Oct 2004; day of year 237 to 288). Ratings were on a scale from 1 (dead, brown turf) to 9 (optimum uniformity, density, and color) and 6 indicates acceptable quality for a home lawn. Means with same letters within each irrigation group were not significantly different (P < 0.05).

The lack of response of TF to water deficit may have been caused by a combination of deep soils at the research site and deep roots typical of TF that may extract soil water from lower in the profile (5,10). Higher quality in KBG than in HBG2 at 60% ET, and the lack of differences between KBG and HBG1 and HBG2 in the control (Fig. 7), are in contrast to results from a study that reported higher quality in a HBG (Reveille) than in KBG (Bensun [A-34]) during prolonged dry down (16). The latter study also determined that the HBG had greater root length density and root mass in the 0- to 60-cm profile than the KBG had, and that greater dehydration avoidance was observed in the HBG than in KBG. Significant variation in drought resistance has been observed among cultivars of KBG and HBG (1,6,8,9), however, and such differences among species and cultivars may have contributed to the contrasting results observed in our study.

Conclusions

Our results indicate that TF may be better suited than HBG in areas of the transition zone where soils are deep, especially if drought resistance is a priority. Susceptibility of HBG to bluegrass billbug in this study also suggests that maintenance costs in HBG may be higher than in TF, although other cultivars of HBG not tested in this study may be more resistant to billbug damage (12). Because some cultivars of HBG also may exhibit higher drought resistance than others (1), further research is needed using new or different cultivars of HBG and in areas with different soils to more completely determine the potential for the use of HBG in the transition zone.

Acknowledgments

This research was funded by The Scotts Co., the Golf Course Superintendents Association of America (GCSAA), the Kansas Turfgrass Foundation, and the Kansas Agricultural Experiment Station. The technical assistance of Alan Zuk and Angela Kopriva was appreciated.

Contribution no. 06-260-J from the Kansas Agricultural Experiment Station.

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