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TURFGRASS SCIENCE

Effects of Ice Cover on Annual Bluegrass and Creeping Bentgrass Putting Greens

Prairie Turfgrass Research Centre, Dep. of Land Science, Olds College, 4500 50th St., Olds, AB, Canada T4H 1R6

^* Corresponding author (jross{at}oldscollege.ca)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Damage as a result of ice cover on putting greens affects golf courses in cold climates. The objectives of this study were to assess cold-hardiness levels and injury of annual bluegrass [Poa annua f. reptans (Hausskn.) T. Koyama] and creeping bentgrass (Agrostis stolonifera L. cv. Penncross) under ice cover maintained for various periods of time under laboratory and field conditions. In the lab, cold-hardened plants of both species were subjected to either snow-covered, ice-covered, or ice-encased treatments and tested for cold-hardiness levels at various periods of time. Ice-encased annual bluegrass plants stored for 90 d were dead, while ice-covered and snow-covered plants had cold-hardiness levels of –4°C and –18°C, respectively. In contrast, at 150 days after treatment (DAT), creeping bentgrass that was ice encased had a cold-hardiness level of –18°C, while snow-covered plants had a cold-hardiness level of –27°C. In the field, annual bluegrass and creeping bentgrass plants were subjected to the following treatments: snow cover, ice cover, and snow or ice removed 45 DAT and then were sampled at various times to determine cold-hardiness levels. As in the lab, ice cover had less impact on bentgrass than annual bluegrass. At 90 DAT, ice-covered creeping bentgrass had cold-hardiness levels of –29°C while annual bluegrass plants were dead at 75 DAT. Snow-covered annual bluegrass plants still had cold-hardiness levels of –16°C at 75 DAT. Removing the snow or ice after 45 DAT had little or no effect.

Abbreviations: DAT, days after treatment • PTRC, Prairie Turfgrass Research Centre

	INTRODUCTION

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WINTER DAMAGE to putting greens is a problem that affects golf courses in cold climate areas (Beard and Olien, 1963). Winter damage can be caused by a number of different factors, but low temperature injury and injury related to ice covers are particular problems. This condition is exacerbated if the predominant grass species is annual bluegrass, a situation that occurs on older greens in the cold climates of the Canadian Prairie Provinces. A previous study (Tompkins et al., 2000) demonstrated that it is important to maintain a snow cover on putting greens to maintain dormancy for as long as possible.

However, in some years, fluctuating temperatures can cause snow to melt and refreeze. Melted water can percolate through snow to the soil surface where it refreezes, producing a layer of ice on the soil surface (McKersie and Leshem, 1994). In addition, ice can form when rainfall occurs on frozen soils that are either covered with snow or not covered. Density of ice cover can vary considerably depending on whether rainfall occurs in the presence or absence of snow. Beard (1964a) reported significant damage by ice covers on annual bluegrass when ice covers were present for longer than 75 d. However, creeping bentgrass was able to survive periods of 120 d under ice cover without damage (Beard 1965). Total submersion in ice causes greater damage than partial submersion or ice cover only (Andrews and Pomeroy, 1975; Beard, 1964a).

Freyman and Brink (1967) in their study on ice cover in alfalfa concluded that CO₂ accumulation was the prime factor in the death of herbaceous plants under ice sheets. In turfgrasses, Beard (1964a) suggested that combinations of freezing and thawing in association with high plant tissue moisture levels might be of greater significance than direct oxygen suffocation or toxic accumulations. Rochette et al. (2000) attributed the cause of ice injury on putting greens to the development of anoxic conditions under impermeable covers, and Olien and Smith (1981) suggested that prolonged ice cover reduces cold-hardiness levels, making the plant more susceptible to other types of winter injury. Another possible explanation for winter injury is that plants with severely injured crowns might not be capable of producing a new root system rapidly enough to meet the water uptake requirements of transpiration (Beard and Olien, 1963). Under these conditions, upper plant tissue may desiccate and die. This theory would explain why turf occasionally appears to be alive when ice cover was removed, but later showed signs of mortality.

Tolerance to ice encasement may or may not be correlated with tolerance to freezing temperatures. In winter wheat (Triticum aestivum L.), ice-encasement and freezing tolerance have been reported to be often, but not always correlated (McKersie and Hunt, 1987; Andrews and Pomeroy, 1989). In contrast, Gudleifsson et al. (1986) reported significant differences between ice injury and cold hardiness in pasture grasses. Flooding may increase tolerance to ice injury but reduce freezing tolerance as a result of crown hydration (Gao et al., 1983). Therefore, in determining whether or not to remove an ice sheet from a putting green, it is important to determine the relative importance of factors such as length of time of the ice cover vs. potential for cold temperature injury resulting from crown hydration when the ice melts. The species of grass present on the green would be expected to have a major influence on this decision.

The objectives of this study were to assess cold-hardiness levels and injury of annual bluegrass and creeping bentgrass under ice cover maintained for various periods of time in controlled and field conditions.

	MATERIALS AND METHODS

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Lab Study
This study was conducted during the winter of 2000. Annual bluegrass (biotype MN 42 provided by the University of Minnesota) and Penncross creeping bentgrass plants were allowed to naturally cold harden on plots located at the Prairie Turfgrass Research Centre (PTRC), Olds, AB, Canada. Plots were established in 1995 and were maintained at a mowing height of 5 mm with regular fertilizer application comparable with typical putting greens (i.e., 0.5 kg N 100 m^–2 per growing month). Fungicides were applied each fall to prevent snow mold formation. Before sampling, temperatures were considered optimum for cold hardening (Gusta and Fowler, 1977), that is, slightly above freezing during the day and slightly below freezing during the night.

Individual samples that measured 10.6 cm in diam. and 7.5 cm in depth were collected on 31 October with a putting green cup cutter. Following sampling, plants were stored in a freezer at –4°C for an additional 3 wk to ensure that plants completed the cold-hardening process. Before the initiation of the treatments, baseline cold-hardiness values were determined for both species.

A split-plot design with four replications was used, with species as main plots and treatments as subplots. Main plots included the species annual bluegrass and creeping bentgrass, while subplot treatments included the control, ice cover, and ice encasement. Field samples were placed in 15-cm pots filled with moistened potting soil and then placed on benches within a walk-in freezer which was maintained at –4°C. The control treatment plants were covered with snow to prevent desiccation. Plants in the ice-cover treatment were covered with 2.5 cm of ice, which was added gradually to the surface by misting with a spray bottle while in the freezer. The ice-encasement treatment was initiated by sealing the holes on the pots and completely saturating the soil and individual field samples. Following this, pots were placed in the freezer and 2.5 cm of ice was added gradually by misting.

Pots were then removed at various intervals of time: 15, 30, 45, 60, 90, 120, and 150 DAT, and subjected to the cold-hardiness test. For this test, field samples were divided into smaller uniform-sized plugs by cutting with a circular saw. Each plug, which consisted of a number of individual plants, was then placed in 12- by 50-mm test tubes.

In a circulating bath (Model LT₅₀, Neslab Instruments, Portsmouth, NH), plants were allowed to equilibrate for 8 h at –2°C. The temperature was then decreased by 2°C h^–1 in a stepwise fashion. When the temperature was in a targeted temperature range as determined by Tompkins et al. (2000), a test tube with plants from each treatment was removed and the temperature was further decreased by 2°C. For example, plants may have been removed at –6, –8, –10, –12, –14, and –16°C. This methodology was developed to prevent the plants from thawing before the cold-hardiness test, as premature thawing may influence cold-hardiness levels. Following the cold-hardiness test, plugs were allowed to thaw at 4°C. Four individual plants were then separated from each plug and were placed in 5-cm-diam. pots with a soilless potting media and allowed to regrow in the greenhouse at 20/16°C day/night temperatures. After 4 wk, plant regrowth was rated for survival to establish the cold-hardiness level. These levels are presented as an LT₅₀ value, which represent the lowest temperature at which 50% of the plants survived.

Field Study
The relative effects of snow and ice cover were tested in a 2-yr field study beginning in 1999 on plots located at the PTRC. The impact of removing the snow and ice 45 DAT was also assessed. Plots, which measured 2 by 2 m, were established in early January of each year in a split plot design with four replicates. Main plots were species: annual bluegrass (biotype MN42) and creeping bentgrass (Penncross). Subplots were snow- and ice-cover treatments maintained as long as possible or removed in mid-February.

For ice cover treatments, 5- by 5-cm wooden water retention forms were placed around the perimeter of the plots. Water was incrementally added to individual plots with a watering can until a depth of 5 cm of ice was achieved. This treatment resembled the ice encasement treatment in the lab study.

Snow and ice were removed from appropriate plots in mid-February, as this corresponded to the 45-d sampling period in the lab study. Snow was removed with shovels, while the ice was physically removed by chopping with an axe and ice chipper. Plants were sampled on all plots at 15, 30, 45, 60, 75, and 90 DAT to determine LT₅₀ values. At each sampling period, a 6- by 6-cm field sample was removed from each plot using a hammer and chisel. Each field sample was divided into six individual plugs, placed in test tubes, and then subjected to the cold-hardiness test as outlined above.

For both the lab and field study, data was analyzed with the MSTAT program (MSTAT-C, 1988). When treatment effects were significant based on ANOVA, LSD was used for mean separation.

	RESULTS AND DISCUSSION

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Lab Study
There were significant differences in cold-hardiness levels between the two species (Table 1) for the duration of this study. Baseline cold-hardiness levels, which were determined following complete hardening, were –21°C for annual bluegrass and –38°C for creeping bentgrass. These values indicate maximum cold hardiness for these two species and were consistent with previous research (Tompkins et al., 2000).

When examining cold-hardiness levels for both grass species, there were significant differences between snow and ice cover treatments on most sampling dates. Plants subjected to the snow-cover treatment maintained cold hardiness for the longest period of time, while ice encasement produced the most rapid loss of cold hardiness. For example, when cold-hardiness levels for both species were combined at 90 DAT, snow-cover treatments had cold-hardiness levels of –27°C, compared with –19°C for the ice cover treatment and –16°C for ice encasement treatment. This demonstrates a significant reduction in cold-hardiness levels when plants are either ice covered or ice encased for extended periods of time.

Differences between snow-cover and ice treatments were greater for annual bluegrass than for creeping bentgrass plants. At 90 DAT, snow-covered annual bluegrass plants still had cold-hardiness levels of –18°C, while ice-covered plants had a cold-hardiness level of –4°C and plants encased in ice were dead. In contrast, creeping bentgrass plants maintained adequate cold-hardiness levels as snow-covered plants had a hardiness level of –36°C, ice-covered plants of –35°C, and ice-encased plants of –32°C.

Both grass species appeared to have a stage where they rapidly lost hardiness. For annual bluegrass, this stage occurred much earlier than for creeping bentgrass. Ice-covered annual bluegrass had a rapid loss of hardiness between 75 and 90 DAT, while ice-encased plants rapidly lost hardiness between 45 and 60 DAT. Conversely, creeping bentgrass began to lose hardiness 90 DAT in both ice treatments, but still retained a moderate level of hardiness (–18°C) at 150 DAT.

Field Study
Baseline cold-hardiness levels, measured at the commencement of the study, were –22°C for annual bluegrass and –42°C for creeping bentgrass. This difference between species was maintained throughout the experimental period (Table 2). However, because of weather conditions, it was only possible to maintain a consistent snow and ice cover on the plots for 90 DAT.

Annual bluegrass plants subjected to ice treatments experienced a more rapid loss of cold hardiness in the field than in the lab. In the lab study, ice-encased plants lost cold hardiness between 60 and 75 d, while in the field study, plants subjected to the ice treatments lost cold hardiness between 45 and 60 d. In the lab study, ice-encased plants were dead at 90 DAT, and in the field, study plants subjected to the ice treatments were dead 75 DAT. For creeping bentgrass plants, the differences between lab and field studies were less pronounced. In the lab study, ice-encased creeping bentgrass plants had a cold-hardiness level of –32°C 90 DAT, compared with –29°C for ice cover treatment in the field study.

There was no benefit of ice removal at 45 DAT to the survival of annual bluegrass plants. In fact, the removal of the ice cover led to a reduction in cold hardiness 60 DAT. Since, the ice was not removed until the 45-d sampling period; no significant treatment differences were observed at 15, 30, and 45 DAT.

Mechanism of Injury
Previous research attributed ice cover injury to the buildup of toxic gasses (Andrews and Pomeroy, 1989), the development of anoxic conditions (Rochette et al., 2000), or to a loss of cold hardiness due to crown hydration during ice melt (Beard, 1964b). The theory that the loss of cold hardiness under ice cover is due to a combination of factors is supported by results of this trial.

The lab methodology attempted to fully hydrate crowns before ice formation (ice encasement) and compare that to ice formation that occurs when crowns were not hydrated (ice cover). When comparing hardiness levels for ice cover vs. ice encasement, there was a significant difference between the treatments and both species. Annual bluegrass had hardiness levels of –10°C for the ice-covered treatment and –2°C for the ice encasement treatment 60 DAT, while creeping bentgrass had hardiness levels of –27°C for ice covered and –20°C for ice encasement 120 DAT. This would indicate that both ice cover and crown hydration were factors in the loss of cold hardiness.

Tompkins et al. (2000) demonstrated that plants did not fully hydrate until they thawed. Plants that are ice covered but not hydrated may be similar to a midwinter thaw that causes ice to form between the soil surface and the snow. Conversely, ice encasement treatments more closely resemble a fall or early winter rainfall that saturates the soil and then freezes when snow cover is not present.

Rochette et al. (2000) reported that for annual bluegrass, conditions of anoxia can develop under an impermeable cover within 60 d and mortality can occur in an additional 40 d. This study supports these findings as a rapid loss of cold hardiness occurred between 45 and 60 d for both ice encasement treatments in the lab and the field. In the lab study, ice-encased plants were dead 90 DAT, and in the field study, plants subjected to ice cover were dead 75 DAT. Conditions of anoxia under impermeable covers are likely similar to conditions that occur under ice. This study supports the concept that anoxia is a major cause of loss of hardiness under ice cover in annual bluegrass.

Species Differences
The loss of cold hardiness due to ice cover is a major contributor to winter injury on annual bluegrass, but it has a minimal impact on creeping bentgrass. Not only did creeping bentgrass have greater initial hardiness levels, but it retained adequate hardiness beyond 90 d when covered with ice.

Previously, Tompkins et al. (2000) found annual bluegrass had a higher crown moisture content than creeping bentgrass, which may explain differences in hardiness levels under ice covers. This condition was observed not only when plants were cold and dormant, but also when snow melt occurred in late winter and early spring.

As mentioned above, anoxic conditions likely occur under ice cover, and annual bluegrass is much less tolerant of this condition when compared with creeping bentgrass. It is possible that annual bluegrass has greater O₂ requirement in the presence of ice or that increased respiration contributes to the build-up of toxic gases under ice cover. Quantifying toxic gas build-up and anoxia for both species might clarify why annual bluegrass has such poor tolerance to ice cover. According to Rochette et al. (2000), recurrently damaged greens had higher soil respiration rates as a result of a larger pool of organic carbon. The decrease in O₂ and the increase in CO₂ were attributed to higher soil biological activity rather than plant respiration. It is not known whether species differences influence O₂ consumption.

One other factor that should be considered is that there can be wide variation in cold hardiness amongst annual bluegrass ecotypes. Dionne et al. (2001) identified up to 9°C variation in cold hardiness amongst ecotypes that were collected from different locations in Quebec and northeastern USA. However, their study did not determine whether this variability existed under ice cover. The same can be said for creeping bentgrass, and it is expected that there is variation between varieties in cold and ice-cover tolerance. This is an area that should be explored in the future.

Comparison of Lab Study and Field Study
In the lab study, ice cover conditions were created by misting during a period of time, while in the field study, water was incrementally applied to the plots during a short period of time. While it is not possible to make direct comparisons between lab and field studies, the treatments that are most similar, ice encasement in the lab study and ice cover in the field study, produced similar results. There was, however, a more rapid loss of cold hardiness in the field, possibly because of the greater variability of temperatures. In the lab study, ice encasement produced the most rapid loss of cold hardiness, while ice cover was intermediate between ice encasement and snow-cover treatments.

Timing of Removal of Ice for Annual Bluegrass and Bentgrass Greens
It would appear that immediate removal of ice would be most appropriate for annual bluegrass and removal at 45 d was too late. Creeping bentgrass, on the other hand, may not warrant ice removal at all. In the field study, cold-hardiness levels at 90 DAT were not significantly different between the ice cover (–29°C) and the snow-cover treatments (–30°C). In the lab study, creeping bentgrass still had hardiness levels of –18°C when ice cover was maintained for 150 d.

	ACKNOWLEDGMENTS

 
The authors would like to thank the Canadian Turfgrass Research Foundation and the Alberta Turfgrass Research Foundation for funding this project.

Received for publication January 13, 2004.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 

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