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Working undercover: Minimizing winter damage to greens

By |  July 23, 2014 0 Comments

By Sam Bauer, Lindsey Hoffman, Ph.D. and Brian Horgan, Ph.D

A look at minimizing winter damage to greens through covers and other prevention strategies.

Editor’s note: This is the second of two articles by the authors focused on protecting turf before and during winter. The first article discussed cultural practices that can be implemented and appeared in last month’s issue.

Winter damage can be caused by a number of different factors; however, some of the most damaging effects are associated with crown hydration and freezing of the crown tissue. Ultimately, this type of injury results in death of the turfgrass plant and can result in costly reestablishment on a yearly basis.

Winter cover protection results vary from year to year, creating headaches for many superintendents.

Winter cover protection results vary from year to year, creating headaches for many superintendents.

In comparing annual bluegrass (Poa annua L.) to creeping bentgrass (Agrostis stolonifera L.), research has shown that annual bluegrass is more susceptible to crown hydration and freezing compared to creeping bentgrass (Hoffman et al., 2014; Tompkins et al., 2000). Along with these two stresses, desiccation can also cause the death of turfgrass plants because of severe cellular dehydration.

Protecting cool-season putting greens from these winter stresses can be a challenging task, especially when managing greens composed of both annual bluegrass and creeping bentgrass. In addition, methods of protection might not yield the same results from year to year.

This is partially attributed to the fact that no two winters are the same; therefore, determining a protection strategy to employ often requires some history of what methods have consistently produced the best results.

Researchers are continually investigating the use of these various protection strategies, but the overall decision of how to protect putting greens during the winter requires experience, constant evaluation and confidence in approach.

Adding some insulation

In locations receiving adequate annual snow cover, snow can be one of the best insulators and may reduce winter injury associated with both desiccation and freezing stress. On wind-swept putting greens, snow fencing is beneficial for retaining snow and helping improve the amount of insulation snow can provide.

Along with snow, manufactured covers are also valuable tools that can be used to protect greens. Covers vary in style based on the intended use. Selection of one particular cover over another requires a thorough understanding of the type(s) of winter injury greens are exposed to and this can vary not only from course to course, but from one green to another.

Impermeable covers, which do not allow for water or air exchange, can be useful for preventing winter injury because of crown hydration by keeping surface moisture isolated from the crown (Photo 1). These covers can be especially important for annual bluegrass, which has been shown to have higher crown moisture content than creeping bentgrass (Tompkins et al., 2000). However, the lack of air exchange under impermeable covers can lead to the buildup of gases at toxic levels and a depletion of oxygen; the rate of this is greatly increased in root zones built on native soils or with high organic matter because of the elevated microbial activity (Rochette et al., 2006).

Permeable covers allow for water and gas exchange at the turf surface, which can be important for root zones with high microbial activity or northern locations where covers are left in place for four to five months or more.

A majority of the insulating type covers are permeable.

The Canadian Turfgrass Research Foundation has provided funding for several complex research projects focusing on winter injury and protection of putting greens in northern climates (Dionne et al., 1999; Rochette et al., 2006; Bertrand et al., 2009). These projects evaluated winter temperatures and gas levels under various winter protection programs, including both permeable and impermeable covers with varying levels of insulation, as well as genotypic differences in winter tolerance of annual bluegrass.

Dionne et al. (1999) found insulating covers to be beneficial in moderating soil temperatures throughout the winter and consequently improved winter survival of creeping bentgrass and annual bluegrass. These insulating covers can be made of wood fiber shavings, straw mulch, foam and bubble wrap, among other materials.

It can be important to install impermeable covers on top of the insulation material to prevent moisture from accumulating on the putting surface. More recently, ventilation systems have been used for improving gas exchange under impermeable covers.

Perfect timing

Timing is everything with covers. Last month we discussed the processes of cold acclimation and deacclimation and their importance on overall turfgrass winter survival.

Covering putting greens essentially creates an environment around the turf that can drastically alter the temperatures between the turf surface and surrounding air. Consequently, this influences both the cold acclimation and deacclimation processes. While research on the effects of covers on these processes is limited, the consensus is that three important factors must be considered for timing of cover installation.

First, the soil should be frozen prior to placement of covers. This enables the plants to acclimate and harden-off, increasing the overall winter tolerance of the turf. Installing covers too early in late fall, when soils are unfrozen, will inhibit the acclimation process by trapping heat at the putting surface.

Second, removing covers too early in the spring can subject the turf to a rapid reduction in temperature, which can be particularly devastating if the turf has already come out of dormancy under the covers. This type of injury typically occurs if covers are removed during a brief warming period (typically late winter or early spring), which is then followed by temperatures at or below freezing (Photo 2). Some superintendents have been successful at reducing damage during the deacclimation process by removing covers during the day and replacing them when nighttime temperatures are low. This is a labor-intensive process, but can mean the difference between survival and death.

Finally, leaving covers on too long in the spring can create an unhealthy atmosphere for turfgrass growth. Under warming temperatures, turfgrass plants can become succulent and more susceptible to infection from pathogens such as snow mold fungi.

Knowing when to install or remove covers is just as important as knowing which type of cover to choose. Monitoring the weather and soil temperatures is the best method for estimating when these processes should occur.

Wrapping up

Cultural practices conducted before covering greens are just as critical for promoting winter survival as would be with uncovered greens. With impermeable covers, moisture will not be replenished in the root zone for the entire time covers are in place. As a result, maintaining moisture percentages at or around field capacity prior to cover installation can be important.

Timing of cover removal is crucial to the success of a cover program. Here, cool air temperatures following removal were enough to shock the turf, causing a setback in turfgrass quality.

Timing of cover removal is crucial to the success of a cover program. Here, cool air temperatures following removal were enough to shock the turf, causing a setback in turfgrass quality.

Soil moisture sensors are useful in determining volumetric water content of root zones. Mowing heights and fertility must be taken into account for the turf species present. Excessive nitrogen fertilization prior to covering can promote unnecessary growth, causing an increase in gas buildup from respiration.

Although covers have many positive attributes, they might not be the solution for everyone. For example, research investigating various covers at the University of Minnesota has yielded inconsistent results from year to year. This might be associated with the timing of installation and removal, which can be difficult to pinpoint.

Another factor associated with the use of covers is cost. Not only is it expensive to purchase covers and insulation, the process of installing and removing covers is time and labor intensive. This can be a major obstacle for courses that have a limited budget and a small crew in the late fall.

One option for this type of establishment is to select the greens that suffer from the most severe winter injury every year. Covering these greens can help minimize the amount of repair necessary in the spring.

An alternative for using manufactured covers is a strategy that has been termed the “poor man’s cover.” This involves heavily topdressing greens with sand in late fall before the onset of winter. Generally, this practice is recommended for golf courses that choose not to cover greens. Sand topdressing with depths between 1/8th and 3/8th inches provides a buffer on the turf surface, protecting crowns from freeze and moisture damage.

In a survey sent to members of the Minnesota GCSA, 85 percent of the respondents who didn’t cover greens applied topdressing sand before winter (Larson, 2010), and we expect this trend to continue.

Protecting putting greens during the winter months also requires some consideration of fungicide applications for snow mold prevention. A large database from seven years of snow mold fungicide trials throughout Wisconsin, Minnesota and Michigan is available on the University of Wisconsin-Madison Turfgrass Diagnostic Lab website. This is a particularly useful resource for determining efficacy of various fungicide mixtures to prevent gray snow mold (Typhula incarnata), speckled snow mold (Typhula ishikariensis) and pink snow mold (Microdochium nivale).

Overall, protecting greens can help minimize damage associated with winter stresses such as desiccation, crown hydration and freezing injury. The decision to cover greens is difficult and depends on a number of different factors including topography and turfgrass composition. In addition, selection of cover type, installation/removal timing, and cultural practices before and during winter are crucial to promote healthy turfgrass stands.

Sam Bauer, Brain Horgan, Ph.D., and Lindsey Hoffman, Ph.D., are at the University of Minnesota where Bauer is a turfgrass extension specialist, Horgan is an associate professor of turfgrass science and Hoffman is a postdoctoral turfgrass research scientist. Bauer can be contacted at for more information.

Bertrand, A., Y. Castonguay, G. Thibault, P. Rochette, T. Hsiang, J. Dionne. 2009. Greens-type annual bluegrass resistance to abiotic and biotic stresses during winter. International Turfgrass Society Research Journal. 11:723-736

Dionne, J., P.A. Dubé, M. Laganiére, Y. Desjardins. 1999. Golf green soil and crown-level temperatures under winter protective covers. Agronomy Journal. 91:227-233.

Hoffman, L., M. DaCosta, and S.J. Ebdon. 2014. Examination of cold deacclimation sensitivity of annual bluegrass and creeping bentgrass. Crop Sci. 54(1): 413-420.

Larsen, Andrew. 2010. Protecting greens in winter: A cost-benefit analysis can help superintendents design a winter protection program that suits their needs. Golf Course Management. 78:86-88, 90, 92, 94, 96, 98, 100, 102.

Rochette, P., J. Dionne, Y. Castonguay, and Y Desjardins. 2006. Atmospheric composition under impermeable winter golf green protections. Crop Sci. 46:1644-1655.

Tompkins, D.K., J.B. Ross, and D.L. Moroz. 2000. Dehardening of annual bluegrass and creeping bentgrass during late winter and early spring. Agron. J. 92:5-9.

Photos: Sam Bauer; Mike Manthey.


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