For many golf courses with creeping bentgrass greens, annual bluegrass invasion is a major weed problem. Consequently, older turf is often overtaken. When this happens, maintenance, rather than eradication, becomes the major consideration. Unfortunately, winter damage to annual bluegrass greens is a significant problem in cold climate areas.

Winter damage to annual bluegrass greens can be due to a number of factors including reduced levels of cold hardiness compared to creeping bentgrass, early dehardening in the spring, reduced cold hardiness associated with crown hydration and mortality under ice cover. This article will examine the manner in which each of these factors can contribute to winter injury in annual bluegrass.

Quick Tip.

Cold hardiness levels During the warmer summer months, plants have little ability to tolerate cold temperatures, and temperatures of 24.8 degrees Fahrenheit may be cold enough to cause plant mortality. In preparation for the colder temperatures of winter, plants undergo a number of changes. This process is called cold hardening.

A number of factors can induce cold hardening including low temperature, shorter day length, reduced soil and plant moisture and plant nutrition (Gusta et al., 1983). Typically, temperatures near freezing are more effective in promoting rapid hardening (Gusta and Fowler, 1979), and a period of below-freezing temperatures may be required to achieve the full level of cold hardiness (Gusta and Fowler, 1977).

Cold hardiness levels can fluctuate from year to year, and soil temperature during the hardening period plays a critical role in determining the hardiness level (Tompkins et al, 2000). Upon exposure to conditions that induce hardening, plants will achieve their maximum levels of cold hardiness at the start of the winter (Gusta and Fowler, 1979). This hardiness level will gradually decrease throughout the winter. Therefore, a plant that can tolerate temperatures of –4 degrees Fahrenheit in December may only be able to tolerate temperatures of 17.6 degrees Fahrenheit in April (Tompkins et al., 2000).

Cold hardiness levels can vary widely for different grass species (Gusta et al., 1980). For example, creeping bentgrass has a much greater ability to cold harden than does annual bluegrass. In Alberta in the western Canadian prairies, creeping bentgrass can cold harden to levels of at least –40 degrees Fahrenheit, while annual bluegrass can cold harden to –5.8 degrees Fahrenheit (Tompkins et al., 2000). Biotypes of annual bluegrass found in other regions may have less ability to cold harden.

As plants cold harden, there is an associated decline in moisture levels in the crown tissue. One difference between creeping bentgrass and annual bluegrass is that creeping bentgrass plants have a lower percent moisture in the crown tissues at levels of maximum cold hardiness.

Dehardening and winter Injury Dehardening is the process that occurs when temperatures warm, and plants lose their ability to tolerate cold temperatures. During the later part of winter, plants undergo a series of thawing and freezing cycles. A plant loses some of its cold tolerance each successive time it is exposed to warmer temperatures.

Dehardening occurs much more rapidly than hardening (Gay and Eagles, 1991). Once cold hardiness is lost, it may be possible to partially re-induce cold hardiness, but this level will never be the same as the initial cold hardiness level (Tompkins, et al., 2000) because the plant has fewer energy reserves to draw on.

While annual bluegrass is not able to attain the same level of cold hardiness as creeping bentgrass, an additional problem is that annual bluegrass can deharden earlier in the spring, making it more susceptible to a late spring frost. For example, in a two-year study conducted at the Prairie Turfgrass Research Centre in Olds, Alberta, the average cold hardiness level in mid-March for annual bluegrass was 8.6 degrees Fahrenheit compared to –20.2 degrees Fahrenheit for creeping bentgrass (Tompkins, et al., 2000).

However, with the advent of much warmer temperatures in April, the cold hardiness differences between species rapidly disappeared. By mid-April, the cold hardiness level was around 14 degrees Fahrenheit for both species.

One way to protect greens from winter injury in the late winter is to maintain a snow cover as long as possible. First, the snow protects plants from low temperatures, preventing injury. Second, as the air temperature warms, the presence of cover can maintain the plants in a dormant state, which helps prolong cold hardiness. This extension of the dormant period may only last for a few days as the snow rapidly melts once daytime temperatures warm, but this may be enough to provide protection from cold nighttime temperatures during this transition period.