WINTER INJURY

Ice Cover Injury

Intermittent ice formation on golf greens and fairways is a common event in northern Europe. Ice cover is often considered part of winter injury caused directly by a continuous ice cover or as part of freeze injury (low temperature kill).

Ice in Association with Freeze Injury

In areas where continuous ice cover for over 45 days is unlikely due to winter weather patterns being broken due to intermittent periods of thawing, ice formation can play a role in freeze injury. Under this scenario a rapid drop in temperature resulting in freezing water around the growing point during late winter or early spring can cause freeze injury primarily to Poa annua.

The critical precursor to freeze injury is the loss of cold hardiness through dehardening and subsequent rehydration of the annual bluegrass crown region. Continuous ice covers as previously mentioned contribute to the decline in cold hardiness. However, the most important factor regulating dehardening is temperature(5). In annual bluegrass the dehardening process can occur quickly when soil temperatures exceed 46F (8C) for 48 hours(6).

What cultural practices can be instituted to minimize ice injury and/or freeze injury? A thorough discussion is found in the 2004 November/December issue of the USGA Green Section Record in an article entitled “Winter Damage” by Keith Happ. Some of the key points are:

1) Produce a healthy plant going into the winter. A weak Poa annua plant with low carbohydrate storage is not going to tolerate ice cover or be resistant to freeze injury as a healthy plant. Shaded areas are more prone to freeze injury than sunny areas, probably due to the carbohydrate status of Poa annua(7).

2) Eliminate poorly drained areas. Poa annua growing in areas where water accumulates is at high risk to rapid freezing during freeze/thaw cycles.

In conclusion, winter injury is normally a combination of several factors one of which is ice cover. A continuous ice cover can cause injury on Poa annua after 45 days. The formation of ice during freeze/thaw cycles in late winter can create a situation where excessive water in and around Poa annua crowns can create freeze injury from ice formed by a rapid drop in temperature.

Continuous Ice Cover Injury

The first type of ice injury is the direct result of a continuous ice cover often referred to as freeze smothering. In the early to mid 1960’s Jim Beard conducted controlled laboratory study where he looked at the survival rate of three cool season turfgrasses under a continuous ice cover and two turfgrasses under field conditions(1,2). He found that creeping bentgrass could survive 120 days of continuous ice cover, however annual bluegrass (Poa annua) loss occurred after 60 days with substantial loss around 75 days. In a more recent Canadian field study annual bluegrass and creeping bentgrass turf was subjected to 45 days of continuous ice cover and then the ice was removed. Seventy-five days after initiating the study and 30 days after removing the ice cover creeping bentgrass still maintained its cold hardiness, while annual bluegrass was dead(3). It would appear from this study that annual bluegrass under a continuous ice cover needs to be removed prior to 45 days.

The reasons commonly proposed for ice injury are the buildup of toxic gases and/or the development of anoxic conditions, and the loss of cold hardiness. It appears that carbon dioxide (CO2) accumulation under ice cover is a major contributor to the death of herbaceous plants(4). Intermittent thawing helped eliminate the CO2 buildup and injury to the plants in this study did not occur(4).

The loss of cold hardiness under ice cover occurs and varies among turfgrass species. Under continuous ice cover annual bluegrass loses its cold hardiness, while creeping bentgrass is not affected(3). The loss of cold hardiness in annual bluegrass is likely due to the anoxia (lack of oxygen) conditions that develop under an ice cover(3).