To whom it may concern:
This spring golf courses across the state have suffered more dead turf than they have in the past 20 years. The reason is usually called winterkill, but the actual causes can be more complex. The purpose of this letter is to inform golfers of the reasons why so much of the turf died this past winter, to acquaint golfers with what could (or couldn’t) be done to prevent turf death, and what it will take to fix the turf.
The four causes of winterkill are snow mold diseases, desiccation, low-temperature injury, and anoxia (lack of oxygen). Snow mold disease occurs every year, and its deadly effects are usually prevented by superintendents' applying fungicide just before the snows of winter. The fungicide usually lasts through the winter, but an early thaw can sometimes cause the fungicide to break down, requiring a second application. Desiccation, or death by drying out, happens to some extent nearly every year. Desiccation occurs when wind blows across uncovered turf and removes too much moisture from the plants. Desiccation is most common when the air temperatures are above freezing (32 F) and the ground is frozen. During the growing season, turf moisture is replaced when roots take up water from the soil, but frozen ground prevents moisture replacement. Desiccation is most severe on shallow-rooted grasses such as annual bluegrass (Poa annua). A good snow cover prevents desiccation. Superintendents often cover putting greens with a woven tarp that allows just enough air and moisture exchange to get oxygen to the turf but helps prevent desiccation.
Turf death from low temperatures depends on the grass type, how long temperatures are low, the lowest temperature experienced by the grass, the time of year, and the age and health of the grass exposed to low temperatures. Grasses become winter-hardy in the fall by reducing leaf growth and storing carbohydrates (sugars), much as a bear stores fat before hibernation. In winter grasses use their carbohydrate supply for energy and to reduce the possibility of their tissues freezing. The high carbohydrate concentration lowers the freezing point of water in the plants in the same way that salt water freezes at a lower temperature than fresh water. As the sugars are used by the turfgrass during its “hibernation,” the freezing point increases and the plants lose winterhardiness, with hardiness essentially lost in March.
Loss of winterhardiness is usually compounded by freeze-thaw cycles in late winter. Thawing snow and ice get more water into the plant, which further dilutes the carbohydrates and makes it easier for the plants to freeze. Research at the University of Wisconsin (Stier et al., 2003) has shown roots freeze before leaves, which explains why grass may appear green as snow melts but eventually will die because no new root growth is possible for absorption of moisture and nutrient. Much of the winterkill during the 2004-2005 winter was due to low-temperature kill caused by freeze-thaw periods in early January and March. The most-susceptible turfgrasses, in order of susceptibility, are: annual bluegrass, perennial ryegrass, tall fescue, fine fescue, Kentucky bluegrass, and creeping bentgrass. This year it was not uncommon to see damage even to Kentucky bluegrass and creeping bentgrass, with 100% loss to some stands of annual bluegrass, perennial ryegrass, and tall fescue.
Shading reduces turf health, and low temperature kill is even more likely in shaded areas. To help prevent low-temperature kill, superintendents avoid heavy fertilization in September and use a dormant fertilization to encourage root growth, decrease irrigation to allow plants to “dewater,” and avoid scalping the turf. All management practices in autumn are intended to make the plants as healthy as possible before winter.
Turf death by low-temperature injury was exacerbated by the greatest ice cover since at least 1993. Much of Wisconsin’s turf was covered with ice this winter.. Ice cover prevents air exchange (snow allows some air to pass), resulting in anoxia, or lack of oxygen, and the buildup of toxic gases such as methane, carbon dioxide, and cyanide by anaerobic (oxygen-hating) microorganisms in the soil.
Research and empirical evidence indicates annual bluegrass is most susceptible to death during ice cover, whereas creeping bentgrass is largely immune. Depending on the time of year and type of freezing (for example, saturated soil versus unsaturated soil), annual bluegrass survives for no more and often less than 45-60 days under ice. Ice formation occurred mostly in low-lying areas such as those at the bottom of undulations on a putting green or fairway Because annual bluegrass is probably the most common type of grass on Wisconsin putting greens, a fair amount of turf was killed this winter. Preventing ice cover is difficult because prevention relies strictly on good drainage, which is usually a function of the course construction. Many superintendents use aerators to punch holes in the ice, or they melt holes in the ice using black sand or products like Milorganite. In some cases, these methods are effective, but turf survival is not guaranteed. Covers or tarps are not necessarily effective. A sandwich method that uses two layers of waterproof tarps, one below and one on top of a thick layer of straw has shown some effectiveness in Canada, but the system can also kill turf if the winter isn’t cold enough.
The diversity of soil types, undulations, air movement, shading, grass type and other natural features explain why some greens or fairways have 100% dead turf and others suffer very little. One golf course may have very little injury, but another course 2 miles away may have a great deal of winterkill. Ultimately, such extensive and widespread injury as we’ve seen this year usually has little or no relationship to turf management.
Recovering from winter injury will take time. Areas with small dead patches of annual bluegrass have probably already recovered by the time you read this as bentgrass or bluegrass grows in the area. Large areas, greater than 1 foot in diameter, will usually need to be reestablished. Re-establishment may occur if annual bluegrass seed already in the soil is allowed to sprout, or the superintendent may choose to overseed the area. Turfgrasses take about 10 days to germinate in cool spring weather and another 7-10 days to develop each new leaf. Most grass plants need a minimum of 4-5 leaves before they can survive regular traffic. Thus, at least 45 days may be needed before regular play can resume. During this period, using temporary greens and designating affected fairways “out of play” will help the course get back
to its normal condition. Playing new turf too soon will either cause it to fail before the end of the summer or keep it so weak that it is likely to die during the winter.
Fairways may be sodded, but putting greens usually are not because it is difficult to achieve a uniform turf surface with sod. Sodding is also discouraged on sandbased putting greens because it is almost impossible to match the sand type of the putting green, and an incompatible layer will result, preventing water movement, weakening the turf and setting it up to fail.
Additional information on winterkill and returfing the golf course is available in the past two issues of The Grass Roots, the official publication of the Wisconsin Golf Course Superintendents Association (Stier, 2005a; Stier, 2005b).
Environmental turfgrass Extension specialist
Director of Communications
Golf Course Superintendents Association of America
Stier, J.C., D.L. Filiault, and J.P. Palta. 2003. Visualization of freezing progression in turfgrasses using infrared video thermography. Crop Science 43:415-420.
Stier, J. 2005a. When ice kills. The Grass Roots XXXIV(2):4-5, 7.
Stier, J. 2005b. Take advantage of Poa annua winterkill: Increase bentgrass on putting greens. The Grass Roots (In press).