One of the most difficult times for a golf course superintendent in Northern regions of the country is that anxious period in the spring when turfgrass conditions reveal how well the turf came through the winter.
As springtime approaches, superintendents are faced with difficult decisions concerning the “when’s,” “if’s” and “how’s” of ice and snow removal, free water removal, green cover use and many other decisions that might dictate how well the golf season starts.
When a course suffers severe winterkill, income suffers, reputation suffers, and most of all, superintendents suffer. Sometimes they get pink slips. Although the affected areas are eventually fixed or will recover, winter damage truly can be the cancer of turf.
Today’s terminology
Many individuals use the term “winter damage,” which covers various mechanisms of turfgrass damage. Basically, winter damage is defined as any injury that occurs during the winter. Winter damage can be inflicted by winter turfgrass fungi (snow molds and cool-season pythiums), crown hydration/dehydration, ice encasement damage (anoxia), direct low-temperature kill and desiccation. Of these, only true winter diseases and desiccation are understood, while crown hydration/dehydration and ice cover damage still aren’t completely understood. Crown hydration and ice damage have many questions associated with the “when’s” and “why’s.” Why this type of injury occurs is partly understood.
In contrast, when it happens is the question to which no one really knows an answer. That’s what makes winter damage so difficult to combat. Some years, superintendents remove snow and ice from their greens, and the resulting turf conditions are perfect. The following year, the same blueprint is followed, and the results are far different with major turf damage.
How it happens
The mechanisms that cause crown hydration (also known as freeze injury) are thought to occur when hydrated plants become subject to rapid temperature decreases. Crown hydration – or dehydration, which is more accurate – happens from inside the plant tissue (intracellular) and from outside the plant tissue (extracellular). Crown dehydration from an intracellular nature is believed to happen when the water around the cells inside the plant suddenly freezes. This draws the water out of the cell, causing dehydration and results in the death of the plant. This is the type of injury that’s widely believed to cause most turf damage during the winter.
There’s certain debate about when this actually happens during the winter. James Ross, a researcher at the Prairie Turfgrass Research Centre in Olds, Alberta, believes the plant must break dormancy and begin to hydrate before the freezing process. Ross believes the transition from winter to spring is the most critical for damage to occur because of plant dehardening.
“Generally, when the plant begins to break dormancy, it takes on water, which hydrates the crowns and reduces the simple sugars that protect the crown through the winter and make it much more susceptible,” he says. “Others believe this can happen anytime during the winter, when some of the wacky weather takes place.”
Extracellular crown dehydration happens similarly, except it’s believed that ice forms outside the plant and pulls water out of the plant by osmosis. This results in a dehydration of the plant and might result in the death of the plant.
For years, winter damage (such as true ice encasement damage) has been associated with a gas build-up that occurs just under the ice surface. Under prolonged ice cover, oxygen is depleted from the plant, which is still under very low levels of respiration and microbe activity. This results in an accumulation of toxic gases that might cause the death of the plant. Recently, it’s been debated whether or not ice encasement damage is a major cause of turf damage.
Superintendents also have been taught the 60/90-day rule for many years. This rule states that Poa annua can survive under ice cover for as long as 60 days, while bentgrass can survive as long as 90 days. This rule is based on 40-year-old research and is being revisited.
Cold tolerance
So, what have the industry learned to date? There seems to be one factor, although not completely understood, which might have the greatest influence on turfgrass overwintering. Plant hardening, or hardiness, can be classified as the antifreeze for turf and might be one of the most influential mechanisms determining winter survival.
This process begins in the late summer and continues into the early winter. The plant goes through the hardening stage by internally adjusting to cold temperatures and by storing carbohydrates. During this period, moisture content decreases in the plant, leaving it with a higher sugar content. This higher sugar (antifreeze) content allows the plant to survive the freezing process better.
It’s believed one of the major differences in winter survival between creeping bentgrasses and Poa annua is their ability to harden and deharden differently. To begin with, bentgrass has the ability to reach a much lower level of cold hardiness than Poa annua. The internal plant mechanisms of bentgrass allow it to harden off at a much lower percentage of moisture content in the crown tissues than Poa annua.
An additional problem with Poa annua is it tends to deharden much faster than creeping bentgrass during the transition from winter to spring. This makes Poa annuamore vulnerable to freeze-type damage.
In a two-year study conducted at the Prairie Turfgrass Research Centre, it was determined that in mid-March, Poa annua’s cold tolerance had dropped to 8.6 degrees Fahrenheit while the creeping bentgrass was cold hardy to -20.2 degrees Fahrenheit (Tompkins, 2000). The same study also found that both grass species eventually had the same cold tolerance by mid-April. These findings lend credence to the notion that most damage occurs during late winter through early spring, when dehardening occurs.
The best line of defense for preventing damage might be a combination of practices that all increase turfgrass health. Raising the cutting height and proper fertilization timing might be the two most important cultural practices. Both will help produce and store carbohydrates in the plant, which will increase the “antifreeze effect.” Another method of defense that’s often overlooked is to try to prolong cold-temperature hardiness by protecting turf with increasing snowcover or the use of covers. However, this can be tricky.
Hard to pinpoint
Many people in the industry have believed that if you have bentgrass versus Poa annua, there’s no need to worry about winter damage because of bentgrass’ greater cold tolerance. Well, we certainly know this isn’t always true. For example, Gerry White, former golf course superintendent of Sebago Lakes Resort’s 18-hole championship golf course in Casco, Maine, was quite surprised to find damage – for the first time ever – to his 10-year-old Poa-free Providence greens one April.
“In the 10 years since the course has been built, we had never had any damage to the greens,” White says. “They had been perfect every year. Suddenly one year, bang! I get hit pretty hard and lost almost all of my 16th green.”
The explanation? White has none, but he theorizes:
“We did have a crazy winter weatherwise, but every winter seems to be crazy,” he says.
So why this one year? No one really knows for sure. When turfgrass suffers winter damage, superintendents and those from the world of academia can only theorize what transpired to result in turf loss. That’s what makes this issue so difficult. If you don’t really know how it was caused, then it’s difficult to find a solution.
Finding a solution, if there is one, can only be accomplished by years of superintendents’ experiences and university research. John Roberts, Ph.D., from the University of New Hampshire, has been looking at winter damage for about 20 years and still hasn’t found the miracle cure. Other universities also are conducting work concerning winter damage. This includes research by Dave Minner, Ph.D., and Nick Christians, Ph.D., from Iowa State University and Darrell Tompkins and James Ross of the Prairie Turfgrass Research Centre.
Although winter damage is still a difficult phenomenon to understand, we certainly know more now than in years past. However, the knowledge that we’ve gained doesn’t ensure winter turfgrass survival. Many superintendents have headed into the winter preparing and conditioning the turf the best they can only to get nailed the following spring with winter damage. That’s why many believe winterkill is truly the cancer of turfgrass.
