Winter is a brutal season for human beings; can you imagine how difficult it is to survive the cold, wind and snow if you were rooted to the earth? That’s what your lawn has to do, and although they’re pretty tough plants, sometimes they do not make it. So, let’s look at what winter kill of lawns is, why it happens and what we can do about it.
First, a definition. Winter kill is a catch phrase that encompasses many different disease elements, some we have control over, most we do not. There is no specific disease called “Winter Kill” that has a discrete pathogen associated with it like a fungus. Most instances of winter kill are associated with the metabolism or the physiology of the turfgrass plant.
The crown of the turfgrass plant is its central growing point – all leaf and root growth starts at the crown – so anything that happens to the crown is therefore catastrophic to the health of the plant.
During the late fall, turfgrass plants undergo a normal process of dehydration that removes a great deal of water from its cells. This dehydration means that there is a much higher level of solutes in the plant cells. Think of it as a glass of saltwater in which you removed quite a bit of water, but not salt, making the remaining water a lot saltier. Now we know that the freezing temperature of salt water is quite a bit lower than that of fresh water so this process naturally lowers the freezing point of the remaining fluid in the cells.
In the spring, the turfgrass plants will take in stimuli from the environment and reverse this process as they emerge from dormancy. The crown of the plant will begin taking water from the soil, hydrating itself. If this happens and we get a sudden and dramatic drop in temperatures, these cells are very prone to freezing and bursting, leading to death of the turfgrass plant.
Desiccation is defined as the loss of water from a plant past the point that it can survive. Plants have the ability to pass water off into the atmosphere in a process called transpiration, a critical process during the warmer months of the year. If the turfgrass is open to the weather (not covered in snow) and the soil is frozen (thus, the plant cannot take up liquid water from the soil) winds passing over the plant will have a drying effect, pulling water from the plant without the plant being capable of replacing it.
Direct Low Temperature Kill
We can rank the turfgrasses that we utilize in our lawns based upon their ability to withstand cold temperatures. First, the reason that you do not see any warm-season turfgrasses, like the bermudagrass you might see in Florida, here in New England is that they do not have any cold temperature tolerance. The warm-season turfgrasses can grow about as far north as Washington, DC, give or take a few miles, after that only the cool season turfgrasses like bluegrass, ryegrass and fescue, can survive. The one exception to this rule is zoysiagrass, a warm-season turfgrass that can be readily found in New England; zoysia has moderate cold temperature hardiness.
But, even the cool season turfgrasses have their limits. There is no specific temperature that will kill a turfgrass plant, it has to do with the level of solutes in the plant, the outside temperature and when ice crystals form and it’s the dynamic interaction between these things that makes the difference. Think of it this way, I would survive a long time outdoors in a heavy jacket and overalls at -10 degrees, but if I were to step out of the shower and go running au natural through my neighborhood at even 32 degrees I would not survive long (assuming the cops didn’t pick me up and throw me in the back of a police cruiser). The point is that an always-changing temperature exists below which the plant will not survive.
You may think that a thick coating of ice would be the death of a turfgrass plant all by itself, but recall that I said that they’re very resilient plants, right? Turns out that turfgrasses can easily survive a long period of time beneath a sheet of ice. However, there is a danger that is caused indirectly by ice cover. Gasses (methane and hydrogen sulfide) that are normally passed off into the atmosphere are trapped beneath the ice. At a certain point, the presence of these gasses can become toxic to the turfgrass plants, causing their death.
The two snow mold diseases, gray and pink snow molds, are normally confined to the dead leaf tissue left behind from the previous season. There are occasions when the disease will become more serious and infect the crowns, or growing points, of the plants. This is far more common on high maintenance, short cut turfgrass such as is found on golf courses than it is on home lawns. However, the possibility does exist and when it does happen, it falls under the umbrella term of winter kill.
What to do about Winter Kill?
Winter kill can be tough to properly diagnose. When the lawn is in dormancy, it certainly looks dead and it’s only the expectation of it greening up in the spring that keeps us from reacting to this blanket of brown as being deceased turfgrass. In most every instance, it’s when the lawn as a whole begins to break dormancy, begins to green up and that green is compared to the still brown areas affected by winter kill that we realize we have a problem.
Dead turf is still dead turf and needs to be reestablished. If the area is an important focal area, repairing using sod should be considered. If the area does not have that much focal attention, seeding is the best option.
Unfortunately, spring is a lousy time to seed lawns. We have just a short period of time to get the work done in order to get the seed up and growing to the point of maturity so that it will survive the long hot summer. Most often, it does not and the area will require reseeding again in the fall.
What makes matters worse is the inability to apply a preemergent crabgrass control in areas where you need to seed. The preemergent crabgrass control will kill a new turfgrass seedling just as successfully as it does a crabgrass seedling. This means that in order to have any chance at regressing a winter kill affected area, you need to avoid using crabgrass control, which almost always results in a big mess.
Fertility has a big role in determining winter survival. Plants should not be ‘lush’ when the point in time comes in the fall for it to undergo the hardening off process, readying it for the cold. Careful planning of fertilization, together with applying fertilizers that assist the plant in hardening off, are key to successful preparation.
- Bob Mann, agronomist.