There are many factors that encourage winter kill to occur they include: grass species, lack of drainage, over abundance of trees near the putting surfaces, and weather. Poa annua is much more susceptible to winter kill than any other grass. At Ridgeway, we have about 80-90% Poa annua on our greens while many other courses have much higher populations of bentgrass. This is why courses in close proximity many have winter kill while others may not. That being said, there are also differing varieties of Poa annua. Poa annua var. annua is a truly annual species of grass while Poa annua var. reptans is a perennial that seeds out in spring and lives on as a perennial. The truly annual species of Poa is obviously weaker and more susceptible to stress. Bentgrasses are much hardier against winter kill and are one of the main reasons golf courses renovate greens to make it the dominate species.
Lack of drainage is a huge factor that affects winter kill because melting snow and water can more easily get trapped on the greens surface. Because it gets trapped, subsequent cold temperatures freeze the trapped water and form a nice layer of ice. Ice, as I will mention later is detrimental to turfgrasses especially Poa annua. Improving drainage will greatly help in minimizing turf loss due to ice formation and crown hydration problems.
Trees that border or in close proximity to the putting surface can increase the likely hood of winter kill. Spruce trees and other conifers are especially detrimental because of the dense shade it casts over the putting surfaces during the short days of winter. Conifers that are to the south and south-east are problematic because the greens are virtually covered in shade for the entire day in mid-winter. Shade inhibits the sun from melting any ice cover and keeps the temperatures colder than that of greens in full sun. Selectively removing trees particularly south of the putting surfaces is a smart idea to allow for more sunlight in winter months.
The most important factor and one that Superintendents have yet to control, is the weather. Some years are better than others for winter kill and some areas get hit hard while others are left unscathed. For instance, the Milwaukee-Chicago area was hit hard by winter kill last year, however just north of the Milwaukee area was seemingly unaffected at all. This year at Ridgeway we received a lot of snow and a lot of melting. This can be problematic because melting snow pools up and refreezes over and over again. This type of scenario usually produces ice cover that sits on the green for extended periods of time. Having talked to several superintendents they have also experienced winter kill this spring. Golf courses with more Poa annua and poor drainage are more affected this year than courses with higher populations of bentgrass and better drainage. Other forms of weather can also be problematic. Winter weather that produces no snow and bitter cold is problematic, it causes dessication. Other scenarios are times when mid-winter thaws are followed by extreme temperature drops that shock turfgrass plants. And in some cases regardless turf species, drainage, and trees the weather causes extensive winter kill to the point nothing could have been done to prevent it.
There are many types of winter-kill, they are as follows: extended ice cover, crown hydration, direct temperature kill, desiccation, and snow mold diseases. In a year where winter kill is present anyone of these factors can cause turf death. Again Poa annua is the most susceptible to all of these forms of winter-kill while bentgrass is almost rarely affected.
Snow mold diseases affect turf over winter and are usually not an issue for Ridgeway because we spray tees, greens and fairways with the proper fungicides that protect the plants over winter. Both bentgrasses and Poa annua are highly susceptible to snow mold diseases thus requiring the same fungicides for proper control.
Desiccation is winter kill that is associated with very cold winters with very little to no snow cover. The plant is then exposed to cold temperatures and high winds. This has an effect on the turf similar to putting your hand on dry ice. There are many ways to protect turfgrass plants from desiccation, one way is by putting out covers over the putting surfaces that help protect the plants from cold and wind. The other method is by applying moderate amounts of sand that act as a blanket and protect the crown of the plants from extreme cold. Both methods are successful against desiccation, we chose to cover our greens, tees and approaches with sand. We have enough covers to use on 8 of our greens at Ridgeway, by putting sand over our greens we were able to cover all 18 greens instead of just 8. The other side benefit is that we are able to incorporate the excess sand into our greens profile. Increasing the amount of sand in our greens profile is one of our higher priorities that our staff and greens committee feel is very important. Burying greens with sand in late fall is a common practice that many golf courses use to protect the plant overwinter while adding sand to their greens profile. A couple of months ago there was a blog post about this exact topic and reading it will give you more insight on why sand and not covers.
Direct temperature kill is another form of winter kill that happens typically when warm mid winter thaws leave the plant unprotected and large swings in temperature shock the plant causing it to die. This happens usually on exposed turf in a short time frame when temperatures go from lets say 45-50 degrees to sub-zero in a matter of hours. Usually sand and covers do an excellent job of protecting the greens from this type of injury. However, in extreme cases nothing can be done to prevent this type of turf loss. Again Poa annua is most susceptible to this type of turf loss.
Extended ice cover and crown hydration are the last two forms of winter injury that I would like to discuss because they are main reasons for turf loss at Ridgeway this year. Continual snows and several thaws left solid ice cover for many days on our Poa greens. Initially, we were able to remove ice with black sand and milograinte (see recent blog post). Poa annua greens can only with stand 30-60 days of ice cover before carbon dioxide reaches toxic levels. The plant then dies from a lack of gas exchange a term called anoxia. Bentgrass can survive 90-120 days of ice before it has issues with an excess of carbon dioxide. As you can see Poa annua is much weaker and can succumb to winter kill due to ice cover way before bentgrass. In fact, it is rare to see bentgrass affected by ice because 90+ days of ice cover is very, very uncommon. Crown hydration is somewhat related to ice cover because can be caused by poor greens drainage. Crown hydration is when melting snow or rain water is taken in by the plant and a sudden change in air temperature causes the plants to burst. This would be similar to taking a plastic soda bottle at room temperature and putting it in your freezer, after a few hours the bottle bursts. This is a good analogy of what happens with crown hydration and turfgrasses. Once again Poa annua is more susceptible to this type of winter injury. Bentgrasses are more resilient and can withstand this type of injury better than Poa. This scenario was the case in late March when the golf course was water logged and free of snow. A wet late season snow storm dumped 10" of new snow. Standing water underneath was exposed to 4 days in a row of temperatures in the single digits. This scenario in combination with excess ice cover early in the winter provided a situation in which the weak Poa annua was unable to fight against winter kill.
Here is an article about crown hydration:
For more information about ice and winter injury look at past blog posts about some of the issues we encountered last winter. There you can also find professional articles related to ice and other forms of winter kill. Also in the next couple of days look for blog posts the show what steps are being taken to bring the greens back from winter damage.