Winter Injury in 2019
Thaddeus McCamant, Staples, MN
The winter of 2018-19 was tough on apple orchards. Deep snow gave perfect cover for voles to girdle trees and gave rabbits a platform to gnaw on branches. While deer, rabbit and vole damage can be detected on the first walk through the orchard in the spring, winter injury often remains hidden for months.
Many growers take the attitude of “wait and see” when it comes to winter injury. I prefer to assess injury in late spring to help growers plan their work. Branch death and tree death from winter injury often don’t show up until the middle of summer. Following the winter of 2013- 2014, some branches didn’t die until the spring of 2015. If nothing else, you can assess winter injury to put your mind at rest. From what I have seen so far, there is not a great deal to worry about.
Winter injury can be assessed by slicing spurs to look at the extent of internal browning. In apples, winter injury occurs across a gradient. Trees can be uninjured, dead or something in between. I rate spurs on a scale of 0 to 3. If a spur completely green, it is a 0 or no injury. A little brown is given a 1. Moderate browning is a 2, and if the spur is dead, it receives a 3. In 2014, I was able to show that orchards rated with a 1 had no crop loss and fully recovered.
Trees are most susceptible to winter injury during the first winter after being planted. Injury to young trees isn’t as easy to detect as cutting spurs, because the tips of the branches dry up and never sprout. The bark remains green until it dries up. As soon as the trees leaf out in the next two weeks, you will be able to see if the trees are alive or dead. In most cases, the new trees die down to snow level, and as long as there is living tissue above the graft union, the tree can be revived from buds. Shoots that sprout from a tree that has died down to snow level often grow 4 feet the first summer.
So far, I have examined a dozen orchards from Faribault to Duluth, along with my rootstock trial in Staples and the few trees at my home near Detroit Lakes. Overall, Honeycrisp, Haralson, Minnieska (SweeTango) and Zestar vary from 0 to 1. There was about the same amount of winter injury south of the Metro area as in northern Minnesota. The glaring exception was in Staples, where Zestar and Honeycrisp averaged 2.5. Honeygold averaged 2.0 in most orchards. First Kiss had the most variability with spurs ranging from 0 to 2. Many new and experimental varieties like Estival, Golden Delicious and Ginger Gold were at a 3.
Here are my conclusions with possible explanations:
Winter injury was worse than 2014 in southern Minnesota and much less than 2014 in northern Minnesota. In 2014, there was a sharp gradient in temperature between southern and northern Minnesota. While temperatures fell to -38 in Staples in December 2013, temperatures in the southern part of the state hovered in the -20’s. This year, every orchard had temperatures below -30. Winter injury was less than expected, considering the temperature. The cold snap at the end of January in 2019 was preceded by a very long period of below-freezing weather. The plants were at their maximum level of hardiness when the temperatures fell in the -30’s. In 2019, Honeycrisp was damaged much less than in 2014, even in orchards where the temperature was lower. In 2014, I suspected that the high amount of injury was because Honeycrisp trees held onto some leaves very late in the fall in 2013. Trees will always be hardiest if their leaves senesce normally. The fact that Honeycrisp is doing better this year supports my original hypothesis. My test plot in Staples is not representative of the rest of the state. The temperature in Staples was -39, which was the same as my house and similar to other orchards. The rootstock trial is the only orchard inspected so far that is exposed to the north wind. Apple trees do not experience wind chill, so scientists who study winter hardiness do not include wind speed in their research. Cold winds can desiccate trees, and every professional I have talked to in this state would never plant an apple orchard exposed to the north winds.
The role of rootstocks: So far, I have not seen any dramatic rootstock effects. In Staples, I have Honeycrisp and Zestar on B9, B118 and Antonovka, and they all look bad. That might change when the trees leaf out. The role of rootstocks in increasing or decreasing the hardiness of an apple variety is still debated. There is no evidence that trees on a hardy dwarfing rootstock like B9 or G 41 are more sensitive to winter injury than trees on a standard rootstock.
Using the information: I am not pruning the rootstock trial in Staples until the trees leaf out. I want to see how many branches died before deciding whether to prune the trees or take the block out. One grower had a Winecrisp tree girdled by mice. I suggested bridge grafting, until I looked at the spurs. He is probably better off cutting the tree below the girdled section. The rootstock trial in Staples and the odd varieties are the exception. The vast majority of the trees in the state look excellent, and some growers will have a snowball bloom and worry about thinning.
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