Preventing damage is key to tree health in winter

By Beckie Gaskill – as seen in The Lakeland Times

UW-Extension outreach specialist Diana Alfuth gave a talk last week regarding how to keep trees and shrubs healthy and protected during the winter. One of the causes of tree damage in the winter, she said, is called sun scald. 

Sun scald happens in late winter when the sun is at its warmest of that season. It starts to heat the bark of the tree and the phloem, which is just underneath the bark. This happens most often when trees are young, and their bark has not developed its thick, corky texture. 

When the cells of the trees warm, they become active during the day. Once the sun moves, or goes down, of course the air cools, as does the tree. With freezing temperatures still possible at night especially, the cells that were becoming active are frozen and killed. There is nothing that can be done to fix those cells, Alfuth said. 

One way to know whether a wound on a tree is from sun scald is the side of the tree on which the wound appears. Sun scald, she said, will always be on the south or southwest side of the tree.

When there is still a good deal of snow on the ground, reflections of sunlight can cause sun scald as well. These bright reflections will work to heat up cells just as the sun’s warmth would. 

To prevent sun scald in winter, Alfuth said, tree trunks should be wrapped up to the first branches at least. Trees should be wrapped up for at least their first four to five winter seasons. After that, the bark will become thicker and will protect the tree itself. Wraps should be of light colors to reflect the sun off of tree rather to collect the warmth, making the tree even more susceptible.

Wraps should be taken off every spring. If they are not taken off, they can constrict the tree growth. Wraps left on have been known to grow directly into the tree. Also, if wraps are left on, moisture can build up between the tree and the wrap as weather warms. This can lead to mold and other issues, she said.

Animal damage

Animals, too, can cause issues for trees in the winter. Deer, rabbits and even small rodents have been known to damage trees.

Deer rubs, which are happening at this time of the year, can cause major wounds to trees. Deer have also been known to snap off small trees completely. This activity can start as early as late August. Even if trees have thick bark and do not need protection from the sun in winter, Alfuth said, the trunks still need protection from deer antlers. 

Rabbits and small rodents feed on bark, she said. They chew on bark when there are fewer food sources on the landscape. They will girdle the trees throughout the winter. Trees need protection from these attacks as well. With no connection from the roots to the tops of the trees, the tree will die. Trees may still leaf out and bloom early in the spring, giving the sense they made it through the damage. However, in June, when temperatures start rising, the tree may not be able to get sufficient water from the roots, and the tree will collapse. 

In this instance, wrapping trees is important, too. The wrap should be placed where the animals can reach. Wrapping should go all the way to the ground in the case of rabbit and small rodent issues. Even pushing the wrap down into the ground half an inch, she said, will help to keep the voles away.

Hardware cloth is a good option in this case, too, she said. It should be left slightly out from the bark but should be wrapped up as far as rabbits and rodents can reach. As food sources become more scarce, trees start to look better and better to these small animals.

When it comes to deer damage, wrapping needs to be taller and stronger, however. These can be purchased, or homemade. Erecting a fence around a tree will keep animals away from the tree, but also not damage the tree. 

“You can just get creative,” she said. Anything that would keep deer antlers away from the tree, without hurting the tree, could be a good option. 

In winter, it will usually be a rabbit or a deer that causes problems, she said. Rabbits will make a clean, angled cut. Deer damage will look more like foliage or branches were ripped off. If missing pieces look carefully removed, it is likely rabbit damage. Understanding what is happening in trees and shrubs can help a landowner or gardener to determine what course of action to take and what type of physical barrier to erect, at least for a time.

Even evergreens such as Arbor Vitaes can fall victim to deer in the winter. The best advice for those in deer-prone areas, she said, was to not plant arbor vitae trees. However, if a gardener does plant them, they will need to be caged or some barrier must be erected, as deer will turn to arbor vitae trees when other food sources become scarcer.

Repellents can also be an option to ward off animals, Alfuth said, but they need to be applied regularly. They can evaporate and wash off, or animals may simply get used to them. She said she, for one, did not enjoy heading out in the middle of winter in the snow and subzero temperatures to reapply repellents, and she said many others likely felt the same way. 

Alfuth also recommended following label directions carefully. Some repellents may be made for use only with certain trees and are made to be applied in a specific manner or timeline.

Other winter issues

Salt damage can be a big issue for trees and shrubs. Alfuth said care should be taken when using de-icing salt near any vegetation. Salt will make its way into the soil and burn tree roots, causing irreparable damage. 

Salt spray can cause problems for plants as well. Salt spray can happen near roadways or other areas where salt is plowed off of surfaces after being applied. It is best to wrap foliage that may be in an area where it will receive salt spray in the winter. Again, this wrapping should be discarded in the spring.

Snow from roofs can damage trees in winter as well. Heavy snowfalls under eves have been known to flatten or split shrubs. Alfuth recommended purchasing a support system for shrubs that may be in areas where snow slides may occur.

Ice storms and ice buildup is another winter issue. Heavy ice storms can cause branches to snap or hang down. In most cases, she said, ice should be allowed to simply melt off of branches. Branches can be gently tapped with a soft broom, for example, in an attempt to free the branches of ice, but often this will do more damage than good. It is better to let nature take its course in these cases.

This presentation, as well as the other presentations in the winter gardening series can be found on the UW-Horticulture website https://hort.extension.wisc.edu. Visitors may also register for future presentations on the website.