While all our trees are truly winter hardy and won't die from the freezing cold weather, winds and snows, there are a few risks of damage overwinter.
If you don't want to read the entire thing or want the one quick easy most important thing, It's put on a white tree guard!
This is probably the biggest overwinter concern and leads the most of the tree death and injuries. The biggest concerns are mice and voles, followed by rabbits then larger animals like deer and moose. They are also all very easy to prevent by using any sort of mechanical barrier. Pests usually start to leave the trees alone when they are bigger and more mature and you see the bark on the trucks and branches go from smooth and young to that more hard, cracked and texture look.
Rodents: Rodents love to nest around the base of trees and chew on the tender bark of the truck. If they chew around the entire tree in a ring, the tree is "girdled" and will die. Be sure not to mound any mulch around the truck to provide good nesting spots. Often it's recommended to avoid mulching with straw in the fall due to this concern as well. We always recommend a spiral tree guard or even split drainage pipe around the truck of the tree. Be sure to press it or dig it slightly into the ground so they can't easily slip underneath, it's best if it's long enough to reach the lowest branch of the tree. Also use a white coloured pipe instead of the black ones and they can absorb heat around the tree and lead to early sap flow causing winter injury.
Rabbits, Deer etc: The concern here is that these animals will often eat the bark and buds on the higher branches overwinter. Once there is a deep layer of snow on the ground the rabbits can actually reach higher then expected, so keep that in mind when building any barriers. Any sort of fencing material is just fine and it's better to keep it a 3-4ft from the trees, so animals can't reach over to the tree, but not too far that they can jump over into the fenced area. This can be a bigger job so it's really only necessary if the pressure from these animals is really heavy. They are also unlikely to kill the tree, but you may certainly lose some branches.
The above all applies to both trees and shrubs, but with shrubs having multiple stems it's usually impossible/impractical to use a tree guard for them. Rodents are also unlikely to girdle every single stem so it's much less likely to kill the entire plant.
I know I said these are all winter hardy and the snow won't kill them, but the mechanical force of the snow can certainly injure them. This can be a problem when you pile snow on top of trees from shoveling, as it's much heavier and large amounts that from a delicate snowfall.
The other issue is when we get strange freezing and thawing cycles over winter (like in the last few years). This creates a sheet of ice along the surface of the snow, this ice can also refreeze onto the trees. When the spring thaw comes or any thaw cycle occurs this sheet of ice can fall and it can pull the tree down with it. This can sometimes cause lower branches and young stems to be pulled down off the tree or snap. This can often be a bigger concerns with small shrubs.
There's no need to go out and do any sort of burlap wrapping and often it can be worse and it creates airspaces that have very different conditions for the plants than they expect. You can simply wrap with some type of wide/gentle material (often fabric strips or old socks and nylons are great) around the entire to shrub to gently hold it together. The other option is to create a small open tent over the shrubs, just something like a 4-5 sticks or bamboo poles.
Sun: The hot sun in the winters or early spring can cause the tree to absorb heat and start sap flowing, then in the evenings or colder spells this area will freeze hard again, leading to an area of cracking or a sunken "canker" area. These are common portals for infections of the tree and can kill them long term. This is much more common in younger and less mature trees. This isn't something that usually happens or is expected but can occur. Choosing hardy/zone appropriate trees is the most important part to prevent this. By using the white tree guards to prevent rodents you are also preventing this type of injury and this is most of the reason you want it long enough to reach the lowest branch. The other option is to paint the trunk of the tree up to the lowest branch, with any type of white latex paint. I've read both recommendations to apply full strength or mixed 1:1 with water. This painting can also smoother some existing insects and prevent some others.
This is not usually an issue for hardy/zone appropriate trees, but can be issues if there's a combination of a few environmental factors. If the ground freezes too fast and too deep it can damage roots of young trees. The same thing in spring when there is intense frost heaves it can break roots. This can occur if there is very little snow on the ground prior to deep freezes. Also if the ground in particularly dry going into winter this can be a bigger problem. If you soil is very sandy or drains fast, it will also freeze faster and deeper.
The best way to help this is by mulching around the trees before winter (in fact mulch is really the key to so many tree issues I can't go on enough about the benefits).
The other way is making sure the ground around the trees is moist heading into the winter season.
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