It might seem counter-intuitive, but one of the biggest threats to young trees during the winter is not the cold - but the sun. During the winter, the tree has shed all of its leaves which were providing shade and protection to its trunk. The sun shines at a lower angle in the sky and the white snow that often covers the ground refracts sunlight up on the bark, too.
Because the tree's young bark cells aren't used to this kind of intense sunlight, they are easily injured or killed. This can cause the tree's bark to turn black and split open. This "sunburn for trees" is commonly called "sunscald."
Trees tend to get tougher and thicker-skinned as they get older (don't we all?), but newly planted or thin-skinned trees like honey locusts, fruit trees, ashes, oaks, maples, lindens and willows are particular susceptible to sunscald. The good news is that prevention is super easy. All you need to do is wrap the bark up with tree wrap
Wrapping the tree is pretty straight forward. Just start at the bottom of the tree and slowly work your way up, overlapping each wrap just a little bit so you cover all the bark. Wrap up to just above the lowest branches and secure with tape. You'll want to leave the tree wrapped up until the weather starts to warm up in the spring. The old timer's rule of thumb is "Wrap at Thanksgiving, unwrap at Easter."
You may need to wrap young trees for several winters until they've developed thick enough bark. After that, they should be strong enough to weather the winter on their own.