The mulch acts very much like the material in the wall of a refrigerator. So, for those kinds of plants, I remove the mulch around the stem in late autumn. Also I wrap the stems of the tender ones up to 2 or 3 feet with masking tape. This helps prevent freezing and cracking and if freezing does occur, the tape prevents the movement of moisture out of the plant so the crack may heal later. In fact, it is a good idea in planting a dogwood, or any young tree, to wrap the whole stem with masking tape and keep it on until its roots are well eslablished.
Many young trees are ne to dry and crack or freeze and crack.
You may wrap the stem with cloth but, if you forget to take it off, it may girdle the tree whereas the masking tap cracks automatically. Some plants grow well together. For example, oak trees and azaleas are suited to each other. The reason is that most oak roots are fairly deep and most azaleas are fairly shallow. Yet azaleas rarely grow well, if at all, near a red maple, tulip-poplar, or other shallowrooted tree.