Here in PA and nearby by areas of the mid-Atlantic or Northeast we had
very warm record setting 70F weather last week and last night we had a
deep freeze, 5F. The danger is that the mild fall and first half of the
winter may have kept the sap flowing in rhododendrons and azaleas. Then
when we get a sudden deep freeze with the sap flowing, the sap freezes
and bursts the stem, the vascular structure and ruptures the bark. It is
almost always fatal. However, if you find a plant with bark split
damage, you can treat the plant as you would a graft, using a rubber
grafting strip, tape, or grafting twine. Then seal the entire graft area
with warm grafting wax or grafting paint. Remove the rubber or twine
shortly after the damaged area has healed. Never allow the binding
material to girdle the stem. These precautions will seal out disease
with wax and provide a rubber covering to provide support and
protection, and can sometimes save the plant.

A friend (rhizo_1) reminded me that "sometimes these ruptures hide
themselves until later on in the season. Perhaps the outer bark itself
doesn't split open until the heat of the summer, or the lesion is buried
in mulch, etc. I've seen both happen numerous times in SC. Of course,
by then it's too late to even think of attempting a repair. I'll be
quite surprised, this spring and summer, if we don't see a lot of
frustrated people writing in to many of the forums about their sick or
dead shrubs."
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Cheers, Steve Henning in Reading, PA USA Zone 6