This is article, written by Cass Turnbull of Plant Amnesty, has some great information about winter injury on shrubs.
A Word about Our Cold Spell from Cass Turnbull…
After suffering through an extraordinary cold spell here in Western Washington, many garden owners will want to know what to do about the damage to many not-completely-hardy shrubs. The leaves of broadleaf evergreens commonly turn brown or black and eventually fall off after cold weather. The plants themselves are probably still alive. To check, use a hand-pruner blade to peel back a little bit of the skin
to see if the cambium layer just beneath is alive (green) and not dead (brown). If alive, it’ll probably flush out with a new set of leaves. So don’t panic if you shrub looks dead. Wait and see. How long? By June you will have an answer. Those that can put on a new set of leaves will have done so by then. If you can’t stand the sight of the stricken brown shrub until June, try running your hands along the branches to knock the brown leaves off. Then, the plant will look deciduous, not dead.
By the end of August, the final report will be in. Freezing weather sometimes does internal damage that doesn’t show up until after the stress of the summer drought. A shrub may look okay through June and July, but then, while it is pumping H2O like crazy trying to keep up with the heat demand in August, some portions can collapse, and you will see die-back. (The non-scientific explanation is my own and may be a little, well, anthropomorphic.)
Many evergreen shrubs that suffer freeze damage, such as escallonia, will die from the tip back. These shrubs respond well to radical size reduction, which in this case means big ugly cuts to the point of green wood. The plants will break bud just below the cuts and many new green-leafed shoots will rather quickly grow out to hide the cuts and provide you with a revived plant by the end of the growing season.
Should we get snow, branches of some plants (choisya, for example) will split, break, or splay flat to the ground due to snow loading. In early spring, get your loppers out and whack everything back to 4″ to 6″ off the ground. Yes, it’s really okay. I promise. I have done this thing many times. As soon as the growing season begins, the majority of cut plants will spring into action. As the renovated shrubs grow up, pinch them back every so often to encourage branching and thickening. By pinching, I mean a very light heading, just nipping the end bud of each branch with your fingernails or hand-pruners.
So if your evergreens that are looking bad, be patient–wait and see.