Over the Arbor Day weekend several people who stopped by to pick up a free potted transplant commented on the amount of winter burn on the trees this year. So I decided it would make a good blog topic. Let's address three questions:
What causes it?
What can I do about it (now)?
How can I prevent it (next time)?
What to do? Nothing! Absolutely nothing--until early June. You need to know if it is just the needles that are dead or if the buds are also dead. If it is just the needles, the tree or plant will be ok, maybe look a little sparse for a year, but will grow out of it. If you trim the dead off now you may be also trimming off live buds. If by early June the buds have not started to grow you know you have a dead branch to prune off, or, in extreme cases, a dead tree.
This photo shows some of the winter burn in the Colorado Blue Spruce. I'm guessing 90% or more of these trees will come out of it, that is, the buds are still alive. However, they may need a couple of years to fill in sufficiently to make a Christmas tree.
What causes winter burn? A combination of prolonged cold, persistently strong winds, lots of snow cover, and clear sunny, but cold, days. Sound like our past winter? These factors dry out the needles, causing them to discolor. Some, if only slightly discolored, may survive and green up again, but most have died. If you see winter burn on the north side of the plant you know that the wind was the major culprit; more often the most severe damage is on the south side and the sun was the major culprit. When combined with the heavy snow cover we had this year, the sun, reflecting off the snow, can slightly warm the needles, which of course freeze again at night. This sequence, repeated many times over the winter season, eventually damages the needles.
It's rather common to see on a single plant: needles that are slightly discolored and will green back up, needles that are totally brown and most likely dead but the buds are fine and will soon cover up the loss, and some buds that also died and the branch or part of the branch will have to be cut out. Loosing the entire bush or tree is the exception, however, if you have lost an entire side you may wind up replacing it anyway. The tree on the left will fully reccover, the one on the right may not make it.
What can you do to prevent winter burn? The easiest thing to do is continue to water your evergreens up until the ground freezes. This may not prevent winter burn but at least it gives the plant more of a fighting chance to avoid it. However, the only way to totally prevent winter burn is to wrap the entire plant loosely with burlap. Burlap is generally superior to almost anything else, although old light-weight blankets also work. The important thing is to use something that is porous, the needles breathe even in winter and tarps or plastic (or wrapping that is too tight) inhibits the breathing.
Some people who had planted our Arbor Day transplants over the last couple of years commented on the fact that the part above the snow line appears to be dead, but the part below the snow line is green and healthy.
Again, wait until you are absolutely certain the top is dead. Then if it is, cut the dead side branches out, leaving the stem. Take one of the live branches and pull it up and tie it to the stem. This will give you a new leader. The tree already has an established root system, so it's better to work with it than pull it and start over again. You may have to retighten the tie to the stem every few days to help pull the new leader in close.