January 2007 was similar to this December-January '11-'12. That cold dry year the ice skating was spectacular voles damage was minimal and many plants suffered.
The process of preparing for winter in hardy plants goes something like this: Plants sense shortening days and cooling temperatures and produce chemicals to start the processes of dropping leaves or closing stomata (the holes they “breathe” through). Food is moved to the roots and important compounds in leaves are recycled and stored away. When freezing begins, water moves out of the cells and into the intercellular spaces (between the cells). This water freezes, but the cell’s contents, with higher concentrations of sugars and salts, have much lower freezing temperatures (like salt water or anti-freeze). As temperatures drop, more water moves out of the cells and solute concentrations in the cells increase, and freezing temperatures drop further. The cell membrane, which is inside the rigid cell wall, actually pulls away and makes room for the ice crystals between the cells.
If temperatures drop too quickly, water cannot move out of the cell fast enough, ice forms inside the cells and in pores of the cell membrane. As you might imagine, jagged ice crystals inside the cells rip them apart and if enough cells die, the plant dies. This is damage we see frequently suffer in spring.
This winter, before it finally began to snow, the days were sunny the nights were very cold, the north-east winds were blowing and several things happened.During the warmer sunny days, plant tissues warmed up enough to thaw and begin photosynthesis. Cells woke up and filled with water. At 3:00 PM in mid January, the warm afternoon sun had the plants thinking it was spring, just before the sun went down. The air temperatures were already below freezing and without the sun on the stems, leaves or needles, the temperatures plummeted and many plants suffered – This damage often shows-up as “freeze-cracking” , split bark or tissue damage on the southwest side of trees.
In many other locations the dry wind and sub-freezing temperatures caused the ice between plant’s cells to sublimate (change from solid to vapor). When the ice around the cells was gone, the cell membrane was exposed and the little moisture remaining in the interior of the cells dried up – This is “freeze-drying”.
This winter, some plants just dried-up. The soils became so dry that even roots died. I lost 2 of 7, 14 year-old currants. Sometimes there is no telling why some one plant dies and another survives. In the wild it is the same story, one manzanita is dead and 3 feet away another is fine and 12 feet further another is dead and so-on. It could have been one branch of a pine 30' away provided a few extra crucial minutes of shade in mid January or the one plant's roots just happened to be under a large rock... it is fascinating and frustrating at the same time.
In home landscapes, many people may have saved their plants by watering in January and people with 3-4" of mulch throughout their garden suffered far fewer losses than most. NEVER underestimate the wonders of mulching.
In the nursery, we lost huge numbers of plants in pots this winter. We tuck the plants together for the winter and put shade around them to trap snow but this winter they froze and dried. You cannot water a frozen container plant because the water freezes and suffocates the roots so we tried to lightly mist them and just raise the humidity but it was largely ineffective. We really need a cheap used snow-making gun for winters like this one (many larger nurseries in the mountain west have them.)
We don’t have a huge variety of broadleaf evergreens that grow well here but there are a few. Manzanita, Huckleberry Oak, Live Oak and Ceanothus and Mt. Mahogany are some of our broadleaf evergreen natives. Many of these suffered this winter, especially those exposed to the north-east winds (see photo of fried manzanita and dead squaw-mat). I have not seen damage on any Mountain Mahogany.
I’ve also seen damage on Native Incense Cedar, Giant Sequoia, Lydia Broom, Hardy Holly, Hardy Rhododendron, Dwarf Alberta Spruce (it often suffers sun scald), Cotoneaster and Bear-Berry Manzanita.
We are still waiting to see what has survived but many are pushing some new growth. Meanwhile, we've fertilized with Biosol and Dr. Earth and with seaweed to stimulate roots.