I lost but a single Gingko during last years wicked weather of alternating freezes and thaws and probably equally important it was due to the prolonged wet period throughout. I nevertheless stress the importance of winter protection. Mind you, this does not involve mollycoddling one’s trees, in fact I insist that most of these plants need a few weeks of frost, just to shut them down and perhaps kill off a few bugs.

That said, not all bonsai will survive the harsh winters that the Pacific Northwest can have, moreover the pots may crack under the expansion of waterlogged and frozen soil.  Both are expensive cost lessons to be learned.

Bonsai that are young without much bark tissue will not survive the USDA climate zone minimums that an adult tree in the garden would. Garden trees normally self mulch themselves as well, bonsai do not. Even in Clallam County we have many different climate zones, depending on the proximity of water, elevation and the factor of wind. You have to be aware of these differences and use common sense when building a cold frame for sheltering your small and shallow planted containers through the winter period.  

In my own one-acre habitat near downtown Sequim I view at least 10 degree F. differences depending upon the site placement of my plants. Windy zones, frost catchment basins or poor light zones. Most conifers need better light than I could provide under my shed. 

Therefore, I have always placed my evergreen plants under a cold frame that had a removable white plastic, not clear plastic covering over it. My deciduous plants would be under an overhang under my barn. These would be well mulched with bark, lavender cuttings or fir bark or whatever is sterile and weed free.
Cold frames are usually constructed on the north side of a building. This may work for deciduous plants, but Pines and Junipers need winter light. I have therefore built mine on the east side of my barn and have it oriented North to South. Orienting them otherwise will cause them the heat up to much. Wind protection is equally important and most of our wind comes from the west, so my barn blocks that. This N/S orientation also gives me the option of leaving the two ends open for air circulation. Doing otherwise would allow strong winds from the west and east to rip the covering off.
These were move able structures as I have multiple uses for them. This year I need to replace the wood and I need to decide on 2 x6 treated wood (unmovable) or lighter plasticized decking product.. Material needed: One inch PVC (ideally spray painted black)and joining, wood, bolts and screws.  These have only lasted for only fifteen years.  Storing the plants outside raises the risk of damage from mice or other small animals, so plan accordingly. Chicken wire, mouse bait and other controls might be considered.  

Fallen leaves could also be used to add extra insulation. For those plants left out in the cold added problems exist, namely over-wet or waterlogged conditions, that can lead to root or basil stem rot. Leaves can create a problem however in holding in too much rainfall, so I have started collecting lavender stems. In five years of over-wintering plants this, pine needles or bark have proved to be the best mulching material for me. 

Shallow pots such as the forest below; create a further problem due to surface tension. These should always be tilted with a foot under one end to provide extra surface runoff.   Mulched at an angle. This can be reversed as the months progress.   
In general, potted trees in the garden should be buried up to the soil level in the pots and further mulched with straw, or bark especially around the trunk space. Wooden pens or concrete blocks might be used to allow for an enclosure, over which white polyethylene can be draped over during severe weather. As this could collapse under a snow load, it should be removed any time a snow event might occur. Rigid fiberglass would be better.
Be it made of concrete as is Bill's, or of wood, they can be easily modified for seasonal needs. In winter a simple 4x4 could be laid in the middle with some 2x2's on either side. The mulched plants nestled within could be quickly covered with a tarp and anchored through the grommets to some weights beneath. I personally like those bricks that have three holes in them. Simple to do! Cover during exceptional freezes or heavy rains, then remove.
Such mulching may mitigate the cold issue, but my experience has been that drainage and excess moisture in pots may remain the biggest problem. By spring’s retrieval the root system becomes a rotten mush that is usually fatal. Many cold frames also create a secondary problem and that is ventilation. A sealed in cold frame under plastic with a snow load can render plants susceptible to a number of fungal diseases, resulting in a weak plant. Moreover some trees such as Pines require those chilling periods and if kept in a too warm greenhouse will only produce weak and spindly growths that are a waste of plant energy. Proper ventilation and a regulated greenhouse temperature is a must. These concrete holding frames would be great for mature Pine or Juniper, less so for young maple trees, elms or even the so called "hardy" Gingko. I would point out that concrete or wooden bins should not be lined with plastic, but with a ground cloth that drains. This is ground cloth!  
Some simple guidelines for protection and daily care in the winter
If living in a place where the winter brings freezing conditions or even snow, one has to take some precautions to bring the trees safe through the winters. Dry powder snow  is good... ice rain is not, nor is heavy wet snow!.Always brush snow carefully off your trees as the accumulations might worsen, freeze and accumulate more. … breaking limbs.  

Be well aware of the species you have as bonsai. Are they in general able to live in the climate you are living in? How cold hardy are they? Chinese Elms, Cotoneaster, Pyracantha, Monterey Cypress, Nothofagus, Podocarpus may need to be put into a protected cold frame or cool greenhouse. The hardiness of Japanese and Korean Maples may vary considerable and this link explains why.

The cold greenhouse 
I always place my bigger bonsais in a cold frame, or in a cold (marginally heated) greenhouse, around the middle of November. Most of my trees need the touch of frost to kill over wintering insects and eggs in the bark, and to rest properly. By late March to April I usually start bringing them out. Usually too early, and I find myself having to bring them back in again.  That error cost me dearly, not only because of shifting weather conditions and frost, but also because of heavy steady rains that these just wakening plants do not need.  


That’s usually around the end of  March, but usually comes in April. It all depends of the weather, and you will find me bringing some of my bonsais in and out a few times, because of shifting weathers. But only in the start and ending of the sheltering period. It can be necessary to protect your plants from heavy steady raining periods in late autumn as well. What may work here in dry Sequim, will not work in P.A. or Pt. Townsend or at higher elevations that get more rainfall.
Protection from cold winds and sun

Less well understood are that bonsai need protections for the cold winds. It is very important to keep any bonsai from wind exposure when their pots and soil are frozen, because their roots are not able to take up frozen water. Try sucking up an ice cube with a straw and you will understand. A cold frame covered with clear plastic may also create a problem, especially with evergreen species. Even dormant plants will continue to   transpire through the bark and buds. Dormancy does not mean a dead stop, the plants may be slowed down very much, but the tree continues to be active through the whole winter period.

Hibernation might be a better word and more easily understood. Finely branched deciduous trees like elms or zelkovia are damaged when cut off from light during the winter period. When you scrape one of these fine branches, you find a green chlorophyll layer under the thin bark. Even though leafless, these fine branches continue to  photosynthesis during above-freezing winter temperatures. When cut off from light at such temperatures, photosynthesis stops but but respiration through bark and buds continues, depleting the carbohydrate reserves of the tree. 

A glass or clear plastic greenhouse will allow enough sun in to heat up the trunk or leaves to a quite warm degree. If the plant is frozen, this can lead to cracks in the bark between the two temperature zones. The tree might also suffer from dehydration, as the leaves will attempt transpiration although nothing is coming up from the frozen earth. 

Smaller bonsai, or shallow potted trays will show the greatest stresses in this regard and should be especially protected.  

Trying to recover root damaged plants. 

Oxygen plus' might be one mitigating commercial chemical that might help recover some root-damaged plants. I have more suggestions that relate more to the mature garden tree, but some of the tips may apply to bonsai as well. The link is to my former  blog "Pacific Northwest Gardening."  You may have to trudge through a 'mole article' but the following blog relates to Climate Zones. 

(C) 2009 Herb Senft.  

I found it amazing that no local member has ever made a comment on any of these web postings, yet a Chinese bonsai site and a customer of mine from Maine would make comments. (Paul P.)
"I live in a condo unit and my small area (12' x 20') is protected on all 4 sides, house and cedar fence on 3 sides.
Plants are placed among the in ground planting, against the house, under benches, and all are covered with pine needles.
The temp. has gone as low as -15 degrees F.and I only lost 1 forest of 7 trees that was in a shallow pot. My pots are now deeper."

"Another thing I do with my J. maples is put them in a lager pot filled with pine needles and cover the whole thing with more pine needles. Also there is a large pine in the yard that stops frost from settling on the plants."

I too have used needles as a mulch, in my own case Cedrus deodora. They do not blow away as do leaves and are easy to apply. Since I also manage a few income chagrined lavender farmers, I can harvest their stems and use them as mulch and as packing material. The double packaging that Paul P. used is great and something I had not thought to mention

Thank you!