Winter months are approaching and perhaps you have still not decided how you will care for your trees; maybe you need a few ideas.  One of the biggest problems in many collections is inadequate winter care.  When we think of bonsai culture we often neglect to consider that winter care is such an important factor of good health.
       Some of my first experiences of wintering over bonsai would start by making preparations in September, digging giant holes, buying mulch, wood, plastic, brick and cinder block.  Although in most cases my trees did well in winter months, my budget & back suffered as a result of my perseverance with a lack of horticultural knowledge.  I can't give the best advise for everyone, since folks that live in extremely cold zones may need heavy protection.  Based on your own situation, with my basic measures used as a guide, you can move in a favorable direction.  I now start preparations much later, perhaps late October to early November with more confidence, and a small fraction of time and money on this task.

Winter temperatures

       Lets look at an interesting term to be concerned with: "heat transfer", a measurement of how heat moves from different substances at different temperatures. We generally don't think of heat as being 28F; but when it is negative 10F outside, 28F is a pretty warm place.  Let's consider three ways that heat is transferred to different substances, conduction, radiation & convection.  We know that our trees need to get cold and not thaw out and freeze again.  What we are basically dealing with is the capacity of the trees to hold a temperature.  Water has a high heat capacity, another term measuring how much heat is needed to increase the temperature.  Making sure your trees are watered before they are put into storage is most important, since it is this water that will prevent the transfer of temperature change.  Water trees the day before putting them into storage; the small air pockets formed in the soil add insulation to the root systems of your trees. Don't try to over water in winter. 

       Copper or aluminum wire, which has relativity low heat capacity, can be very damaging to tree limbs in winter, since most of the area of wire is exposed to the air and not conducting heat from the limbs of your tree and temperatures will change rapidly.  Before storing away your trees in extremely cold areas remove any wire. 

Setting Up Storage Area

       Your storage area should be a place that reduces heat transfer. Stay away from areas that have heat, such as the foundation of your home.  Although this area may seem like a great place... all it takes is one winter thaw and you can cause some severe damage to your trees.  Avoid keeping your dormant trees in a greenhouse.  Hoop houses that are set up for cold frames can be acceptable areas.  Protect your trees from the sun, the greenhouse effect of containers, and objects heating up that from the sun. 

Here’s a list of materials you’ll need

Plastic tarps – find relatively inexpensive tarps at your building supply house.  I use a discount warehouse that sells blue 10 X 20 foot tarps for about $4.00.  In spring I pack them up and use them again next year. Don't use clear or translucent materials… the less light, the better.

Wood strapping – Your local building supply house should have a good price on these.  I pay under a $1.00 each  and use about 20 pieces for two 8 feet benches.

Staple Gun & construction screws - Your work will move relatively fast constructing frame with use of a handsaw and power drill to drive screws.  I’ll often use some batten material to keep tarp from moving under staples.

Bonsai benches – My benches are very basic benches of heavy construction. Typically built about 3.5 feet high using heavy rough milled 4X4 & 2X10 lumber. The four legs are constructed with 4 X 4 material with 2 X 4 bridge supports. The tops of benches are constructed with 2 X 1 2 lumber about 8 feet in length.  A nice alternative is to stack concrete blocks with 2 X 1 2 lumber.

I gather 2 to 3 benches together using the wood strapping to frame in a base to attach my tarps.  Use two tarp layers with ½ inch  air space by adding additional strapping where needed and covering again with 2nd layer.  Make sure layers are secure and pulled tightly so they provide more resistance to outside air movement. After construction is finished, add a good layer of pine straw to bottom of storage area.

Wet and Dry Mulch

       When you place your trees in storage area, pack around pots moist mulch with a good amount of water content.  I use pine bark mulch most the time. Around the outside perimeter, use dry pine straw and add precautions for rodent control.  I cannot say enough about controlling those little rodents that will invade and destroy the many trees in your collection.  Now,  as I'm putting in the trees and mulching around them with moist mulch, I add about 4 inches of dry pine straw  on top of trees.  The dry pine straw  helps to dampen air flow  and provides a good insulation barrier.  When we think of insulation barriers, we consider materials that are poor conductors, such as materials that have dead air space. The pine straw and the double layer of tarps covering your storage area provide great insulation barriers.

The Last Minute Rush  

       I have found there is no better way than the last minute to put your trees away for the winter.  Meaning, a day early is a day too soon.  The trees and storage area must come to a cold state; it is much like packing a cooler with ice.  In our climate zone, I watch weather reports looking for that one cold week.   At that time, I might move trees into the storage area but won't actually lock down the sides of the tarps until temperatures drop into freezing zone.

More Insulation for your Trees

       The snow is finally covering your storage area.  Snow is one of natures best insulators from those dangerous low temperatures we get up north.  It's time to get busy and pack more of this great stuff around your storage area. In time, the weather could change and we need to protect our trees from these changes.  One of the reasons my benches are so strong is not just for esthetics… but to take massive amounts of snow weight.  The sides of the storage structure are double-layered tarps, and with a 2 foot wall of snow outside, we have very little change inside.  Temperatures could drop to negative digits and trees will stay at 25F.  As spring approaches,  the snow helps stabilize warming temperatures.  You want your trees to stay dormant and frozen as long as possible.  It could be the last snowstorm of the season; yet, it could be your last chance to shovel more snow over your storage area.

Spring Care

       Things can get chaotic if trees wake up too early.  The spring frost can take foliage off trees and often trees get stressed from the early season changes.  The first trees I remove from storage will be my pines.  Other deciduous trees will follow in time.  I then keep an extra tarp on hand to cover trees if I have a late spring frost.

        When your first flush of growth starts, when the maples push into leaf,  when the candles push on your pines, you will see the results of your taking the extra steps in proper winter storage and can look forward to a fantastic growing season ahead.