Over the years I have been writing articles for Bonsai4me.com I have been asked by enthusiasts in colder climates than my own, how they should provide Winter Protection for their bonsai. The problem enthusiasts have is that the majority of their trees are only frost hardy to approximately -10ºC but cannot be overwintered in an artificially heated environment.
My personal experience is limited to the relatively mild British Winters where temperatures very rarely drop below -8ºC, however, I recently received an interesting e-mail from Chris, a bonsai enthusiast in Alaska who explained to me how he provides protection for his bonsai in one of the coldest inhabited places in the world. Chris insulates his bonsai under a thick layer of snow that provides a reliably 'safe' constant temperature so effectively that his trees are able to tolerate the long Alaskan winters.
Chris explained to me that he doesn't see his trees from late October to mid April as they are covered by a thick layer of snow but he knows that they will be safe under the snow even when the air temperature drops to -40ºC.
Chris has two indoor/outdoor wireless temperature gauges with one of them placed under the snow at ground level along with his trees so that he can monitor the exact temperatures his trees are exposed to.
Above ground, air temperatures will stay at -20ºC for long periods of time, but even after weeks of those temperatures, the ground temp always remains steady at -3ºC to -4ºC. Cold but not killingly cold. Even after extended days of air temperatures near -40ºC, the ground temp never seems to drop below -4C because the snow acts as such a good insulator against the extremes of cold.
In this image of Chris's garden in Autumn it is just possible to see where he has added wind protection around his bonsai's winter quarters in the form of burlap (it's around 60cm high). The wind is not an issue once the snow gets to a certain level, but before that time, a little protection is a good idea in his area.
Chris uses wooden garden boxes to store his bonsai and then allows the insulating layer of snow to build up over and around the boxes..
Inside the wooden garden boxes he digs a hole in the garden soil for the tree to sit in and then lines the soil with several inches of gravel, putting the tree on top of the gravel. In the Spring when the snow starts melting there is a massive amount of water that needs to be drained off away from his bonsai so that they do not end up standing in cold water for any period of time, the gravel layer allows the excess water to drain away quickly and safely.
Next Chris adds weed guard on top of the pot (to stop the garden soil mixing with his bonsai soil) before finally adding a layer of garden soil as a blanket. The last preparation he makes is to top the layer of soil covering the bonsai with plastic (about 50% coverage) to minimize the amount of snow melt that will eventually seep into the bonsai soil.
After these preparations have been made in the Autumn (October) the bonsai then disappear under the snow until Spring arrives in April.
This image is taken from the same place after a one metre snow storm and it is possible to see how the snow blankets the entire garden!
A close up view of one of the snow-covered garden-boxes
One piece of advise Chris has for anyone contemplating burying their plants in snow for long periods of time is to watch out for mice, voles, and shrews who tunnel under the snow for the duration of winter. If allowed to they will completely strip the bark off deciduous trees. It's critical to protect the trees (particularly maples and birch, though pines do not appear to be as appetizing) with wire mesh. Finally, Chris tells me that he doesn't have to worry about moose until Spring!
Chris would like to hear from any other bonsai enthusiasts in Alaska and can be contacted at firstname.lastname@example.org