Bonsai Nut
Nuttier than your average Nut
We should probably create a resource for overwintering trees. I would do so, if I weren't gutting our master bathroom atm. 
First, consider tree species. Some trees will die from the cold... some actually will die without it.
Second, do not focus on the branches, or the trunk. Focus on the ROOTS. Most deciduous trees have evolved so that branches and trunks can survive brutal conditions - as long as the roots are protected. The problem with bonsai (being in a container) is that the roots are far more exposed. When people talk about mulching, etc, it isn't to protect the top of the tree... it's to protect the roots.
Third, deciduous trees take a long time to go into dormancy... and a long time to come out of it. Spikes in temp - either high or low - are not good. Trees will suffer more from a warm spring followed by a late cold snap, than they will if the temp stayed brutally frigid all spring.
Within this context, a cold frame or unheated greenhouse will buffer swings in temp and weather conditions. It is not a perfect solution for all species at all times, but it is one more tool that you can use to protect your trees from Mother Nature's worst behavior
First, consider tree species. Some trees will die from the cold... some actually will die without it.
Second, do not focus on the branches, or the trunk. Focus on the ROOTS. Most deciduous trees have evolved so that branches and trunks can survive brutal conditions - as long as the roots are protected. The problem with bonsai (being in a container) is that the roots are far more exposed. When people talk about mulching, etc, it isn't to protect the top of the tree... it's to protect the roots.
Third, deciduous trees take a long time to go into dormancy... and a long time to come out of it. Spikes in temp - either high or low - are not good. Trees will suffer more from a warm spring followed by a late cold snap, than they will if the temp stayed brutally frigid all spring.
Within this context, a cold frame or unheated greenhouse will buffer swings in temp and weather conditions. It is not a perfect solution for all species at all times, but it is one more tool that you can use to protect your trees from Mother Nature's worst behavior
