I frequently hear people say they have not lost any Bonsai to what must be some of the hardest Winter’s on record here in the UK. In particular where I am down in the South West we peaked at -14c; maybe many others in Scotland will scoff with -20’s day after day but for here is was a record.
What can you do for protection? And, what is the likelihood of losses?
Unheated greenhouse is fine for evergreens, a garage or shed for deciduous, the latter not requiring light in anyway shape or form for probably a good 8 weeks so you can safely wrap them up in say burlap or similar for added protection. I do use background warmth to keep the greenhouse at 0c or perhaps -1 or -2c. It may seem cold but with -10c outside the trees will really appreciate it. I use one of these cheap to run convector heaters set on a thermostat, I have used paraffin heaters but they are not so easy to regulate. Same applies to the garage.
You may even have an unheated back bedroom which to all intents and purposes is at freezing, this then is fine for the deciduous trees, but watch for heat increase if the sun is coming through windows.
So what about losses? What tends to happen to our Bonsai which may seem fine after the Winter?
What can happen is the tree will carry on as per usual, buds swell, leaves appear, but perhaps in a slightly lesser form. The tree is, or has been weakened by Winter (Root damage and or rot through standing water), without knowing what is happening it will close down as quickly as it started growing and die. This then is the true test on Winter losses. Oh sure, obvious damage is apparent by looking. I myself have a specimen Firethorn with many almost black leaves. It is being tended to as I type in the hope it will make it. Trouble is this particular species is not that tolerant to a repot so chicken and the egg spring to mind. It is however fully protected, thinned out and showing good signs of recovery all-be-it slowly.
Repotting time for a weakened tree is all the more important and you should aim to take much longer looking for damaged roots or rotted roots and remove them. Be sure to seal any large cuts that you make to reduce the chance of infection (bit like us really, just apply common sense). Make up a good quality soil mix that is appropriate for the species and give a good dose of vitamins (say superthrive). I use trace elements in the soil when repotting and would restrict additives in inert soils to just this on a winter damaged or weakened tree.
As a final word; if the tree is not due a repot but you feel it may have been damaged by freezing conditions, take the tree out of the pot and have a look what is going on. You are looking for nice plump creamy to white root tips, these are more obvious on some species, and will look like honing down to a slight bullet point.
In this case you will almost certainly be fine, if they are black or jelly looking you have a problem and should seek further assistance if you are unsure on what to do next.