Without a greenhouse or garage protecting bonsai from the extremes of winter cold can be tricky. Having only a small courtyard garden this is a problem that I have encountered, all of my bonsai are relatively hardy particularly as I am most interested in native trees, but last night the temperature droped to -7C though and this gave me cause for concern regarding my shohin trees (less than 25cm tall from the rim of the pot). Due to their small root masses they are most vunerable to freeze and thaw and the associated drying that can occur (small pots dry out quickly by evaporation from the soil surface and it is impossible to water when it's this cold). Two of these trees are Acer burgerianum and have fleshy roots said to be vunerable to freeze and thaw.
They can't be brought inside as the heated house would bring them out of dormancy (certain death at this time of year) and though they are sheltered from the worst of the cold by the walls of house and courtyard this might not be enough. The answer? Well, it's an experiment but I have carefully lined a box with polystyrine tiles (a good insulator) thawed the trees out on the cold windowsill of my bathroom, watered them as they were looking a little dry already and packed them up carefully. The box is back outside in a sheltered possition, I just hope that is enough! I am also concerned about my Cedrus brevifolia (a native of Cyprus) but I don't have a big enough box or enough polystyrine so it will have to take its chances... after years of warm winteres I have been caught off guard... I guess I'll find out in spring whether these trees can cope!