Ask any bonsai enthusiast here in the UK about what they do to protect their trees during the winter and you will usually get a very confident answer; although there does seem to be variation in what people say. I will offer one piece of advice straight away, the smaller the tree the more protection you should give it, this is all dependant on conditions of course and there are no right or wrong answers. The more options you have the better off you will be, the more precautions you take the better off the tree will be. You can extend the growing season by using good protection and lessen the possibility of ill health or worse death. The vast majority of trees do require a period of dormancy however.
Some people do not use any protection for their trees over winter at all. Taking the view point that trees live outside in nature anyway, isn’t strictly true in bonsai of course as trees do not grow inside pots in nature and protecting the roots is key. Never the less larger trees, especially pines and junipers can be okay outside all year long, as long as the temperatures aren’t too extreme. Just because you can however, does not mean the tree will be any better off throughout the rest of the season. On the other end of the scale, try to take into consideration trees with fleshy roots like Yew, Redwood and Gingko, these should be given a better level of protection as this root type is more susceptible to damage.
For broadleaf trees, apparently keeping them in a well ventilated garage during dormancy is okay, as they do not photosynthesise. I’ve never tried this and obviously will not work for conifers as they require light all year round, unless grow lights are used in replacement. Grow lights are being used increasingly, and with LED technology may not cost much to run.
Another popular technique involves storing bonsai underneath the shelves one might use to keep the trees on during the growing season. A piece of tarpaulin for example can then be draped over the top of the shelves and around the sides overnight and in particularly cold spells helping prevent frost damage and control water levels. It is important to open the tarpaulin when possible to allow air to circulate inside and around the trees during the day (trees need to breath too).
A greenhouse is the ideal solution; but also the most expensive. It doesn’t necessarily need to be heated – the ideal to give a good constant average temperature which doesn’t fluctuate too much. Again opening the greenhouse during the day for aeration needs to happen, I didn’t do this once for a few days and a good amount of next year’s buds went mouldy and were no longer viable. While I am on this subject, try to leave as much room as you can around your trees, especially conifers to allow for better air circulation.
Most tropical trees should not be exposed to any temperature below 10c and therefore a heated greenhouse, conservatory or indoor space is required in the UK.
If you provide your trees with the best possible conditions you will reap the rewards.