Everybody who has bonsai knows the worry at the end of winter. A tree is a little slow in responding to warmer weather. No bud is swelling. No green line is showing. Has the bonsai died over winter? This post is part of a
Why do bonsai get damaged by low temperatures in winter?
The main damage to trees in winter caused by the cold is the result of ice crystals forming in cells, rupturing the cells and thus destroying the living plant tissue. The temperature at which ice crystals form, is different of different plants, and is dependent on the time-specific hardiness of the plant. Typically, it is roots of bonsai that get damaged by low temperature, rather than the above-ground parts of the tree. This is because the roots of trees are normally firmly buried in the ground, and subsoil temperatures normally do not drop as far as air temperatures: Plants protect the roots less than the above-ground plant. If you have frost damage above-ground, it normally is not the cold, but wind or sun damage.
How do plants protect themselves against frost
Plants have a number of mechanisms that stop them from dying from cold. As days shorten and temperatures drop, deciduous trees drop their leaves, and reduce the amount of water in ranches and trunk so it cannot freeze. Furthermore, plants produce a natural form of anti-freeze that lowers the temperature at which water in cells crystalize. All in all, this effectively protects plants against frost levels that occur at their natural ranges. Note that plants that originate from regions without frost, typically cannot handle ANY frost. Ficus, for example can die from as little as 1 degree of frost for a night.
How does frost resistance in bonsai vary?
Each plant has a natural cycle of frost resistance. When the days get shorter and temperatures drop, plants start to create frost resistance as outlined above. The plant will however not create maximum frost resistance at once. Rather, every time the temperature drops, it will build op higher resistance. So a -10C frost in October may do massive damage, whereas many species will be able to deal with it come January after several frost cycles.
After some 90 days of cold weather (Daytime temps below 14 degrees or something) the hormonal balance in the tree shifts from winter to spring modus, and warm days start triggering a reduction of frost resistance, and even budding may occur in some species. This is very clear in the warm winters of the past few years, where in February some plants showed buds starting to swell. This means that frost resistance has been reduced and plants can get severely damaged by a subsequent period of deep frost. (note that this mainly is valid for deciduous, rather than evergreen trees, who are a lot hardier overall)
What to do to avoid bonsai mortality of cold in Winter
The most simple thing to do is to know the temperature range your trees can handle under normal circumstances. An olive, for instance, will suffer from frost, but may survive up to -5 degrees Celsius. An oak however will withstand -20 Celsius when properly dormant. So check the species specific hardiness, and take care to realize that root-temperatures are most important. In order to insulate the roots of the plants in pots, I typically place them on the ground, rather than on the stands when temperatures really start dropping. This connects the roots to the ground, reducing spikes in the root ball. I than normally spread some leaves between the pots, and put a wind shelter around all the pots, typically in the form from some leftover bricks I have laying around.
When you want to go one step safer, you can take the bonsai out of the pot, and place the root ball in the soil in your garden. That way the trees roots get most protection against the cold. Naturally, if you have to opportunity, placing the trees in an unheated garage will also shelter them against the cold. Do not however, place them in a frost free room unless the species is not frost resistant. A period of low temperatures has been shown to have a positive influence on tree health.