The hibernation of Bonsai does not differ essentially from the hibernation of other, in pots held woods. In our opinion, hibernating a bonsai is uncomplicated. However, a few basic points must be considered.
Is the bonsai hardy ?
Depending on the home climate of a tree species, bonsai are more or less winter-hardy or frost-resistant. Roughly speaking, the Bonsai tree species can be divided into 3 groups according to their winter hardiness:
- Not hardy: Tree species from tropical and subtropical areas are not hardy. In their homeland, the temperatures don't sink below 3-4°C (exception in the mountains). They are genetically unable to resist frost. It is not possible to adapt bonsai from such areas to temperatures below 3-4°C. This group includes many room bonsai such as Ficus Bonsai and Crassula Bonsai.
- Low frost hardness: Temperatures around freezing point are tolerated for a short time. Such bonsai tree species come from areas where light frosts rarely occur. We call such bonsai Kalthausbonsai. This means that they need protection from severe and prolonged frosts. Typical representatives of this group are Mediterranean tree species such as olives and pomegranates, but also tree species such as the Chinese elm.
- Absolutely hardy: These bonsai come from areas with strong frosts in winter. Such bonsai can tolerate winter temperatures of -10°C or lower.
Bonsai overwintering - temperature and metabolism
The metabolism of plants is temperature-dependent. A temperature 10°C higher doubles the metabolism. All processes in the plant run faster and consume more energy. Reserve substances are degraded more quickly. These reserve substances may then be missing when budding winter-hardy bonsai. At the same time, hardly any energy can be generated. Many outdoor bonsai have no leaves in winter (for photosynthesis) and the supply of light is low due to the short days.
Do not overwinter them winter-hardy Bonsai too warmly. A cellar room is not a suitable place for wintering frost resistant bonsai.
The same applies to room bonsai. The temperature in a heated apartment is high, the metabolism of your room bonsai is running at full speed. At the same time the days are short, often cloudy and in addition the light supply in a normal flat is usually very low. Help your room bonsai by a low temperature in an unheated room. 8-10°C are perfectly sufficient for Indoor bonsai.
Frost resistant Outdoor Bonsai overwintering - Temperature and Winter Hardness
Most native tree species are extremely hardy. Healthy outdoor bonsai have stored many reserve substances (in the form of starch) in trunk and roots in late summer and autumn. Starch has little ability to attract water. As a result, the cells contain little water with lots of dissolved substances. The freezing point of the cells drops significantly. The bonsai is ready for the deepest frosts.
If the days become longer again at the end of December, the bonsai slowly become sensitive to higher temperatures. I.e. if the temperature is too high over a longer period of time in the middle to end of January, the bonsai starts to sprout. The starch is broken down into glucose by enzymes. Glucose is highly soluble in water. The cells of the tree begin to absorb water. The freezing point of the cells rises rapidly and significantly. The tree loses its frost resistance. Even very hardy trees can now be damaged by frost.
Important: Do not overwinter outdoor bonsai too warmly. Greenhouses and foil tents should be well ventilated in winter when the sun is shining. Often there are +20°C in the foil tent during the day and -10°C at night. No bonsai likes that. When overwintering by sinking the bonsai into the soil - choose a shady place for the winter quarters.
If a bonsai has started sprouting too soon due to high temperatures, it must be protected from frost.