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Most species of bonsai and nearly all temperate climate woody plants require the cold. During the Autumn, as daylight hours become shorter and temperatures drop, trees react by hardening up their immature growth. Stem tissues begin increasing their levels of sugars and carbohydrates which by the time the first frosts arrive, act as an antifreeze to ensure that the plant itself will not freeze. Deciduous species lose their leaves to reduce moisture loss and all growth stops for 4 or 5 months. (Coniferous species have thin, waxy needles that reduce transpiration to a minimum and this allows them to stay evergreen).
My bonsai after a snow storm in March 2006DORMANCY
Eventually as winter arrives, trees have completed their natural defensive system against the cold of winter; dormancy. In the Spring as temperatures rise, new buds on the trees will start to extend and unfurl their first growth in Spring, completing an entire years' growth cycle.
Some bonsai beginners feel that their trees may perish if subject to the harsh conditions of the winter months and bring their trees indoors to 'protect' them. This continuation of heat and light through the winter prevents dormancy in temperate trees. The resulting continual growth throughout the year goes against the trees' internal clock which is requiring a dormant period, the clock can be tricked to an extent; the tree will continue to grow inside. It may even grow continuously for as long as two years after which, whatever the season or conditions, deciduous species will drop all leaves and evergreens will stop all growth. This out of season dormancy usually results in very sickly trees and even death.
HOW LONG A DORMANCY DO TREES REQUIRE?
For areas with mild winter temperatures, providing a natural winter dormancy can be difficult;it should be noted that all temperate woody plants require a dormant period where temperatures drop to less than 10C continually for a period of between 260 hours and 1000 hours (depending on individual species). This equates to a dormant period of between 11 days and 42 days of continual sub 10°C temperatures.
TROPICAL AND SUBTROPICAL BONSAI
The only exceptions to this need for dormancy are tropical and subtropical species. These are outdoor trees only during the warm summer months in many temperate areas of the world. To keep them outside during the winter would be fatal. Tropical and subtropical trees have very modest dormancy requirements and in their native habitats are capable of continuous growth all year round at 20°C or above.
For this reason tropical and subtropical trees are resilient to indoor growing conditions during the Autumn, Winter and Spring.
The truth is that nearly all temperate woody species are reliably hardy down to -10°C (15F). Below this temperature all that is required is some protection for the root system which is not as frost hardy as the top growth. To become reliably hardy to these temperatures, trees must fully harden up in the Autumn by being grown outside. Trees are able to survive sub-zero temperatures by virtue of being dormant. Trees that are purchased during the winter should be kept in similar conditions to those that they had been kept in by the retailer, even if this was indoors. To put a tree outside in the middle of winter when it has previously been grown indoors and is still active would kill it.
My Privet (Ligustrum) Bonsai
THE EFFECT OF FREEZING ON BONSAI
The rootsystems of our bonsai are the most susceptible part of the tree to damage from the cold. In nature, a trees roots' are buried into the ground and are rarely subjected to freezing temperatures. Whilst the surface of the ground may freeze, this will only affect the top few inches of the soil. Below this the cold is unable to penetrate deep enough to freeze and the trees' rootsystem remains unaffected by the above ground temperature. Bonsai however, have their entire rootsystem above ground level in an often shallow pot where the soil is easily affected by prevailing air-temperatures.
The top growth of trees in nature however, is subject to the full force of winter and is able to withstand temperatures far lower than the rootsystem ever could. Though the rootsystem hardens up in the Autumn, it is to a lesser extent than the topgrowth.
Damage to topgrowth only usually occurs when the ambient temperature rises during the day whilst the water in the ground or pot is still frozen. This situation can often arise in greenhouses during the winter and also outside in areas where there are large fluctuations in temperature between day and night. As temperatures rise the leaves start to transpire but the roots are unable to take in replacement water from the frozen soil, causing the top growth to dry out, resulting in dieback. This problem can also be aggravated by wind which also results in moisture loss from leaves and shoots.
When we see the soil in our bonsai pots is frozen in winter, it is easy to think likewise that the tree itself is frozen. In fact, it is the water in the soil that is frozen, not the soil and importantly nor the roots of the bonsai itself. If the roots of the bonsai were to freeze, it would be fatal.
During the Autumn, the tree stores a mixture of sugars, sugar alcohols and proteins that act as an antifreeze, so whilst the water in the soil may have crystalised into ice, the tree itself is still fluid. It is not until the temperature of the soil drops below -10°C that there is a threat of the rootsystem freezing.
There is a variation in frost-hardiness between different species of trees and naturally shallow-rooted trees such as Azaleas are hardy to far lower temperatures than species that are typically deep-rooted. Some species such as Trident Maples and Magnolias are more susceptible to frost damage and protection from temperatures warmer than -10°C should be given.
When outside air temperatures drop below -10°C, the pot needs to be afforded some protection to stop the temperature of the soil dropping to the same level.