Germany is located in the temperate climate zone with the typical alternation of warm and cold throughout the year. Native plants have adapted to these conditions and thus characterize the seasons of spring, summer, autumn and winter. In spring, nature awakens to new life. The first leaves and flowers appear. Summer is the main growing season and the fruits ripen. In autumn, the plants finish growing, and beautiful autumn colors emerge as the leaves store nutrients. In winter, the plant goes dormant and stops growing completely.
Native trees and shrubs generally overwinter without any problems. Species with sensitive leaves shed them as they are not frost-hardy. Sometimes they remain dry on the tree and thus form a natural evaporation protection for the entire plant. The roots are well protected from frost in the soil. Alpine woody plants sometimes disappear completely under masses of snow and are thus protected from cold and drought.
A bonsai in a pot is initially defenceless against the cold. The roots often cannot tolerate sub-zero temperatures and some of the above-ground parts of the plant are not frost-hardy either. Plants that naturally originate from warmer climate zones are particularly at risk here. The frost hardiness of the respective plant species therefore determines the choice of winter habitat.
In addition to low temperatures, dryness in winter is a particular danger for bonsai. A frozen plant continues to evaporate water and would dry out as it cannot be watered during long periods of frost. Adequate evaporation protection is therefore particularly important. There are various options here:
In a greenhouse, the bonsai are protected from the sometimes very dry winter air. For sensitive species or very low temperatures, the room should be kept frost-free with a heater.
Bonsai can also be buried very well in winter. The surrounding soil protects them from drying out and low temperatures. However, the place should be as sheltered and shady as possible. If you don't have a garden, you can also wrap the plants in a large box with peat and protect them from the cold, sun and wind with white foil or fleece. Bonsai can also be successfully overwintered under a covered garden table.
In nurseries, plants are often overwintered under foil or fleece tunnels. When using foil, however, it is important to ensure that it is removed in strong sunlight due to heat build-up.
Some unheated cellars and garages are also very suitable for overwintering. At low temperatures, there is no need to worry about a lack of light, as the plant is in a dormant phase.
Even in winter, the soil must not dry out and it must be watered as required. However, waterlogging should be avoided at all costs for all plants.
Too much heat in the winter quarters can be dangerous. Plants start their metabolism at a temperature of around 10 degrees. However, as soon as roots and shoots begin to grow, almost all plants are sensitive to frost. This means that the bonsai would be damaged at night when temperatures fall below zero. For this reason, all winter quarters must be sufficiently ventilated or shaded when exposed to sunlight so that temperatures do not rise too quickly. In spring, the plants must be protected from late frosts.
Plant species from the subtropical climate zone (e.g. Chinese elms, olives, firethorn) are not frost-hardy and also require winter dormancy. The light conditions in Germany are also insufficient in winter. It is therefore best to keep the plant in a cool place. The hallway or another unheated room is usually sufficient.
Plant species that are adapted to the tropical climate do not go dormant in winter and should be kept warm all year round. However, as there is less sunlight in winter, it is essential to choose a bright location. Ideally, the plant should also be provided with special plant light during the dark season. Indoor bonsai should continue to be fertilized in winter. However, it is sufficient to do this once a month.
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