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The wintering of bonsai does not differ significantly from the wintering of other woody plants kept in pots. In our opinion, wintering a bonsai is straightforward. However, a few basic points must be observed.

Wintering of outdoor bonsai

Outdoor bonsai are hardy bonsai from tree species that can withstand even the lowest temperatures in winter without problems. They are genetically adapted to frost.

Although these hardy bonsai trees can withstand frost well, they should not be overwintered outdoors if possible. When it is frosty, the bonsai soil in the bonsai pot is frozen and can no longer supply water. Despite the pronounced winter hardiness, a bonsai kept outdoors can suffer damage from the lack of water.

The problem of lack of water in frozen ground can be avoided by overwintering the bonsai trees in an unheated greenhouse or foil tent. Sinking the bonsai into the garden soil (shaded, sheltered from the wind) also works very well.

In any case, you should avoid wintering outdoor bonsai indoors or even in the apartment. It is far too warm there and usually far too dark. Under these conditions, the bonsai use up their reserves and become weaker. They may even sprout out. Then even the hardest bonsai suddenly become frost-sensitive and can freeze to death even at temperatures just below 0°C.

Witch hazel (Hammamellis) flowers

Which types of bonsai are hardy ?

Depending on the home climate of a tree species, bonsai are more or less hardy or frost-resistant. Bonsai tree species can be roughly divided into 3 groups with regard to winter hardiness:

  • Not hardy: Tree species from tropical and subtropical areas are not hardy. In their homeland, temperatures do not drop below 3-4°C (with the exception of the mountains). Genetically, they are unable to withstand freezing temperatures. It is also not possible to adapt bonsai from such areas to temperatures below 3-4°C. Many indoor bonsai belong to this group, such as ficus bonsai, Carmona, Serissa, chinese privet and pepper tree.
  • Low frost resistance: Temperatures around freezing point can be endured for a short time. Such bonsai trees come from areas where light frosts rarely occur. We call such species mediterranean bonsai. That means they need protection from severe and prolonged frosts. Typical representatives of this group are tree species from the Mediterranean region such as pomegranate, chinese ash, mulberries and olives, but also tree species such as Sageretia, podocarpus bonsai and chinese elm bonsai.
  • Absolutely hardy: These bonsai come from areas with heavy frosts in winter. Such bonsai can tolerate temperatures of -10°C or lower during the winter. This group includes: maple bonsai, apricot, azalea bonsai, hornbeam, larch, redwood bonsai, elm trees and juniper bonsai.

Temperature and metabolism

The metabolism of plants is temperature-dependent. A 10°C higher temperature doubles the metabolism. All processes in the plant run faster and use energy in the process. Reserve substances are broken down faster. These reserve substances may then be missing when winter-hardy bonsai sprout in spring. At the same time, hardly any energy can be obtained. 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 hardy bonsai too warm. A basement room (cellar) is not a suitable place for wintering bonsai.

The same applies to indoor bonsai. The temperature in a heated apartment is high, the metabolism of your indoor bonsai is in full swing. At the same time, the days are short, often cloudy and the amount of light in a normal apartment is usually very low. Help your indoor bonsai by keeping the temperature low in an unheated room. 8-10°C are completely sufficient for indoor bonsai.

Temperature and hardiness

Most domestic garden bonsai are extremely hardy. Healthy outdoor bonsai have a lot of reserve substances (in the form of starch) stored in the trunk and roots in late summer and autumn. Starch has little ability to attract water. As a result, the cells contain little water with a lot of dissolved substances. The freezing point of the cells drops significantly. The bonsai is ready for the deepest frosts.

If the days get longer again from the end of December, the bonsai slowly become sensitive to higher temperatures. This means that if the temperature is too high for a longer period of time in mid to late January, the bonsai will begin to sprout. The starch is broken down into glucose by enzymes. Glucose is easily soluble in water. The tree's cells 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 warm. Ventilate greenhouses and foil tents well in winter when the sun is shining. Often it is + 20°C during the day in the foil tent and -10°C at night. Bonsai trees doesn't like that. When wintering by sinking the bonsai in the garden - choose a shady place.

If a bonsai started sprouting too early due to high temperatures, it must then be protected from frost.

Overwinter by sinking the bonsai into the garden

Overwintering of outdoor bonsai by sinking them in the garden is easy to handle and costs neither a lot of effort nor money. If the points listed below are observed, this method is very safe for the bonsai and easy to do even for the layperson

  • The wintering location should be sheltered from the wind (garden corner, house wall, hedge). Bonsai constantly evaporate water in winter (even without leaves). Wind increases evaporation significantly. In the case of permafrost, the frozen ground cannot supply water. The sunken bonsai can suffer from a lack of water. A sheltered location reduces the risk. If there is snow - simply cover the bonsai with it. Snow is a perfect protection against drying out.
  • Choose a shady location for wintering bonsai. This also reduces evaporation. In addition, sunshine with permafrost can cause frost cracks (mostly on the trunk). These are caused by strong temperature differences between the sunny and shady sides and can degrade the quality and health of the bonsai.
  • Dig the bonsai deep enough, but not too deep. In deciduous trees without leaves, a bonsai can be buried almost to the first branches. If it is not buried deep enough, the substrate in the bonsai pot (or the root ball) quickly freezes and cannot supply the bonsai with water. The root ball should be clearly covered with soil.
  • Check the bonsai regularly for mouse bites when overwintering outdoors. Mouse bites are not common, but occasionally they do. Tree species popular with rodents are: Above all apple tree (Malus), sometimes also Japanese maple (Acer palmatum), linden (Tilia), dawn redwood (Metasequoia) and spindle tree (Euonymus).
  • Sinking the tree with or without a bonsai pot ? Both are possible. If the pot is not frost-proof, the tree must removed out of the pot.
  • Tip: Whether with or without a pot - wrap the bale with a jute bale fabric. Excavating and cleaning up in the spring is so much easier and more convenient.
  • Be careful when using tools. A branch breaks off quickly with the spade. Often the use of the hands (rubber gloves) is faster and safer.

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