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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 bonsai, chinese privet bonsai and pepper tree bonsai.
  • 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 bonsai, 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 bonsai, larch bonsai, 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.

Experiences from Martin Schillings

Hibernating bonsai is, as can often be read in the current literature, actually not as problematic as the bonsai beginners in particular, but also enough of the more advanced among us think.

All "forms" of overwintering known to me have their advantages and disadvantages. So for years I have let my trees without exception (apart from indoor bonsai) without pot, let them hibernate in the garden without a single frozen branch or tree.

For the past two years I have put part of my collection in our gazebo. I left another part on the shelf or put it in a box (3m long and about 1m wide) covered with bark mulch and brought it through this time of year to protect it from the cold winds.

In the following I give briefly my (!) Experiences with overwintering bonsai, based on the following values:

  • Maximum minus degrees: approx. -15°C rarely, briefly below
  • Maximum maximum temperature: approx. +10 -> + 15°C
  • Precipitation amounts: far too much
  • Snow depth (no nonsense): last year for about two days 2-5 cm. After that, it usually thaws away again (skiers don't get far on their boards with us)
  • Sunlight: far too little for humans and animals (with us it's the season of long faces and the sale of antidepressants in pharmacies is increasing enormously)

Overwinter - sunk into the ground without a pot

  • Covered with approx. 3-5cm garden soil
  • Advantage: Moisture all over the period in all parts of the root ball. Additional cover with leaves or similar unnecessary.
  • Disadvantage: If the substrate is loose (and that's how it should be), you can rarely avoid repotting in the spring. Even firmer bales disintegrate in the bed over time. Something like that goes into the money somewhere (Akadama is good and expensive). No winter work possible.

Hibernate bonsai - with a pot in the ground

  • Advantage: No new potting, work that can be carried out in winter is feasible if the ground is not frozen through and for this reason simply does not want to "give up" the tree!
  • But these are the only advantages if you really put the tree with the pot in the ground
  • Disadvantage: Due to the generally loamy garden soil, the drainage holes in the pot clog, rainwater runs off very slowly or not at all, resulting in waterlogging and thus root rot. If moisture (assuming that the tree is protected from the rain by foils!) comes to the tree, then only from above.

Hibernate - with bonsai pots under plastic tunnels

  • The floor is hard (concrete slabs / wood / pallet)
  • Advantage: No new potting, work that can be carried out in winter is easy to do
  • Disadvantage: more frequent control of the substrate moisture is essential

Overwinter - With the pot in a plastic tunnel covered with bark mulch

  • Advantage: Additional (for us rather unnecessary!) Protection in freezing frost. Work that can be carried out in winter is easy to do
  • Disadvantage: (empirical value!) Bark mulch tends to get moldy when it is damp and "covered" by the plastic film

Hibernate - with the pot in the greenhouse (cold house)

  • Advantage: good protection (if adequately ventilated), no new potting. Work that can be carried out in winter is easy to do
  • Disadvantage: The temperature in our greenhouse easily rises to over +20°C when the sun is shining

Hibernate - with a bonsai pot in a dark, cold room (e.g. cellar)

  • Advantage: good protection, no new potting. Work on the bonsai that can be done in winter is easy to do
  • Disadvantage: Not suitable for pines and other "evergreen" trees

Overwinter - with the pot in a bright, cold room

  • Advantage: See above "evergreen", but the temperature should be well below + 5°C (maximum +8°C for trees such as Trident Maple / Acer palmatum Deshojo or Seigen)
  • Disadvantage: none from my point of view

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