Indoor Bonsai Trees’ Need For Dormancy

Can all bonsai trees be kept indoors?
A tree’s natural habitat is, of course, outdoors. We are often asked if a tree we categorise as ‘outdoor’ can be kept indoors – it is tempting, especially when the tree might be in flower. Bonsai trees sold in this country have origins in tropical, sub-tropical, and temperate climates and all can be kept indoors if given the right conditions.
The conditions that need to be met include: dormancy, watering, light, temperature, and humidity.
Trees from temperate climates (like ours) do need a dormant period in winter and keeping these trees indoors all year round will result in their death, usually in 2 years.

What Is Dormancy?

This is a survival strategy that temperate trees have evolved to stay alive in the winter. The tree has a biological clock which is governed by temperature and light levels, it tells the tree to slow activity and prepare soft tissue for freezing temperatures – that is, winter.
If you try to trick the tree into thinking it is eternal summer it will grow for two years and then go into shock, losing leaves, and this will probably be fatal.
Trees that hail from the tropics and sub-tropics need little or no dormancy. They are capable of continuous growth at temperatures of 20°C and higher, although their growth may pause at certain times of year. These trees do not need to spend any time in a cooler environment, although some sub-tropicals like the Chinese Elm and Pepper Tree will thank you if you do give them a rest period in a cooler room or similar place. The time to do this is when the leaves drop.

How To Help trees During Their Dormant Period

If your tree needs dormancy move it to a place where the temperatures will range between 0-6°C. However, try not to move your tree from a warm room straight into its winter home; that’s going from one extreme to another and would be stressful for your tree. If possible keep the tree in the garden in late autumn/early winter. The hours of daylight will decrease and temperatures will fall, thereby warning your tree that winter approaches.
If you have a garden keep your tree outside for 6-8 weeks, but protect it from sharp frosts. A garage or shed is fine – the tree will not need light while it is resting (but don’t forget to water it about once a week). If you live in a flat, place your tree in the fridge! The temperature inside will be perfect. Failing all else, move your tree to the coolest room and away from a radiator.

Cold Hardiness

Perhaps this is best explained by how much cold the tree will endure. Trees start to prepare for winter in autumn when they build up sugars and carbohydrates which act as a sort of anti-freeze. Most temperate trees can survive a couple of degrees of frost.

Summary

Don’t let the above put you off owning a bonsai! If the advice on dormancy is too daunting choose a tree that will live happily in the same conditions all year: the best choice is probably the Ficus bonsai or a Chinese Elm. Then, as you gain experience and confidence, experiment with other species. Any type of tree can be kept indoors IF you give it what it needs:

  • Light: nearly all trees need bright light. Remember that the glass in a window will cut out about a quarter of the light shining on the window, so even a south facing window may not supply sufficient sunlight.
  • Water: give your tree water when it needs it, try not to follow a set timetable.
  • Humidity: spraying your tree helps, but better still is to add water to the under-tray – add a few pebbles so the pot is not standing in water. Top up when dry.
  • Dormancy: As explained above, if your tree is from a temperate climate it must have a dormant period in winter.

Enjoy your tree!

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