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is no such thing as an indoor bonsai or an indoor tree. All bonsai
are naturally outdoor trees and as such are better cultivated
outside. The only exceptions are tropical species that can not
tolerate lower temperatures at certain times of the year and without
an artificial source of warmth would perish.
It is a relatively common sight to see indoor displays of Bonsai in books and magazines but it should be understood that these are only temporary. In Japan, Bonsai are commonly brought inside for 1 or 2 days at a time and are shown as a part of a tokoname display before being returned outside where they can recover.
There are two main reasons that temperate climate bonsai are not kept inside for any length of time. Firstly, all deciduous and coniferous trees need a period of dormancy which is only triggered by cooler temperatures. Without this dormancy, trees can continue to grow for anything up to 2 years before going dormant whatever the season or temperature; this enforced dormancy can often be fatal.
Secondly, it is difficult to provide adequate growing conditions for trees indoors. Inside, light levels are lower, humidity levels are very poor and trees suffer from lack of air circulation. For species that are hardy to frosts, it is far better to cultivate them outside all year round where conditions are conducive to their health and vigour.
Tropical species of bonsai on the other hand require temperatures above around 10-15°C (depending on individual species). In cool temperate areas of America and Europe this can mean that many tropical bonsai can only be kept outside for 2 or 3 months of the summer, the rest of the year adequate care must be provided inside.
Indoors, light levels are very poor. Though imperceptible to the human eye, light levels drop rapidly the further you are from a natural source of light. It is said that light levels halve every 50cm further from a window you are. This means that for most tree species light levels are too low, even when stood on a bright window-sill. It needs to be understood that glass filters out many of the UV rays that plants require for the process of photosynthesis; many trees species can fail to receive enough light on a south-facing window-sill even though the heat of the sunlight is burning their leaves.
Trees that do not receive enough light will fail to grow strongly if at all. Any growth that is produced will tend to have long internodes and become 'leggy'. Leaves will be over-sized in an effort to catch maximum light.
Some tropical species however are used to growing on the forest floor (ficus/ Serissa/ sageretia amongst others) in their natural habitat where there is naturally little light and these will cope with lower light levels indoors.
Trees grown indoors should be placed on sunny window-sills and/or provided with overhead fluorescent lamps. This should be sufficient for many tropical species but will probably still be too dark for ordinary woody temperate species.
Ordinary fluorescent lamps or aquarium lights kept about 6 inches above the tree for 12-16 hours a day can be used to supplement light levels.
Do not place trees on window-sills that are closed behind curtains at night, the temperature in between the window and curtain can rapidly drop below room temperature during the night.