Bonsai art’s charm lets us consider our plants as an ornament for our houses, but we usually forget the bonsai is a living creature that needs peculiar environments to live in.
It’s a plant before a work of art, and we must take this concept into account when we approach the bonsai art. Whether a bonsai should be put outdoor or indoor it’s easy to say: no trees were born to live indoor, and bonsai is no exception. However not every place in the world has the right climate for every plant. For example a ficus bonsai cannot live in the outdoor in Norway during winter and a red-deal will suffer and die if cultivated in Nairobi. So where to place a bonsai depends on where you live.
As general rule you can consider as outdoor species those native of your place (or a place with a comparable climate) and indoor all the tropical species. This is a superficial distinction and, whether a bonsai is an outdoor or indoor one, should be analyzed carefully. In every case, all bonsai, even those considered indoor, should be set outdoor during the summer. This is because our houses are lightless and heating systems create a dry environment, very stressful for the plant.
Those bonsai native of tropical places are naturally adapted to a continuous vegetation growth, at the opposite, those native of temperate climate stop vegetation during fall and start again when spring comes. When we adapt a tropical species to live in a temperate climate, we have to force it to have a rest during the winter, when it stays indoor. This is because growth during indoor periods will produce internodes too ample, leafs too big and weak, as well as poor lignifications, all due to the lack of light among other things.
To achieve this cycle, we have to force a strong vegetation when the bonsai is outdoor so it will need a lignifications rest when we’ll put it indoor. Another school of thought says even when indoor, the bonsai should have a vegetation growth. This is because its natural cycle is like that. I don’t recommend this practice for the reasons I listed before, unless you have a greenhouse reproducing the tropical climate with the right temperature and humidity tax.