As we know, all trees are outdoor trees and the terms "indoor bonsai" and "outdoor bonsai" only describe the place, where you grow your bonsai. In addition, to grow indoor bonsai is very difficult, since normally outdoor trees do not grow indoors if you do not grow them in the conditions, similar to their natural environments.
Hence, to grow indoor bonsai demands appropriate light, temperature, humidity, watering and dormancy requirements. Outdoor trees require a cold dormant period during which they accumulate energy for a new spring growth. Such species like juniper, pine, maple and the others, commonly referred to as temperate climate species, have a biological clock that tells them to slow their activity for cold temperatures. Species that have a dormancy period, while living in the nature, cannot be deprived of it in artificial conditions of living. If you allow such species as Japanese maple to grow in warm temperatures, it will grow continuously only for two years. After a maximum period of an extensive growth any temperate climate tree will automatically take its dormancy period regardless of the season or condition. It can be very stressful for a plant and almost hundred percent fatal.
Therefore, if you are planning to grow an indoor bonsai or even a bonsai garden indoors, your bonsai tree will need a cold dormant period. After a normal growing season, you should keep it in decreasing the temperature and shortening the day length. Most temperate climate plants need a dormancy period of about one thousand hours of forty F degree temperatures. After this period the plants will break the dormancy and begin growing again, as you bring your plant indoors.
Tropical and subtropical trees have modest or no dormancy demands. Particularly, most tropicals grow slower or not at all in particular times of the year, but it is not the period of dormancy, and, commonly, tropicals are capable of a continuous growth at seventy F degrees + temperatures.
Subtropicals, such as Chinese elms and Ulmus parvifolia, have little of any dormancy requirements. All depends on the environment they have been evolved in. Plants, having grown in colder areas, drop their leaves and go dormant. In milder environments, they are evergreen and show an extensive growth, except some occasional periods of slowing down.
Thus, in order to determine whether you can grow a plant indoors or not, it is necessary to consider its natural environments. Plants from the areas, receiving regular freezing temperatures, cannot grow continuously indoors. It will be your indoor bonsai if only you manage to satisfy its natural requirements for the dormancy period. Although it is not easy, many bonsai lovers are successful in adapting temperate climate species by giving them a dormant period annually. You can do it by putting a tree into a refrigerator, a cold garage or outside if the temperature allows. Then you can bring your plant back indoors to provide warmer temperatures for its future growth.
Growing indoor bonsai demands more than only learning how to manipulate its dormant period. It is far more important for a beginner to learn how to properly water, prune, fertilize and repot your indoor bonsai. It is better to start with tropical trees' growth, since they are not so "fault-finding" as their temperate climate relatives.