The best indoor bonsai are produced from species native to tropical or subtropical regions. Ideally, these trees should be placed outdoors in summer and brought indoors for the rest of the year. Otherwise, they should be kept indoors in a sunny window. In autumn, dormancy is induced by placing them in a cool, brightly lit room.
Proper watering is absolutely crucial. Before watering a plant, touch the surface of the soil with your fingers. If it feels damp, mist the bonsai instead of watering it. If the soil feels dry, add water until it runs out the drainage holes in the bottom of the pot. How frequently you need to water a plant depends on a number of factors, including room temperature, pot size and type of potting mix.
Bonsai need light in order to grow, and this is particularly true of those that bear flowers and fruit. When grown indoors, they should be placed in a south-, east- or west-facing window. The pot should be turned a half turn once a week so that all parts of the tree receive light.
Ideally, all bonsai should be moved outdoors in summer. It is important to acclimatize them, however, so as not to burn their leaves by exposing them to the sun too quickly. For the first few days, it is best to place them in the sun for only a few hours. If it is not feasible to move them outdoors, place them in a window where they will receive fresh air and plenty of sunlight.
Bonsai should be fertilized lightly, once a week, from May to September, at one-quarter to one-half the dosage recommended on the product label. Alter the type of fertilizer you use depending on the season. In spring, apply high-nitrogen fertilizer (28-14-14), in summer, a balanced product (20-20-20) and in fall, a high-potassium mixture (15-15-30). Don't feed a plant when it is dormant or sick. If you keep your bonsai indoors year-round, keep fertilizing until the plant stops growing.
Ambient temperature is very important, particularly from October to April. During this period, tropical bonsai (see Selecting species) require nighttime temperatures from 12°C to 18°C, 20°C to 25°C in daytime. Subtropical bonsai prefer cooler temperatures, 4°C to 10°C at night and 12°C to 16°C during the day. Outdoor bonsai will not survive if kept indoors in a warm room year-round. In winter, they need to be placed in a location where the temperature will remain near freezing.
To maintain or improve the shape of a bonsai, you will need to prune the branches and new shoots on a regular basis, along with the buds from time to time. In addition, young trees should be repotted once a year and the roots trimmed. The type of potting mix, container and tools you use are all important considerations. We recommend that you consult the bibliography at the end of this leaflet, and that you join a bonsai club or take a course to learn more about these techniques.