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Tropicals and similar 'houseplants' can be successfully trained for bonsai and grown indoors year round. In mild climates, temperate bonsai should remain outdoors year round. In cold climates, temperate climate plants should be grown outdoors during the warm seasons of the year, but will need winter protection (the subject of another article). It is possible to grow temperate climate plants indoors in winter if they are first given the required period of dormancy.
The urge is strong for beginners to grow their bonsai indoors. Although a few traditional species for bonsai may be grown indoors year round if they are given a dormant rest period, you should be aware that this requires some skills usually obtained from growing bonsai for a few years. Below are some procedures that should be followed for indoor bonsai.
Consider that, outside, the light comes from not only the direct sun, but from 180 degrees of sky PLUS all the reflected light of objects in the other 180 degrees. Light from a window is little better than a point source of light. If you measure the light level with your camera (not pointing it directly at the sun, but obliquely to get an average reading) you will find that the level inside is two to three f-stops lower than just outside the window. One f-stop would be half as much light, two f-stops is 1/4 as much light, etc.
From experience I can tell you that most woody plants will perform best at full sun to 50% full sun. I get 50% by growing plants under shade cloth. Less than this amount, performance falls off, and at 70% shade, plants get leggy and problems can begin.
You can correct this by putting your bonsai in the sunniest window of your dwelling, but not too close to the glass or it will experience excessive heat buildup. This light may be too intense for some tropicals that are used to growing on the forest floor, but for most woody temperate climate plants it is still insufficient. Couple this with an overhead fluorescent lamp for these species. Keep the lamp about six inches above the plant. Twin forty watt fixtures are inexpensive to purchase and use. Special bulbs are not necessary. Keep the lamp on 12 hours a day to augment the sunlight. If you lack a window with sufficient light for even low light tropicals, you can safely use fluorescent lamps as outlined above as the only source of light.
Without this dormant period, deciduous species will continue to grow for as much as two years, then go dormant no matter what the season or temperature. This can be very stressful for the plant, and it in some cases fatal.
Outdoor watering in sunny summer weather is easy, the plants dry out every day, so you water them everyday. Indoors is more difficult. You should only water when the plants need water, not to schedule. The interval will vary with the light level, temperature, degree of colonization of the roots in the pot, amount of foliage, and the humidity.
There are three basic methods to determine if the plant needs water. My favorite, not necessarily the best, is to simply pick the plant up. Dry plants are significantly lighter than well watered ones. You can easily learn the difference. The second is to scratch the soil with your finger and see how dry it is under the surface. It varies somewhat with soils and volume, but if the soil is dry down past a quarter of an inch, it probably needs watering. If the surface is still moist, it most definitely does not need watering. The third method is the infamous Persiano Pick, a method devised by one our illustrious Internet Bonsai Club (IBC) members. Use a piece of chopstick or wooden skewer as a sort of dipstick. Leave it in planted in the pot. To test for water pull it out and check the moisture on the stick. If the stick is dry or dryish, it's time to water. All of these methods take a little practice, but you should be able to satisfactorily learn one or all of them in about a week.
Air circulation in summer can be as easy as leaving the window open to get a little breeze. In winter you might want to get a very small fan to gently waft the air about your plant.
Humidity is greatly over emphasized for temperate climate plants kept indoors. For many beginners, misting is mantra, a way of showing that you care, but many plants could care less. Many tropicals need high humidity, temperate climate plants do not, but it is important not to let levels approach desert aridity, which can happen inside during the winter. Misting plants once or twice a day in my opinion is a worthless procedure. The only thing that happens with misting like this is that you build up salts on the leaves as the water drys. If you mist the entire area around the plant to bring up the humidity, then you are doing something useful, but in the house this is usually not possible. Instead, create humidity by placing your plant on a bed of small stones in a large flat tray filled with water. Do not let the pot actually touch the water. The flatter the tray the better, this will help keep down algae and other critters because nearly the entire volume of water will evaporate each day.
Mites, aphids, woolly aphids, mealy bugs, scale, and nearly all potential pests can be controlled if caught early and sprayed with an appropriate insecticidal soap. These soaps may be phytotoxic to some species and cause leaf damage, so try only a few leaves first to make sure. Repeat sprayings are necessary to get the new hatchlings. Once every five to seven days is sufficient if done three times in most cases. For very difficult cases you may have to resort to chemical insecticides. Always follow the instructions exactly. Systemic insecticides can prevent infestations of some species of insects, but you must be aware that you are living in the continual presence of a deadly chemical. This can be of particular concern for indoor plants.
Fungal and other diseases are, in my opinion, more difficult to diagnose and treat. Without an organism, such as insect, to identify, diagnosis becomes very difficult, even for the experts at times. Diseases in plants are most often the result of the presence of an organism, and not the organism itself. This is especially true of fungal diseases. Most fungal diseases of container plants are caused by creating environments more favorable for the organism and less favorable for the plant, so the place to begin is with the environment. If you are getting root rot, shift to better draining soils, water less often, give it more sun (if appropriate), optimize growing conditions to get the roots to quickly colonize the soil. If you are getting leaf fungal diseases, decrease the humidity, stop misting, give it more light and air circulation. If these do not work, seek help in identifying the problem so that you can choose an appropriate fungicide, if one exists.
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