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Growing Bonsai Indoors

by Brent Walston


One of the most common misconceptions about bonsai is that they should be grown indoors. With the exception of tropicals and sub tropicals, all bonsai should be grown outdoors. Temperate climate woody plants must go through a period of cold dormancy in order to survive. This dormancy completes a yearly cycle. In deciduous trees this is a very obvious phenomenon, however, temperate evergreens such as Juniper also need to go through this cycle.

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.

The Need for Strong Light

The major problems in indoor growing are the lack of intense light and a cool dormant period for temperate climate plants. Even if you kept your plant in an unobstructed south facing window, I doubt that the light would be sufficient for many species of woody plants. Most people just don't understand how dark it is in the house, even in front of a window.

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.

Providing a Dormant Period

The other major problem is the lack of a cool dormant period. Temperate climate plants, even evergreens, need a dormant or rest period that is activated by cooler temperatures. For most plants, this is a period of at least six to eight weeks at temperatures of 30F to 40F or lower. Most temperate woody bonsai can tolerate temperatures down to 20F without any protection. Many plants will of course lose their leaves at this time. Other species such as Chinese Elm, Ulmus parvifolia may or may not lose their leaves during this dormant period. At mild temperatures (above freezing) they will remain evergreen, losing all of their leaves during the following season, but not all at once, so it appears evergreen. Kept outside in zone eight or less they will nearly always be deciduous (depending on the cultivar and severity of the winter). Temperate evergreen species also need a cold dormant period, but will not lose their foliage.

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.


Another problem is watering. Virtually no one with extensive experience in bonsai recommends watering by immersion, or watering to a schedule. This is a marketing scheme to make raising bonsai seem easy. Watering by immersion occasionally is fine, for instance if the plant is extremely dry and needs immediate resuscitation, or if it has just been repotted, or if you want to soak it in fertilizer solution. Watering by immersion tends to accumulate salts in the soil. Watering from overhead will help flush salts and waste gases from the soil. Water overhead from a watering can with a fine rose head, 'throwing' the water at the plant and waiting a few seconds for it to absorb the water, before giving it another toss. Do this until water pours from the drain holes. This will reduce runoff and keep from eroding the soil. For a single plant this might take thirty seconds. Never let a plant sit its own drain water (the rare exceptions are Wisteria and Bald Cypress growing outdoors).

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.


You can fertilize to a schedule, and that is probably the best way. Most soluble fertilizers recommend that you fertilize full strength every other week. Simply water it in. Always use them as directed. Forget the half strength stuff for bonsai, this is a myth, use them as directed. Begin to fertilize when the plants start actively growing and stop at the beginning of the dormant period. For tropicals continue to fertilize as long as new growth is evident.

Air Circulation and Humidity

Secondary considerations, but important ones, for indoor growing are air circulation, humidity, and insect/disease control.

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.

Insect and Disease Control

Insect and disease control will actually begin when you do all the above, because you will have an active healthy plant that will be able to resist them. However, there are bound to be some problems. Watch very closely for spider mites. These are almost microscopic, and can be best seen with a five to ten power hand lens. They live on the undersides of the leaves, so that's where to look for them. Turn a leaf over and look for webbing or debris. A healthy leaf will look absolutely clean (unless you have been misting a lot and building up salts) except for possibly hairs on some species. If you see any debris or webbing at all, start searching for mites. They are little spider-like creatures and they will have very tiny translucent round eggs. Red spider mites are easy to see with the lens and visible with the human eye for those of you with eyes under forty. Two spotted mites are smaller, translucent except for two dark spots on their back, and can even be hard to see with a five power lens, you have to search for them.

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.

And finally

Follow these tips and you should be able to grow suitable species indoors for bonsai. It will not be possible to grow every species indoors. You should carefully choose only those with a moderate chance of success. Start out with the easiest, Ficus, a really good tough plant for indoor growing.

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