I have been growing bonsai indoors on windowsills and under artificial light for over twenty five years. During that time I have learned, and unfortunately had to re-learn, some crucial concepts. What follows is an assortment of ideas that I have gleaned over the years. Many of the conclusions seem obvious, but they were not so obvious at the time. I hope these ideas will help growers save themselves time, effort and mangled trees.
Indoors is not indoors!
Growing trees indoors is a difficult task, and part of the problem is that indoor growing conditions can be quite variable. One window in your home drops to 48 degrees Fahrenheit at night, while another windowsill may be a constant 85 degrees! One spot in the living room is extremely dark with almost no natural light and another area has southwest light streaming in from the window. Trees that grow well in one room of a house may not be happy in another room in the same house. Finding the right microclimate in a home is a huge part of winning the battle. A cool basement is the ideal place to grow boxwood, crape myrtle, Cotoneaster, Chamaecyparis, Serissa, and citrus, while the warmer bedroom is the best place for Ficus, Schefflera, Wrightia religiosa, and buttonwood. Ask friends who are successful in growing trees indoors about the lighting, humidity, temperature range, soil, and water conditions that work for them. Use these suggestions as a starting point for your indoor growing set-up and modify these to suit the types of trees that you grow.
All plants are not created equal!
One key element to successfully growing bonsai indoors is selecting trees that will survive indoors. Most trees will not survive indoors for long periods of time, while a few trees are proven indoor survivors. Temperate trees, those requiring a cool dormancy period, such as maples, larch, pines and junipers will usually not live indoors. While tropical trees such as Ficus, Brassaia/Schefflera, Sageretia, and Portulacaria are quite happy in most homes and will not need chilling, or a prolonged winter resting period. They also will not have a leaf drop and sit in leafless condition for weeks while waiting for the start of their spring growth period.
Select trees that will be happy under your home conditions. If you do select sub-tropical or cooler type trees modify your home to make these trees happy.
One way to find suitable trees that may work for your indoor situation is to go to the produce section of your supermarket and buy some fruit; try guava, lemon, kumquat, tamarind, and fig. As you eat the fruits, save the seeds and plant them. Some of these will survive, and may make good plants for indoor bonsai. Next, go to your local plant nursery, and select any small-leaved tree from their terrarium selection. Trees that you can find are cotoneaster, chamaecyparis, boxwood, myrtle, elm, and ivy.
Grow these plants in your home, and over a year or two some will survive while others will die. Select the healthy and growing survivors and concentrate your efforts on these few trees. Propagate the vigorous trees and discard the weak trees. Make sure to have at least three or four specimens of each of the strong varieties.
Lastly, purchase some pre-bonsai and finished indoor bonsai from a reputable bonsai nursery. Ask specifically if these plants can be grown indoors or whether they will require a dormant period or an outdoor summer growth period. These developed trees can be admired immediately, while your young, new experimental trees will take time to mature into respectable bonsai.
Light and more light!
Growing trees in a dark corner of an apartment is doomed to failure. Over the last five years, I have become more convinced that the most critical element to long term success with indoor bonsai is adequate light. Given enough light many trees will grow indoors and become wonderful bonsai.
Windowsill growing is borderline in most homes as window light is often dim and unreliable. For most indoor growers, supplemental artificial light is the only way to go, and my recommendation for anyone with a small bonsai collection is to use POF, plain old fluorescent lights. POF are inexpensive to purchase and to run. At the local hardware store purchase four foot long fluorescent fixtures. Use simple chains to hang these over the bonsai growing area.
Do not bother to search for special plant bulbs. The normal daylight spectrum bulbs work just fine and they are much less expensive. The key is to have the leaves nearly touching the bulbs. Fluorescents are relatively weak in energy and they must be left on for a 16 to 18 hours each day. A simple electric timer will cycle the lights on and off automatically.
Other types of lighting can work including incandescent bulbs, metal halides, halogens etc. The key factor is to increase the amount of light available to the trees. Trees in dark areas are literally starving to death.
I encourage those who have tried to grow trees indoors and failed, to try again with supplemental lighting. The experience will be an “enlightening” one for you and the trees.
Semi-tropical or less than tropical trees need special care!
Most tropical trees will grow year round in the normal temperature range of a typical home, 60-90 degrees Fahrenheit. However, if you are trying to grow semi-tropical trees, you likely will experience great difficulty unless you allow these trees to slow down their growth in the fall. Trees such as juniper, cotoneaster, holly, elm, boxwood, pomegranate, serissa, and azalea can be grown indoors, but many successful growers of these plants grow them in cooler temperatures than exist in most homes.
To succeed with these plants try a cool basement, or a really cool window in an unheated, spare bedroom. Besides cooling down the growing area, another trick for success with these trees is to raise the humidity level. A humidifier can be placed near the plants or the plants can be surrounded with a plastic tent to increase the humidity. Leave the top of the tent open to keep fungal problems from developing. Cooler temperatures also help by keeping the relative humidity higher. Another technique is keeping trays full of water near the trees, but do not allow the trees to sit in this water.
The last suggestion is to allow the soil of these sub-tropical trees to become definitely drier during their winter rest period; inactive trees need less water. Begin watering them normally when they resume active growth.
Select trees from the survivors’ list, and make sure to find the microclimate in your home to make them happy. Increase the light and humidity levels and let your trees rest when they want to rest. These five tips can help you successfully grow bonsai in your home. Future articles will highlight other useful tips. Please let me know of other ideas that make your growing more successful.