You may have contemplated a bonsai tree inside a friend's home, or seen an indoor bonsai display in a magazine or book. While most bonsai trees are grown outdoors, there are some species, mostly tropical and sub-tropical, that can survive or even thrive indoors.
If growing a bonsai tree indoors is what inspires you, consider tropical and sub-tropical species such as weeping fig, Fukien tea, Hawaiian umbrella tree, jade, and baby jade, bush cherry, desert rose, and starflower. Other options are Norfolk pine, boxwood, and Chinese elm — all typically outdoor species that can also flourish indoors with the proper care. Ask a specialist at a bonsai nursery for advice on which species to buy for your home.
All indoor bonsai need special attention and near-to-perfect growing conditions to thrive. Here are some things to keep in mind:
1. Light is the most important factor when growing a bonsai tree indoors. In most homes, light levels are both unreliable and too dim. And even on a sunny windowsill, the glass can filter out many of the UV rays your bonsai needs to grow. Most bonsai experts use some form of supplemental artificial light for indoor bonsai trees. The easiest are ordinary fluorescent lights that are both easy to find and inexpensive. Aquarium lights are another option. The most important thing to remember is to place the lights about six inches above the bonsai and leave them on for 12-18 hours a day. You can use an electric timer to switch the lights off and on.
2. Having the right temperature is critical for indoor bonsai. Tropical and sub-tropical species can handle a standard room-temperature environment-between 64-75° Fahrenheit during the day and slightly cooler at night-all year-round, but cannot tolerate cold. Some sub-tropical species need a cooler temperature during the winter so that they can go dormant. Place these trees, such as Chinese elm, in a cool basement or unheated spare room or garage for at least a couple of months if you want them to survive. If you keep your bonsai in completely windowless or low lightroom you will need to provide artificial lighting.
3. Having the right humidity levels and proper air circulation is almost as important as providing the correct amount of light. Indoor heating and air conditioning can reduce humidity to the point where a tropical species just can't survive. Misting is one simple way to not only offer moisture to a tree but also to clean its foliage of dust and dirt. Many bonsai gardeners also use humidifiers or keep their bonsai trees over a humidity or gravel tray. Gravel trays contain water, damp sand, and gravel. Others place cups or trays of water near their trees. Don't let your tree sit in water, but do water regularly and when the soil becomes dry to the touch.
4. Even though your tree is indoors, it is still vulnerable to some pests and diseases. Be on the lookout for spider mites that cannot be seen with the naked eye but create fine webbing between leaves. Fungus gnats are attracted to wet compost, so make sure your bonsai pot drains well and don't overwater. Standard plant insecticides will get rid of both of these pests.