I'm not going to keep you in suspense - while most bonsai are more easily grown outdoors, some species of trees can grow and thrive indoors. If you'd like to keep a bonsai tree indoors, you just need to be sure you get these few things right.
Good Species for Indoor Bonsai
Tropical varieties that tolerate low humidity are generally best suited to indoor growing in a typical home.
Try these species for indoor bonsai:
- Willow Leaf Ficus (Ficus nerifolia)
- Ficus (Ficus benjimina)
- Miniature Jade (Portulacaria Afra)
- Chinese Elm
- Lavender Star Flower (Grewia occidentalis)
Avoid these species for indoor bonsai:
- Atlas Cedar
Where to Place Indoor Bonsai
Indoor bonsai need natural light to thrive. For all of us in the United States, south-, west- and east-facing windows are all candidates for bonsai growing. Skylights near windows or kitchen windows that project outward to make a small greenhouse provide even more light. North-facing windows will typically receive only indirect light, which may not be enough for many tree species.
Additionally, the quality of light for a given window or placement within a home will change as the angle of the sun changes throughout the year. While a south-facing window will receive a lot of light most of the year, during mid-summer the sun will track directly overhead and not shine directly in. Windows that face southwest or southeast will be more constant.
To optimize an indoor bonsai's growth, the proximity to a light source is very important. The distance of the plant from the window will affect the radiant heat as well as the amount of the light. For south-facing windows shading in the afternoon or morning by positioning the tree to the east or west is also a consideration.
Direct sun coming through a window can be scorching to some plants. If you get full sun and your windows are not shaded, you may want to add a layer of diffusing material to provide diffused light. Many older greenhouses use whitewash on the exterior of the roof to diffuse light. Consider scrim material as a diffusing option or just sheer curtain fabric. Think of this as the indoor equivalent of shade cloth.
Unlike outdoors, the light source movement is restricted indoors - to encourage uniform growth regularly rotate your bonsai in relation to the light source. Place a reminder in your calendar to rotate your tree 180 degrees weekly, or 90 degrees every few days.
If you don't have great natural light available then you will need supplemental lighting. There are many good lighting systems available at different price points. The color temperature, type of bulb, positioning and duration/hours per day that you use all have an effect on the plants.
Air is Important for Indoor Bonsai
For tropical species an indoor air temperature between 65 and 75 degrees Fahrenheit will be nearly ideal. Keep in mind that air near a window can be significantly more variable than the air at your thermostat - use a thermometer positioned out of direct sunlight to measure the air temperature near your plants. If the indoor temperature exceeds 85 degrees and you don't have an air conditioner, keep a close eye on your bonsai for signs of stress.
Alternately, if you like to keep your home cool, you may want to provide your bonsai trees with supplemental heating. Lighting sources can provide some heat, but generally it's not in the ideal place - close to the leaf tips. Instead, consider inexpensive seedling heating mats to gently warm the root zone. I prefer the HydroFarm brand for evenness of heat and toughness in their construction. Consider using a thermostat as well, which will automatically turn off the mat once the root zone reaches the programmed temperature.
The inside of many houses has very little air movement. Because the metabolism of plants depends on gas movement out of tiny pores on the leaves, more air movement is better. Good air movement can reduce insect attacks, fungal problems and increase overall health. Consider cracking open the window or use a small fan.
Watering, Fertilizing, and Pest Control
Watering Bonsai: Season, temperature, sun/lighting, plant metabolism and soil type among other things will affect the frequency of watering your tree. Water when the soil is damp, but before it dries out. Water based on the amount of moisture in the soil, not on a schedule, and when you do water, wet all the soil, either by soaking, overhead watering or alternating the methods.
Watering can be challenging because the plants either need to be placed in trays to catch the excess water, or transported away from wood furniture and other things that shouldn't get wet. Use some sort of tray if you plan to water in place!
Fertilizing Bonsai: Apply fertilizers on a regular basis. For indoor growing, organic fertilizer can cause odor [and fly] problems; use a balanced mineral-based fertilizer either in liquid form (properly diluted!) or in the form of time-release pellets. There are many types that work; I use BioGold or Dyna-Gro "Foliage Pro" or "Bonsai Pro."
Pest Control: Pest problems indoors can be tricky to eliminate once a pest is established. If you don't want to use chemistry on your indoor plants, first try spraying the pests off with water. You can smother many types of insects by mixing soap with water, applying a spray to all leaf surfaces, and then rinsing it off. If that doesn't do the trick, consult integrated pest management websites for pest specific advice and best practices. Pests can arrive on your tree in a variety of ways; check branch junctions, the under side of leaves and other hidden places on your tree regularly.