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"And the LORD God made ... trees that were pleasing to the eye ..." Gen. 2:9, New International Version.

"Bonsai isn't just something I do; it's part of what I am." Remark to my wife and daughter.

Tropical Trees in an Indiana Winter

The term "tropical," of course, generally indicates trees that are native to the earth's warmest zones. How does one keep them alive and happy in a location where winter is four months long, and we can expect the temperature to get down to -15° F at least once? (That's -26° C.)

When one keeps tropical trees as bonsai in a non-tropical climate, two of their characteristics, in particular, must be kept in mind. (There are other things to consider, too, but these two are the most important.)
  • First, with almost no exceptions, they are genetically programmed to grow continuously year-round, with no regular dormant period.
  • Second, they lack any of the mechanisms that protect temperate-zone natives from serious cold; no antifreeze analogues in their tissues, for example. Expose them to freezing temperatures for more than a brief period, and they die.
Keeping tropicals alive and healthy during winter in the earth's colder zones means, primarily, giving them adequate levels of heat, light, and humidity. There are other considerations as well, but warmth, light and humidity are the Big Three.

When I was single, my tropical trees spent the winter under lights in my living room and bedroom. When I married, I discovered that my wife didn't care for utilitarian-looking light fixtures in the middle of her d├ęcor, and some other place had to be found. I now keep my tropicals in a home-built basement enclosure that I call the "Bonsai Crate."

I call it that simply because it is about the size of a large crate. Inside dimensions are 54" wide by 73" long by 75" high (1.37 m by 1.85 m by 1.90m.) To reduce heating costs, I built it as compact as I could; the ceiling, for example, is just high enough that I don't duck reflexively when I walk in. It was built using standard lumber from a local home-improvement chain, and basic home carpentry tools. As of early 2012, I think a structure like this could be built almost anywhere in the USA for $250 or less.

1. Frame with outer plastic "skin" in place.
The Crate holds about two dozen tropical bonsai or bonsai-to-be of different sizes. Its capacity also enforces a limit on the number of tropical trees I acquire or propagate (a limit I admit I need.)

The wooden skeleton (built using the framing techniques I learned on a summer job in 1970) is covered with glazing both inside and out. This "double-skin" design traps a layer of dead air between the skins; dead air is good insulation, and free! Picture 1.

The outside of the frame, and two-thirds of the inside, is covered with the same sheet plastic that is sold for winterizing windows. On the inside the ceiling, and the walls next to where the trees sit, are covered with silver-matte projection-screen fabric, acquired free from my former employer. This fiberglass-based fabric is durable, water resistant, and reflects more light than almost anything else available. Picture 2.
2. Silver-matte fabric.

3. One flap of the door is open inward, the other out.

The door (Picture 3)  is a double flap of the same silver-matte fabric, stiffened on tops and free sides with thin lumber and closed with Velcro.

4. Single fixture on right holds 4 bulbs; others hold 2 apiece.

Two benches, each 24 inches by 48 inches (61 cm by 1.22 meters,) are set in an "L" configuration. They hold the trees 38-40 inches  (about 1 meter) off the floor, the height that suits me best. I use standard 48-inch 40-watt fluorescent bulbs, usually a mixture of "Cool White" and "Sunlight." I don't use specialized plant lights. Picture 4.

5. Heater and high-tech humidity set-up!

A tray of water on an old heating mat (made for seed-starting) provides humidity; I frequently enhance the humidity further by spilling a little additional water on the floor. A standard oil-filled space heater, set quite low, is enough, together with the light bulbs and humidity set-up, to provide plenty of heat. Picture 5.

The fan from an old room humidifier keeps the air moving gently. Picture 6. Heater, lights, humidity rig and fan are all on a timer set for 18 hours on and six off. At one time, I left the lights on around the clock; but I have learned by experience that Portulacaria afra ("elephant bush") needs a few hours of darkness every day to stay healthy. I bought a timer made for outdoor use because of the expected humidity levels. Picture 7

6. Formerly the fan side of a  humidifier.
7. Timer and power strip near ceiling to avoid splashes.

Temperature in the Crate is usually in the mid-to-upper 70's F (around 21° C.) It is easily adjusted. Humidity is usually high enough that a light haze is briefly visible when the door is opened, and my glasses (if I'm wearing them) fog over when I walk in! That level of humidity also encourages aerial roots from the species that are prone to throw them. Light levels are adequate, but not much more: by the end of the winter, most trees are producing leaves visibly larger than the leaves of the summer before, and internodes are getting longer. Eventually, I may upgrade to metal-halide lights, but at present I can't afford them.

I aim to move my tropical trees into the Crate when overnite lows start frequently falling below 45° F (7° C.) That is usually by mid-October. I don't do it earlier because I want them to have natural sunlight as long as possible. In the spring, they go back outside once overnite lows are staying consistently above 45° F (7° C.) That is usually in the second half of April.  I do have to be ready to move them back in if a sudden colder spell looms. We can get frosts in early May.

This is my third winter using the Bonsai Crate. Within a year or two, it will have to be replaced: the constant humidity is slowly deteriorating the wood of the frame. The next iteration will be built with pressure-treated lumber to resist rot longer.

First season.  Late autumn 2009. Looking almost straight ahead from door.

First season. Late autumn 2009. Side bench.

Third season. Late winter, 2012. Looking straight ahead from door.
Third season. Late winter, 2012. Side bench.