A bonsai is a small tree or shrub grown in a shallow pot. Pruning is what makes and keeps it small, but pruning is only a small part of the art of bonsai.
The art - pronounced BONE-sigh - began in China almost 2,000 years ago, then was carried to Japan during the Kamakura period (1180-1333), where it was brought to a state of perfection.
A bonsai planting portrays, in miniature, a natural theme such as the rugged beauty of a gnarled pine on a windswept slope, the tranquility of a grove of larches, or the joyousness of spring in the cascading branches of an old fruit tree bursting into bloom.
To evoke such a mood, the pot must be chosen with an artistic eye; likewise for the manner in which branches are shaped, and the choice of groundcover beneath the tree. And with all this, the plant must be kept healthy with careful attention to soil, fertilizers, watering and shelter from the cold.
Getting a bonsai started
Most bonsai are created from plants that, given their druthers, would grow into towering trees or billowing shrubs.
You can purchase bonsai or, even more engaging, start your own by digging up a smallish wild plant or even a seedling tree that popped up in your yard. This plant will need its first pruning, of its roots, before it even goes into a pot. Wild plants and seedlings, even if small, often have surprisingly far-reaching roots. These roots must be untangled and shortened to fit the plant into its pot.
Certain trees have a taproot - a main root that plunges deep into the ground - in addition to shallow feeder roots. Cut back the taproot to fit the plant into its shallow pot.
Prune right from the beginning
The top of a new bonsai also might need to be cut back to bring it down to bonsai size, which is usually under 4 feet. (Bonsai are classified according to form and size, and the smallest are less than 7 inches high.)
But you cannot simply lop back a stem or trunk; the plant will look like a lopped-back plant instead of an ancient tree in miniature.
To shorten a trunk artistically, cut it back to within a few inches above its desired height. Trim the bark from the portion of trunk above the highest remaining branch, and pare the stub to a taper. Then bend the next highest branch upwards, tying it right up against the tapered stub, with some padding to prevent the string or wire from marring the branch.
After a few weeks, when the branch can hold the upright position without assistance, remove the ties and cut back the stub, with a sloping cut, to the base of the now-leading branch.
Another way to artistically shorten a trunk is to create a "broom"-style bonsai: a trunk capped by a fan of stems. Begin by cutting the trunk back to where you want the branches to begin. Rather than a flat or slanted cut, leave the cut surface of the decapitated plant with an asymmetric V-shape.
Next, wrap rubber strips tightly around the trunk at the top, to prevent it from swelling and ruining the form. Many new shoots may attempt to grow from where you cut; rub off all but a half-dozen of them. As the shoots grow, pinch their tips to promote branching. This broom style is especially suited to the growth habits of deciduous trees like elm and sycamore.
Add some age to a young bonsai
To create an "old" snag of wood on your young bonsai, snap off a branch or the top of the trunk. Pull down a strip of bark from the snag as far as you want. Let the exposed wood dry out and then paint it with full strength lime-sulfur solution (available at nurseries and gardening centers) three or four times, every two weeks, to preserve it.
There you have before you the beginnings of a wizened tree. You're not finished pruning though - that remains an annual affair.