When one sees a tree that could be 6 or 60 years old that is 12 to 18 inches tall growing happily in a small, shallow pot, one may ask the question  “How can this be possible?”  Is there a secret learned in the Orient that permits such? No, there is no such secret.

Well, does one starve a tree when it is young so that the tree growth is stunted?  Some people have suggested something like that.  But no, the trees are kept well fed and healthy thereby promoting growth and helping to protect the tree from disease and pests.

Well then, does one let them stay on the dry side depriving them of proper food transport to inhibit growth? No, the tree is watered regularly and more often than a tree growing in the ground because of the nature of the potting medium that is used.

Well, does one deprive the tree of light to cut down on growth?  No, reducing the light only makes the tree gangly as it reaches out for the light. These trees need light as much as an ordinary tree does.

So, how does one keep a tree small, especially an old one, in spite of its age?  One way of accomplishing this is by reducing the growth of the tree by selective pruning.  As an example, if a tree is getting taller than is intended for the tree style, cut off the apex to a point just above one of the side branches. Then  turn this side branch up and wire it in place thereby creating a new but shorter apex.  In time, the cut and reduction will hardly be noticed.  This can be done time and time again to keep the tree in the size and form that is desired.  The same applies to shortening of branches.  Find an offshoot of the side branch and cut back to it, wire the offshoot and bend the branch in the desired direction.  This too can be repeated time and again. In bonsai, less is better.  That means that leaving only the branches and foliage that will complement the style of the tree. It also means that branch reduction results in leaf reduction which in turn results in less foliage to produce the food the tree needs for growth.

Is that all there is to keeping a tree short.  No, like the potted perennials that one may grow, the bonsai trees do become root bound.  Does being root bound slow down the growth of the plants?  Yes, it also does the same to trees.   BUT just as that is not necessarily good for the health of your plants, it is also not good for the health the tree. So what does a bonsai enthusiast do? In the late winter (end of February or early March in the Memphis area just as the new buds are beginning to swell), the tree is carefully removed the tree from its pot, the root system is reduced by cutting  out large, old roots but saving the young, tender hair-like roots Then the tree is repotted in fresh potting medium.  This provides room for new root growth and the tree expends energy to regrow the root system.  Energy used to regrow the root system is energy not available to provide for other growth (leaves, extension of branches or apex and tree girth).

Between the reduction of the apex, side branches, and foliage; to reduction of the root system  (anywhere from once every year  to every 4 or 5 years or more), the tree is kept smaller but  continues to age.  That is the secret of it all.  Simple, isn’t it?  When you reduce the foliage, you reduce the food being produced and thereby slow down the tree growth.  When you reduce the roots, you have taken away the energy storehouse, which is used to grow the leaves, branches and apex.  Roots must be redeveloped and require energy to do that.  Do both (foliage and root reduction) and the growth of a tree is really slowed.  It is just a matter of knowing how and when to do that.
So there is no secret of the orient that is practiced to grow, style and maintain a small tree, a bonsai.  Anyone can do it with some time and patience.