How gardeners manage to grow miniature pines, firs, and oaks in flowerpots for half a century has always been more or less of a secret [sic].  It is the result chiefly of skillful, long-continued, root-pruning.  They aim first and last at the seat of vigorous growth, endeavoring to weaken it just as far as possible without destroying the life of the tree.  They begin with the young plant, say a seedling of a cedar or hemlock, when only two or three inches high, and cut off its tap-roots as soon as it has other rootlets enough to live upon, and replant it in a shallow earthen pot or pan.
       The end of the tap-root is generally made to rest on the bottom of the pan, or on a flat stone within it.  Alluvial clay is then put into the pot, much of it in bits the size of beans, and just enough in kind and quantity to furnish a scanty nourishment to the plant.  Just enough of water, light and heat is given to keep it alive, but not enough to excite a vigorous habit.  Gardeners usually pride themselves on the shape of their miniature trees, and they use strings, wires, and pegs, and various other mechanical devices, to promote symmetry of habit, or to fashion their pets into odd fancy figures.  Thus, by using very shallow pots, the development of the tap-roots is impossible, and by using poor soil and little of it, and little water, rapid growth is prevented.  Then, too, the top and side roots, which are within easy reach of the gardener, are shortened by means of a pruning knife, or are seared with a hot iron.  In this manner the little tree is headed off on every side and is allowed to grow just enough to live and look healthy.  Accordingly, each new set of leaves becomes more and more stunted, the buds and rootlets are diminished in proportion, and at length a balance is established between every part of the tree, making it a dwarf in all respects.  In some species this end is reached in three or four years, while in others ten or fifteen years are required. 1 

1    Scientific American, Vol. 108,  May 10, 1913, pg. 432.