Training Pots
Most plant material for bonsai has long roots that will not fit into a bonsai container. For this reason a training pot is used. The training pot is larger than a bonsai container and holds the heavy roots, which are gradually cut back, for a period of years until small, fibrous roots develop.
All kinds of containers are used for training pots: clay saucers, plastic containers, and wooden boxes of many different sizes. Many of these clay and plastic pots are available at garden centers. The azalea pot and the bulb pan are especially suitable. The pot should be just large enough to accommodate the tree’s root system. It should be similar in shape to the bonsai pot, which will eventually replace it.
For example, an upright tree, destined for a low, flat container, should be grown in a fairly low training pot. A cascading tree, to be planted later in a high bonsai pot, should be trained in an ordinary flowerpot.
Make sure that all training pots you use have drain holes at least 1/2-inch in diameter
