Most immature bonsai plant material has long roots that do not fit easily into a bonsai container. For this reason a training pot is used.
The training pot is larger than a regular bonsai container and holds the heavy roots These are gradually cut back over a period of years until small, fibrous roots develop.
All kinds of containers are used as 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.
Choosing a Pot
Choose a pot in which to display your bonsai when the training of your bonsai is sufficiently advanced. The size and shape of this pot will depend on the size and shape of the tree.
Trees trained in the cascade and semi-cascade styles look best in round or rectangular pots. Plant the trunk in the center of the pot with the branches sweeping down over the side. Place upright trees slightly off-center (one-third the distance from one end) in oval or rectangular pots. Place trees with thick trunks and dense foliage in deep, heavy pots.
The branches of a bonsai tree should be in harmony with the shape of a pot. If the branches are longer on one side than the other, place the trunk off-center within the pot.
The color of the pot should ideally contrast with the tree’s foliage. Use white, tan, or green pots for trees with brightly colored flowers or fruits.
Use unglazed pots with pines and deciduous trees.
Generally, bonsai containers come in five shapes: round, oval, square, rectangular, and hexagonal. In each shape there is a wide variety of sizes.
Bonsai containers can be obtained from some of the larger nurseries. Chinese or Japanese hardware stores, and stores that specialize in imported items, also offer containers for sale.
Bonsai plants should be anchored to their containers until the roots take hold. One method used to anchor the plant is to tie it down with wires leading up through the screens that are placed over the drainage holes in the container.
After tying the plant to the container, carefully adjust the plant’s elevation.
At the end of the first year, the tree is usually transplanted from its training pot (or from the ground) into a new pot that is more suited to its dimensions. Always retain some of the original soil, and trim the roots if necessary. Cut away any abundant growth of new roots at the base of the trunk before repotting.
If only a few roots have formed around the taproot, prune these roots slightly.
Prune the taproot again at the end of the second year, and cut it short at the end if the third year. This final cutting should be done when the new roots have appeared at the base.
Repotting of bonsai plants is usually needed when soil insects damage the plants, or when soil is in poor condition.
Sometimes, however, soil condition can be corrected without repotting and disturbing the roots of the plant. This is done by adding new soil around the outer surface, or by removing plugs of soil and replacing them with a free-draining bonsai soil mix.
The health of trees grown as bonsai depends largely on the care in changing the soil in the pots and the proper pruning of surface roots.
A healthy bonsai puts out new surface roots every year. The growth of these roots makes it difficult for vital water and air to penetrate the soil. The surface roots will be nourished, but the main root near the trunk may die. Therefore, periodically cut back the main root and thin out the surface roots.
A tree’s rate of growth determines the frequency of repotting. Pines and spruces, for example, need repotting only once every 3 to 5 years. Flowering and fruit-producing trees, every year or (depending on the variety) every second year.
You should note that it may be necessary to repot quick-growing species, such as willow and crape myrtle, at least twice a year. This is a serious commitment and should be taken into account in initial tree selection.
These intervals apply to healthy trees that have received proper care.
Repot your plants in the early spring when the first new buds appear. A secondary season occurs in late summer or early autumn when, for a short time, the roots check their growth. It is dangerous to repot in late spring and early summer when the leaves are just open and still tender.
When the tree is in a dormant state it is unable to establish itself in the new soil and root diseases are likely. For this reason, bonsai must never be repotted in winter, except when kept in greenhouse culture.
Soil mixtures vary a great deal depending on geographical area and personal preferences. There are many conflicting ideas within the bonsai community on the type of mix to use. Many growers find that bagged potting soil is usually satisfactory for potting bonsai plants, whereas others are convinced that only a proprietary bonsai soil mix will do.
If you use bagged soils, make sure they contain sphagnum peat moss and coarse perlite in equal quantities. Bagged soils are available in most garden supply houses.
Specialist bonsai soils can be purchased easily from Amazon as well as other sites such as www.bonsaiboy.com.
Generally, mixing soil should have rapid drainage, a structure that permits fine roots to develop, and contains decaying humus and mineral nutrients. It should also be free of root rot and have a pH similar to the tree’s native soil, so research this carefully.
Try to avoid high levels of dry fertilizers in the soil mix and screen bagged soil to remove the fine clay particles. Use formulated bonsai fertilizer if you want to err on the safe side.
A good basic soil mixture consists of one-third clay, one-third humus and one-third sand.
If you live in an area where humus is not available, then obtaining a specialist bonsai soil mix from your garden store or nursery is the only practical answer to this.