Repotting 

The periodical change of pots is an indispensable operation, in order to allow a safe and harmonious growth of the trees.
In nature vegetables grow and die in the same place, if men do not intervene. They are able to spread their roots in depth and in a horizontal direction to search for new nourishing stuff.
The pot is an artificial, impassable micro-cosmos, where this expansion is impossible.
So the root growth, which follows nature laws anyway, must be controlled and directed, helping the development and the increase of some root parts. Moreover the soil, which the tree is put in, tends to lose its nourishing principles as time passes and needs to be changed. Finally the growth of the trunk and the branches impose the use of bigger pots.

In spite of common opinions, repotting is not a simple change of container; it is a very delicate operation, full of risks, which requires certain knowledge of vegetable biology and growing techniques. The people, who try this way without the proper notions, will surely flop a lot of times. Repotting a bonsai-tree, especially if it is old, can cause its death. A centenarian Pinus mugus yamadori has very different needs in comparison with a tomato plant!

The most suitable periods for repotting are two: spring and autumn. During the vegetative reawakening, after the winter inactivity, the first buds will appear. In this moment the plant is able to bear the stress better and to emit new little roots. For Mediterranean plants the operation can be postponed to the following month, whereas for the tropical species we can wait until the beginning of June. As a matter of fact, Ficus, Carmonas and Bouganvilleas need a stable temperature, between 15 and 20°C, to get over the transplantation without any risks.

Flower trees are different: they must be repotted only in spring, after the flowering. When the first cold arrives, the color variation will announce the relenting of sap movements and vegetative processes: this is the right moment for repotting. In this period it is important not to penalize the root ball too much, because it will have just a fortnight or twenty days to store reserves before winter comes.

The first important repotting operation generally consists in the passage from the earth or from a big cultivation pot to a bonsai-tree pot. This passage imposes a drastic reduction of root balls and vegetation. Removing a part of the roots will cause a diminution of nourishment absorption, with a consequent impossibility to keep the foliage intact. There is a balance between the aerial and the underground parts of a tree: its roots assimilate what leaves transform through the photosynthesis. A few nourishing stuff will cause absolutely unpredictable loss of vegetable portions. The tree can lose the branch we have already decided to cut, or the fundamental one for the aesthetic of the future bonsai-tree. So it is up to us to prevent a possible disaster, selecting the interesting branches and removing the ones we do not need.

If you want to repot your bonsai-tree, you must let the soil get dry (do not exceed!), in order to help the separation of the roots and reduce to the minimum the weight the root hairs will bear during the operation. These very little structures are fundamental, because they are the only ones able to absorb water and nourishment from the soil. So you have to preserve their integrity as best you can.
The previous pot must be placed on a flat floor or, if it is a little pot, on a table. With one hand you will firmly keep the base of the trunk, lifting the same plant from the support (be careful not to spoil the bark), while with the other hand you will hit the pot edge to make it fall. From now on you will have to proceed quietly with great delicacy. After putting the plant on the table, you will remove the soil among the roots, using a wooden stick (the ones you use at the Chinese restaurant are perfect!). You will see smaller and smaller roots, which must be combed and spaced out.
It is necessary to distinguish between the plants that grow by seed directly in the earth and the plants that are already in a container, the so-called pre-bonsai trees. The former present a main taproot, which is directly perpendicular to the surface and from which the secondary and tertiary roots started. The latter, on the contrary, have no taproot, which has been cut to put them in the pot. As time passed, they developed direct horizontal and vertical little roots. The same situation can possibly be observed in cuttings, which are able to emit roots only like the spokes of a wheel: it is the best situation to easily repotting and allows the development of a round nebari, a characteristic of the most valuable bonsai-trees.

When you repot a plant with taproot, you will have to cut it, being careful not to endanger the root ball. You should preserve a certain amount of little roots and capillaries. If these structures are placed on the taproot, this one cannot be totally removed, so you need to leave the plant in a big container for some years. To help the development of other horizontal roots, we recommend putting a wooden board in contact with the cutting surface of the taproot; this expedient will force the growth of roots like the spokes of a wheel. Pre-bonsai trees will easy to repot: their root ball will be only reduced and freed from dead or less vital roots.
However you will wash the roots to remove the soil. This operation can apply also to conifers.
After twenty years of work, we can state that this cure is not really necessary. You just have to mix a certain amount of root-spores with the soil: they will colonize the roots again in a short time.

Now a constantly humid light cloth, useful to prevent delicate capillaries to dry, will protect the root-loaf. You can prepare the pot. You will cut as many little squares of net as the number of potholes. You will apply them on these holes and fix with copper wire. Through the holes one or more metallic wires will be fixed to the biggest roots or to the trunk base, in order to anchor the tree to the pot. In fact the tree could start moving in the pot, causing trauma to the little roots.
Before positioning the tree, you will prepare a thin layer of akadama, on which the roots will rest. Then you wire the tree and pour the soil from a small container. To obtain a homogeneous distribution of the soil, you can use a wooden stick: you will fix one of its extremities into the soil and rotate it.

The mould will reach the lower edge of the main roots, which should be in sight. Even if the repotted plant has not strong roots, you will leave them uncovered: the exposition to U.V. rays and air will help their enlargement and the development of a thick bark. At the end of this operation you will water the tree with abundant water, in order to remove all the powdering component of the soil: you will stop when the water coming out of the holes is completely clear.
Finally you will put the tree in a cool greenhouse in autumn or under the shadowing net in spring to avoid further damages.

SOILS

Soil is the main source for nourishing stuff and plant support. It is an under-layer for the several microorganisms that synthesize many growing substances. It is composed of a mixture of organic stuff, deriving from the decomposition of dead animals and vegetables, and inorganic matter from the smashing of the rocks. The glucose organic stuff (simple sugar, starch and cellulose) is synthesized by the microorganisms, which are present in the soil, and available as carbon, nitrogen and other substances for the roots. These products of bacterial degradation, together with some immutable vegetable derivatives (such as lignite), constitute the humus, whose composition varies according to the basic materials, the microorganisms, the soil pH, the temperature and the aeration. Moreover water and gases are present in variable proportion into the soil.

Water melts mineral salts and helps the root hairs to absorb them by osmosis or to transport them along the sap vessel of the plant to the leaves. This is the place for photosynthesis: a catalyzed by U.V. rays reaction that transforms the organic stuff into inorganic matter in presence of O2: simple sugars, such as glucose, and complex sugars, such as cellulose and lignite, amino acids, proteins and plant hormones. Sugars have a lot of functions: they are the energetic source for the growth of the plant and a reserve, stored in the roots for winter rest; finally they intervene as cellulose and lignite in the constitution of the green and wooden parts of the vegetable.

Soil composition and the consequent balance between the organic and inorganic components, water and air, sometimes influences the growth and the outliving of the tree. Excess or lack of one of these elements can cause hard growth or death of the plant. Stagnation can happen in impervious, loamy, and poor in gas soils, which do not drain. The proliferation of microorganisms leads to root rottenness. Clay is a very rich soil, but its fine micro-porous structure causes a compact mass in presence of water; on the contrary, when it is dry, it tends to crack and break the root hairs. The same situation will happen in winter, when the frost increases the volume of wet clay. Lack of water can happen in sandy and gravelly draining (macro-porous) soils, which are poor in nourishing stuff. These kinds of soils are normally mixed with akadama clays for yamadori growing, because they do not stand any stagnation. In this case it will be necessary to fertilize, as it is explained in the related chapter. The most used soils in modern cultivation are three: Akadama, Kanuma and Etna lava reddish mould.

Akadama

Japanese people discovered this special clay mould. It is especially suitable for all bonsai species cultivation, apart from acid-loving plants. It combines the great fertility of clay with a homogeneous granular structure, which eliminates the problem of impermeability of this type of soil. Its grains hold a big amount of air and do not impede the growth of little roots.
To enjoy these characteristic, you should sift it before you use it, to remove all the powder that fill in the holes. This clay can absorb water without disintegrate and can drain its excess, keeping a humid root microclimate without any stagnation. Its structure and the air it keeps make akadama a good thermal isolator, which avoids the roots to overheat in summer and to freeze in winter. It is rich in microelements, immediately available to the plant, but it has little organic stuff, which could be added with some manure, oxblood or mixture like Bio Gold or hanagakoro (see Fertilization chapter). Akadama must be used pure: you must not mix it with cheaper moulds to reduce costs, as its contact with other turfy substances can cause porosity alteration.

Kanuma

This volcanic mould comes from Japan, too. It is similar to akadama, thanks to its granular structure and its microelements. However it has a different pH, which makes it suitable for acid-loving plants, such as Azaleas, Heathers and Rhododendrons. It must be used absolutely pure, to better enjoy its characteristics.

Etna lava mould

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