'Nebari is a Japanese word which means the lowest part of the trunk of a tree and the visible of the roots. Nebari is just not surface roots, but it also takes into account the way a tree flares out at the base into a strong root system and grips the soil, giving an impression of great age and stability. Roots do not grow in straight line, instead they become twisted like the branches as they encounter spots of nutrients, water, hard or soft soil pockets. Competition for nutrients and water in the soil will result in the main roots spreading out evenly from the trunk, without crossing one another.
As a seed germinates, two types of roots form- 'Taproot' and the 'Surface roots'. The Taproot grows in the early stages of life of a plant and is a long root going deep into the soil looking for water and providing stability to the plant. The surface roots grow just below the first layer of soil and spreads out all around the tree. As the tree matures the surface roots replace the tap root as they have quick access to the nutrients and the rain water as it comes down. These surface roots form the basis for the Nebari.
A 'Good Nebari' is a mature root system that radiates out from the base of the trunk at the same level and appears to grip the soil with no gaps between the soil and the roots.
The root system should be well distributed and the roots should grow outward and the inwards in all the different directions without crossing each other. If there trunk is slanting to one side, the nebari should have its roots growing to the opposite side of the slanting trunk and also the roots must be shorter on the side the trunk is slanting.
Precaution: Whenever a major work is done on a tree, ensure that the tree is healthy and in top condition or it may lead to its death. Young plants will have no trouble in recovering while older trees have a bigger risk. The ideal time to work on the nebari is end of winter or early spring, when the tree is repotted. When working on the roots of the tree, it is important to work in the shade as the fine roots tend to dry out quickly. To avoid this, place a bucket of water nearby, so that the roots can be dipped once in a while. Also, the soil should not be washed away completely.
Simple Pruning Method
Pull out the nursery plant and examine the roots- healthy plants have light brown, yellow or whitish roots while unhealthy plants have black roots. Remove all the dead and damaged roots and all the roots that are growing downwards, forcing the tree to depend more on surface roots. The radial surface roots are also cut considerably so that numerous fire roots are produced near the cut, giving a better ramification to the root system. Plant the tree in a wide, shallow container, using soil mix with plenty of sand to allow the severed roots to sprout. To develop good, strong surface roots. Keep them covered till new roots are produced.
Flat Surface Method
If trees are grown from cuttings then a good nebari can be created right from the start. Select a cutting with good root distribution around the trunk. Prune the tap root completely and trim all other roots leaving a section of root in place. On the areas that have been cut, new roots will form. Place the cut area on the top of a root-barrier like a flat stone, tile or a CD and plant the tree in the ground or in a large training pot. Protect the plant from wind and strong sun. The new roots that will grow from the cut area of the old roots are forced to grow outwards because of the flat barrier underneath, resulting in evenly distributed roots around the main stem. Later on choose the evenly distributed good roots and prune the other unnecessary roots.
A process, generally used to propagate new plants from an existing one is a very useful tool to create a reasonable nebari. This method is very effective with deciduous and ficus trees. In this method, a strip of bark is removed off the tree above the ground at the desired height. Root hormone is applied on the wound and the area is wrapped with sphagnum moss and covered with plastic. The moss is kept moist to promote growth. When the new roots are established, the trunk and the new root system is removed and new plant is repotted.
In this method, a piece of copper wire is tightly wrapped at the base of the trunk slightly above the soil line. The wire should go all around the trunk and should cut about halfway into the bark. The thicker the trunk is, the thicker the wire should be. Rooting hormone is dusted around the wound and the wound is then covered with sphagnum moss just above the torniquet and kept moist. The squeezing pressure partially blocks the stream of nutrients from the roots to the tree, forcing the tree to grow new roots first above the wire. Once the new roots are established and the new trunk has thickened, the old roots and trunk are removed and the new tree is replanted into a pot. This process produces some spectacular results.
Crafting of roots to the base of the trunk is another method to improve the nebari of a tree. The best time for this is spring or early summer. A two years seedling is taken and the top portion is cut to form 'V' shaped wedge. A matching 'V' shaped groove is carved at the base of the tree where the root is required. The seedling is entered into the groove with the cambium layers of seedling and that of the tree properly matched. The grafted areas should be well sealed. The advantages is that mature roots are immediately placed and in perfect position.
Another method is to make a channel at the trunk base where the root is to be inserted. Sufficient soil is removed from where the scion will be planted. A pencil thick sapling or scion is inserted into the channel and fixed securely into position. The scion is covered with soil is speed up the maturing of the bark. The scion is allowed to grow freely and vigorously until the scion merges with the main trunk base. Later, the top of the scion is pruned leaving the base as the new root.
Trees With One Sided Roots
A suitable root system is necessary and desirable to give the bonsai a stable appearance in its pot, but many root flaws cause the tree to appear unstable. Sometimes cuttings or nursery plants have one-sided roots which needs to be improved upon.
The techniques is to drill small holes in the bark where roots are needed and the holes are dusted with rooting hormone. Once this is done, the holes are covered with moist soil. After a few months new roots will grow from the places where the bark has been drilled. This is a slow process and takes time for the new roots to develop.