Bonsia Supplies + Tips Search! Enter Your Topic Below:

Chapter 3. How to Repot Bonsai

Yuji Yoshimura

Root pruning is a key-technique

Why is it necessary to change soil and cut roots?

There are two reasons; 1. The root systems of dwarfed potted trees get pot-bound just like any other plants growing in pots. Repotting improves root aeration and drainage, and provides new soil into which the fine new absorbing roots can grow. 2. The other reason is to encourage the growth of fine roots by cutting off some of the larger roots. From the cut ends ("stumps") of the larger roots, many fine roots will grow. This procedure keeps the bonsai root system always young.

How often should bonsai be repotted?

Roughly speaking, deciduous trees should be repotted once in two or three years; evergreen conifers, once in three to five years. If the roots grow very rapidly, like willow or crape-myrtle, they need an additional repotting at the end of each summer.

If the tree is not "pot bound," repotting can be delayed. For example, if there is as much as 1 or 2 inches of soil around the rim of the container that is not filled with roots, then repotting can be delayed one to two years longer.

For a tree in an 8- to 10-inch container, after repotting there should be about 2 inches of new soil between the root mass and the outer rim of the container. The new roots grow into this fresh soil.

If the tree is planted on a stone so the roots more or less clasp the stone, the stone and the roots are never separated from each other during repotting.

When is the best season to repot?

It is best to repot all kinds of bonsai in the early spring just as the new buds swell and start to grow, but before the leaves appear. Evergreen needle trees (conifers) also  can  be  repotted just  at the beginning of autumn, because they have a period of autumn rot growth. Deciduous trees can be repotted in late spring to summer, if it seems necessary. In this case, cut off all the larger leaves, letting only tiny new ones remain. If deciduous bonsai are repotted in late autumn or winter, they must be kept at spring temperatures after repotting.

If the repotting consists only of shifting the plant to another container, with little or no pruning of the root system, it can be done at any season.

After care

Repotting, with its accompanying drastic root pruning, is a shock to the tree which therefore needs special attention for three weeks or more after the repotting operation. Its after-care is similar, in a way, to that of a cutting—which must be kept in a humid atmosphere, with proper soil moisture, and protected from direct sun and wind. And it should not be out in heavy rain because the surface soil may-wash away. However, in the case of early spring transplanting of deciduous bonsai (before the buds burst), it is not necessary to protect the trees from direct sun. It is very important after repotting that all the soil, including that close around the trunk, be watered twice daily with a fine spray. Never fertilize until a month after repotting.

Strands of copper wire are threaded through the drainage holes of the container, as shown in middle sketch, before the tree is replaced, as above. Upon finishing step "6" {see text), twist wire tight with pliers (lower right), so that roots are firmly held in position (lower  left). This  is  step  "7"

Eva Melady

Usual method of repotting

Do not water bonsai just before repotting; it is difficult to remove the old soil if it is too wet. New soil must be dry and generally is a mixture of clay, peat moss, leaf mold and sand, in equal parts. Bach of the soil components should be sifted in order to separate the different sized particles (see chapter 14, "The Care and Management of Bonsai").

The most necessary tools and materials are as follows:

Sharp scissors, a round brush (see illustration), pointed chopstick, wire clippers, watering can (preferably one with fine spray), copper wire in a range of sizes, string, and dried and powdered moss which has grown on rock, i.e., a low growing moss.

The important ten steps:

1. Take tree out of container and clean out container with brush.



2. Prepare container. If the root mass of the  bonsai  will  fill  one-half  to  two-thirds of the space in the container after roots are trimmed, use copper wire to tie the tree in place. Arrange wire as illus trated, above; tie (later) as shown. If the root system is small, it is better to tie with string after the repotting is finished (see drawing chapter 3, lower right).



3. Loosen the soil of  the root mass. Use chopstick to loosen up the outer one-third of  the  soil of  the root mass. Also loosen soil of the underside of the root mass (approximately the lower third).



4. Trim roots. Use large sharp scissors and cut away the outer third of the fibrous mass; also cut off about half of the roots on the underside. A little tree should not  be  repotted  if  it does not have a tightly packed root system, that is, if it is not completely "pot bound."

Sectional diagram through container showing method of holding a new transplanted specimen tightly in position.  Stick sharpened at one end and thrust through root ball.

Heavy cord, tied around the main stem several times above the container, and tightly drawn under the pot, will secure the plant and keep the main stem in desired position.

Dry soil is firmed between tree roots with chopsticks, using a rapid up-and-down motion (step "6")

Surplus soil is brushed away to bring final level slightly lower than container rim (step "8")

5. Position tree in container.   Place tree in center if the container is round or square, and one-third in from the end if a rectangular or oval container.



6. Get new soil in around the roots (see above). This is done by rapid jabbing with a chopstick. The soil should not be pressed down by hand.  Only the chopstick can make the soil go into the spaces between the roots. The jabbing motion should not be to the same depth each time. That is to say, the first motion should go down deep, and the next time a little shallower than before, etc.  Also jab sideways as well as straight down. Generally speaking, this jabbing motion takes time. For instance, it was necessary to do it 15 minutes for the tree pictured at the left above. As the soil works down in and disappears, during the jabbing, constantly add more—until no more is needed.



7. Tie tree to container. Wires must be pulled up first then drawn together over the root mass, and twisted tight. There are several different ways of doing this  (see drawings).



8. Brush away excess soil. If the surface soil is higher than the rim of the container, all the water will run off when the tree is sprinkled. Keep soil one-quarter inch lower than the side of the container. It is desirable to insert a protective rubber strip between wire and roots, so the wire will neither contact nor cut into the roots. Visible wire may be cut away after one growing season.



9. At this point only one finishing touch is needed. Spread a very thin layer of topsoil over the surface of the soil already in the container, and sprinkle dried powdered moss, through the fingers, onto the topsoil.



10. Water. Place the newly planted bonsai container in a tub of water in which the water level is the same depth as the container. At the same time, gently water from above.

Are You Ready To Move Onto The Next Lesson? Click Here...