Tips & Care: Bonsai Root Pruning and Repotting

Root pruning is an fundamental part of good bonsai care. There is no tree that can survive in a container indefinitely without some sort of root work. 

Root Pruning is done to:

• Promote a fibrous root system
• Promote good surface roots
• Produce a balanced system
• Remove any deformed roots
• Check for pests and disease
• Balance the ratio between foliage and roots

Bonsai need water rich, nutritious material to grow their roots. In the earth this is no problem, roots can travel many tens of feet from the plant stem in search of nutrients and water. 

Roots should be examined at least every two years. Depending on the age and species, and the size of the pot, roots can be become root bound.

Roots will naturally curve somewhat inside the soil, when they encounter a hard obstacle such as the side of the pot, they curve inward. Eventually they have nowhere else to go as more roots occupy the available space.

The idea is that you should prune the roots and repot your bonsai when they show symptoms of decline and/or chlorosis, or when the bonsai is rootbound (it begins to push out of the pot).
The frequency of root pruning depends on the tree species, its container and the environment. Remember that root pruning damages the tree, restricting its ability to take up water and nutrients, so it has to be done at times of the year that the stresses on the plant are minimal. For temperate climate plants there are two times of the year that these conditions are optimal, late fall and early spring. Tropical plants can usually be root pruned and repotted during periods of 'quiescence' or slow growth.


Before that happens, they should be trimmed with very sharp scissors. Use a chop-stick or knitting needle and gently brush away the soil from the root ball.

Remove about a third of the soil to expose the roots you will be cutting. (your scissors will stay much sharper if they do not have to cut through the potting soil and the grit it contains)  Place the root ball in a bucket of water to prevent the roots from becoming too dry.

Then, trim back existing roots to about two-thirds to one-half their current length, eliminating a few younger and a few older roots.  

Be sure to leave a good number of main roots to serve as anchors and to continue to supply the plant with nutrients. Repot into a larger pot suitable for larger and older trees. To prevent the tree from tipping over after repotting you will need to secure the root ball in it's container with wire.

Anchor two lengths of wire inside the pot, either by securing them through the drain hole or the two anchor points some bonsai containers provide.

Place a piece of wire mesh over the drain hole, apply a layer of gravel for drainage and then a layer of soil slightly mounded near the middle of the pot.

Place the root ball on top of the mound and then bend the wire across the root ball to firmly anchor it in place. 

Continue to add soil until the roots are covered and the soil is just below the top rim of the container.

Position the rock planting slightly off-center and rotate until you have an attractive position.  Then press the planting into the loose media. Using one pair of ends, tie down the rock planting on one side, then the other. The rock planting should be secured firmly. If not, pass a loose end under the other tie; pull taut to take up any slack, and knot. Repeat if necessary. The ties will insure that the planting will not shake. Ties can be removed when the roots are established in the media. With the spoon firm down the surface of the visible media which should have sunk down about 1/4" below the pot's inner rim.

The first watering: Fill a pan with about 2" of water and allow the newly potted plant to sit in the water for 30 minutes or so.  This will allow all parts of the media to become totally saturated.  Allow the plant to drain, then give the surface a finishing leveling by gently patting with a spoon.

Watering a recently repotted bonsai is best done by the same saturation method.  The water pushes out all air from the porous media and replaces it with water.  New air rushes in as excess water drains out.  This repetitive sequence of total water saturation and air exchanges is the ideal condition for healthy root and plant growth.  Watering from the top does not wet all parts of the root-ball thoroughly.  Proper watering is the key to success!  Place your palm on the surface of the media.  If it feels wet,  don't water.  Water when it feels slightly cool.  Don't allow it to completely dry out!  You can also judge the amount of water in the container by the weight as a recently watered plant is noticeably heavier.