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I have had a number of old deciduous trees go through my collection over the years. Often they arrive in various states of neglect or dis-repair due to ageing owners, lack of interest or lack of knowledge. What has been interesting is the results of this lack of technique and or application.

I think most people are aware of what they should be doing. Most beginner books say something along the lines of “cut all downward growing roots off to form a flat root-base”. In essence that is correct. But why?

• Is it to get the roots to fit in shallow containers? Partly yes.
• Is it to encourage a flaring nebari? Partly yes.

There is one more reason that is rarely discussed or talked about when talking about root work.

What we want roots to do in a bonsai container is counter intuitive to the plant we are growing. We want a shallow root system close to the soil surface. This area of the soil profile is horticulturally the worst place in the pot to grow roots, and here in lies the problem.

The top few centimetres of soil are those most likely to experience temperature changes and drying out, both of which are no good for roots. Yet this is the area of soil we need them to occupy to form a good shallow root-base.

Correctly pruned root base

The above image is a vertical slice through the center of a well-formed nebari.The hairs on either side are roots and the triangluar shape is the nebari and lower trunk.

Idealy you want roots on the outside radius of the root base so that as the tree grows the nebari slowly gets larger. These roots however are occupying a zone that is not ideal for them. In order to keep them active in this zone you must force them to stay there.

To do that you remove all downward growing roots. These roots are more protected under the trunk and head straight for the cooler more temperature and moisture stable soils lower down in the pot. If you leave them on year after year the bonsai will preference these downward growing roots which are in better horticultural locations and the roots you actually want to keep around the edge of the nebari will slowly start to die back.

Incorrectly pruned root base result.

What you end up with is something like the above drawing (this shows a tree which had a well-developed nebari and was then incorrectly maintained). All the roots on the edge have died back and only roots underneath the trunk are growing. Now the nebari will not grow in size around the edges but instead grow underneath the trunk and slowly make the root base less and less shallow requiring deeper and deeper pots to house it.

These downward growing roots often push the trunk up and show the edges of the nebari so that it appears somewhat like a blob of melted candle wax. Not ideal. Not natural.

If left in this state for a long time the edges of the nebari can begin to develop areas of dieback that will often spread from the edge of the nebari up toward the trunk. All in all a bad result.

So if you have deciduous trees that are beginning to form nice root bases be sure to ALWAYS take off the downward growing roots. It is not just to fit the tree into the pot but it is mainly to ensure your roots grow where you need them to and this will keep your nebari developing correctly well into the future.