Bonsai soil – avoid drainage layers

There is a well-settled myth in bonsai that is one of the hardest to kill. The use of so-called “drainage layers”.

Drainage layer is a no go because it has the opposite effect as it is supposed to.

The idea may seem logical and clear, but we have to look at the facts and physics. When a layer of smaller stones, gravel or pebbles, often between 1- 5mm is placed in the pot there is air between them.

This space makes the water “glue” and builds a wall or a bridge of water, hindering it from running out of the drainage holes until a certain point. Capillary action stops the water flow.

If you notice how water drops on your window start as small droplets and then grow as they run down the surface and merge into larger drops you get the idea of what happens.

Also, notice how a glass of water can build up above the edge. This gives an idea of how water is able to build up water tension.

This building of a water bridge in the soil causes poor root growth and root rot. Therefore, I have totally skipped the use of any kind of drainage layers, and through the last seasons, I have experienced how well the roots are distributed and developed healthy both at the centre of the pot and the bottom. And water runs freely when still using an airy soil mixture.

It´s important that the soil is mixed with soil taking up oxygen well because this is essential for healthy root growth. This should be Akadama, pumice or similar qualities with a fine pore structure taking up water and nutrients easily, and releasing these slowly. Whatever soil source you use, just have this in mind.

For Shohin bonsai and Kifu there is no difference in the soil added. I use the same blend in the pot without having layers of different blends. The only difference is a thin layer of clean Akadama as a top soil layer, keeping humidity a bit higher, and preventing evaporation from being too fast.

For larger bonsai as well I will also hesitate to have any difference in the mixture. I see some have a coarser bottom layer, but again, I see no benefits from this. All there is to do is avoid any kind of “drainage layer” building up water-blocking areas.

Problem solved. A win-win water situation.

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