Bonsai School – Glossary of Growing Mediums


Japanese origin


A clay soil dug and dried in Japan. It is baked quite hard. Screened and separated into various particle sizes though most importers only bring in the most useful 3-6mm size.

Different qualities are available in Japan but the only one used by us for bonsai is the Double Red line grade, being as it is harder, thus taking longer to break down into powder. Often used in Japan [ where conditions may be markedly different ] on it’s own , but usually in the UK. as part of a mix.


Another type of soil , a little less alkali, more sandy in nature and often used for coniferous species. Only rarely available in the West in a screened particle size, often a bag contains a wide range if particles that must be sieved into various sizes before use, again commonly only part of a mix.


Yet another natural soil, dug up and dried for use. Commonly a wide range of particle size in each bag. This is quite acid in PH and is used for acid loving species such as Rhododendron [ Azalea ] .

There are other Japanese growing media but these are the 3 main ones.

UK or European source.


Almost any crushed stone can be used , we use Granite . Screened into useful particle sizes , commonly 3-6mm. No insulating characteristic, in fact the opposite.


Sounds Japanese but is in fact UK produced. A fired ceramic product so very hard and will not break down. Quite porous so will absorb water readily and holds lots of air when dry. Used as well as or instead of grit . A good insulator.


A natural rock product crushed and screened into various particle sizes. Also very porous holding water or air if dry.  Often used in Europe on it’s own for collected [ yamadori ] to encourage rooting in vulnerable trees.

Calcined Clay.

A fired clay product, so it is usually quite hard in nature , very porous, so good for holding air or water. Manufactured to do a variety of absorbing type jobs including cat litter. However, some products are quite soft and break down quickly into a powdery mush which is a very undesirable characteristic to have in our growing mediums.

Pine bark.

Clean, sifted pine bark produced mainly for orchid growing, most useful in the 3-6 mm grade.

Very slow to break down and possibly somewhere for beneficial fungi and bacteria to colonise.

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