Bonsai School – Growing Mediums

By Peter Snart

Growing mediums for Bonsai

Let’s take a look at the variety of growing media available and their mixing potential, being, as it is, the start of the potting season.

In the early eighties, when I first got interested in bonsai, I had a fairly typical beginner’s view of bonsai soil. My opinion was also influenced by my earlier forestry training and years spent planting trees in a wide range of soil conditions throughout the country. I had planted literally thousands of trees of a number of different species into almost every different type of soil , and by and large they seemed to grow!  There were a number of things, which at that time, I was totally unaware. First is the really big difference between the way the roots of a plant respond to growing in a relatively shallow and small ceramic pot compared to open ground out in the landscape. Second is the difference between a tree that may be doing little more than just surviving , particularly whilst it gets established in the ‘ wild ‘  and a tree thriving and growing at or near it’s maximum potential as a developing bonsai! Which is surely what we would like our trees to do . There is no virtue in taking ten years to achieve exactly the same as can be attained in half that time , we are not masochists !! at least most bonsaists are not !

So I used loam with some added grit and congratulated myself on not buying any of that fancy, expensive, Japanese growing medium. But as I began to learn a bit more I could see that the root systems of my bonsai were just not developing in a very desirable way , the roots pretty much kept the trees alive but it was taking a long time to get anywhere with such as ramification and at last I learnt that to do that my trees needed a much more ramified root system which was just not happening but why ?

The most significant factor was undoubtedly the poor drainage characteristics of the growing medium so let’s have a look at how this might impact on the roots themselves. To do this we need to concentrate on Autumn and Winter conditions. Both loam based soils and peat have the capacity to retain a lot of moisture and even mixing grit has limited beneficial effects on this. A useful analogy to consider here is the mixing of concrete!  In doing this one might commonly take something like 20mm Decorative Aggregates, which we would all agree is pretty free draining , water poured on to it will just pass rapidly through the gravel. But, add to the gravel some fine sand and some even finer cement powder and mix thoroughly, now add water and you get a real sticky mess ! Unfortunately adding grit to loam or peat works in a similar way . But what actually occurs during the winter with this soaking wet growing medium ?  Well, when the whole lot freezes that’s when you can get significant damage to those nice fine roots that are so important to our bonsai. Now this will rarely result in the death of the plant , though on occasions it will, but what will happen is that a proportion of the root system will either be killed or will suffer damage. The consequence of this is that the tree spends a large part of the next growing season recovering from this set back instead of growing and developing in the way that we want our bonsai to do.

In a well draining medium there will be minimal water retained at the time that the freezing occurs and thus there will be less damage. Of course, a free draining mix has implications for Spring and Summer care.  There may be less rainfall , sunshine and higher temperatures and the trees will be growing well and therefore using significant amounts of water from the soil , this will all result in the medium drying out more quickly which may necessitate more frequent watering but this is a chore worth suffering for the better over wintering of the trees.

So some years ago I stopped using any organic material, except clean, graded pine bark which breaks down very slowly ,  in my standard mix resulting in a big improvement in the overall health of the bonsai. However, it is important to stress that this change may alter dramatically the way you undertake the day to day care of your trees. If by necessity, or indeed by inclination, you are in the habit of only looking at your bonsai say, once per week , you might find that in a dry spell they will dry out just too quickly for the good of the plants. You might have once been able to go away for a few days confident in the knowledge that they would not dry out, that may no longer be the case !

Let’s suppose then that you have abandoned both peat and loam as a basis for your mix, what alternatives are you left with ?

Grit ? Actually most tree species will grow perfectly happily in just grit on it’s own if you provide all the nutrients. Just think how readily seedlings sprout up in gravel areas. The trouble is again related to Winter conditions because grit will provide no insulation to the roots at all during cold weather and if you add to this problem the fact that our ceramic pots are also poor insulators it can make the fine roots just too vulnerable.

So we need to add to the grit another component that will act in some way as an insulator. The Japanese use a product called akadama , in a large part on it’s own and with excellent results. Akadama is a clay product mined in Japan, sieved and graded into a variety of particle sizes and dried into a semi-hard grain. There are different qualities available that differ mainly in their hardness. The double red line quality is the only one suitable for use in our climate. Now, over the years akadama has had a bit of a bad press and mostly totally unjustified . It is true that in our alternately wet then cold winter conditions there is a risk of the frost breaking it down into a fine silt relatively quickly which is why we have always used it in conjunction with a grit and maybe another granular type product as well to help maintain a free draining character. It is also likely that soft, poor batches of the product have contributed to the bad reports. It is also incumbent upon the bonsiast to mitigate the affects of heavy or persistent rainfall during winter months, in what ever way is most practical for them.  Because dry akadama will contain at least some air, and air is a very good insulator, then the roots will be better served in cold conditions by the mix of akadama and grit , the grit helping to maintain spaces between particles as the akadama slowly breaks down.

Kyodama is a ceramic based product that can replace, or be added as well as, grit. Kyodama is very porous , capable of holding a lot of either water or air.

Something else that might have to be considered when thinking about growing mediums breaking down is re-potting frequency.  Our old well formed bonsai need potting only infrequently but young trees still having their root systems developed so as to suit a plant that will spend it’s life as a bonsai may need  more often to have the roots pruned and the soil changed.

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