by Terry Erasmus August 14, 2015 6 Comments

Why do we need repot?

Unlike trees growing in the open ground where their roots have a large soil volume in which to grow, our bonsai grow in a very restricted space. As the bonsai tree continues to grow, so does it's roots. If we do not wish to upsize the pot then we need to reduce the root ball so that it can fit. A tree will actually begin pushing itself out of its pot after a couple years, this is especially so in species with strong root systems like Chinese maples.

After a few years the structure of your growing medium will begin to collapse, holding more water and reducing oxygen at the roots. This slows the growth of the tree down and can lead to various problems such as root rot. If the tree is not repotted it will slowly deteriorate in health and die.

Lastly, the kind of roots we want in bonsai cultivation are the fine, feeder roots which gather nutrients and water. We do not want the thicker storage roots which eventually develop after a number of years. So when we repot these are removed and the fine roots are encourage.

There are probably more reasons but these are the most important ones that come to mind.

Chinese elm bonsai tree forest

Image: New leaves and swelling buds on deciduous. At this time, or even a little earlier, is the best time to start repotting ie. when buds begin to swell.

When to repot

The "when" is a little tricky to answer precisely. For those of you new to bonsai I would suggest you speak with someone who has been growing bonsai in your area and ask them about repotting times. If this is not possible there are several other potential sources of local information including your local bonsai nursery or local club. 

Generally speaking we want to repot the tree when it is just showing signs of activity, as it emerges from winter dormancy. In deciduous trees this is evident by the swelling of the new buds on the branches. Just as the buds are about to burst and the new leaves reveal themselves is the optimal time. At this stage the tree has pushed a lot of sap up from the root system and thus by trimming the roots now, the tree will have enough energy to continue growing. In converse, repotting too early can severely weaken the tree as you deprive it of the energy needed to grow or it could even kill it. Repotting too late and the tree does not have sufficient roots to take up water in order to support its foliage.

Evergreens are a little different. Very often with deciduous trees they begin showing signs of growth quite early in spring, when it is still quite cold. However with evergreens it is better to wait until it is consistently warmer before repotting. Of course they do not have buds which swell, but rather new growth in the form of new foliage in junipers and extension of candles in pines. One way to check that the tree is active is to lift the entire tree and it's root ball from the pot and inspect the roots. You should see a lot of new roots, with tips white in appearance.

Japanese black pine bonsai tree roots

Image: The roots of this Japanese black pine tree are showing signs of growth indicating that now would be a good time to repot.

How often should I repot

Frequency of repotting should probably also fall under this "when" section. Normally younger trees are repotted more frequently as they are faster growers and fill the pot with roots rather rapidly, so possibly every 2 - 3 years. Mature trees should not be repotted too often, perhaps only every 5 years. This is as you want their growth to slow down or perhaps they are very old and lack energy so you don't want to stress them unnecessarily. I generally will only repot if I see that drainage has been impeded or if the tree is about to pop out of the pot. Don't repot just for the sake of repotting as everyone else is doing it. Repot because you have a reason to do so.

A more indepth explanation around this is possible but for now I think this simple statement is all that's necessary. 

Growing mediums for growing bonsai trees

We cannot have a blog post about repotting and not mention anything about growing mediums. They say that for as many bonsai growers there are in the world, that's how many variations of growing mediums there are! However there are a couple basics which I believe will help you either to select the right growing medium or to help you mix your own.

Firstly, notice that I do not use the word "soil." Although trees do grow very well in soil, this is in your garden. In a pot, soil creates too many problems. These include insufficient drainage (potentially leading to rotted roots), deprivation of oxygen and more.

The next fundamental is that there are essentially 2 components to all growing mediums; organic and inorganic. Organic includes material which will degrade. Inorganic is a component such as crushed stone, which will retain its structure.

Organic components

This includes mature milled pine bark, peat, compost or other such ingredients. Organic material is used largely for moisture retention. How much you use will depend on how often you are able to water. Keeping a trees roots wet for extended periods can result in diseases which attack the roots, such as root rot. It also slows growth down as the roots need oxygen to grow and if the air pockets are filled with water it is deprived of that oxygen. Whatever your choice of organic medium it's a good idea to sift the fine particles out as they tend to clog the mixture.

You can also add milled seaweed to your mix when repotting your tree. This organic component does retain some moisture but it is primarily a slow releasing source of nutrients to your trees.

Inorganic components

Such as crushed silica sand, river sand, LECA, Groperl and vermiculite help to maintain the mediums structure as they do not degrade like the organic component. Some also absorb moisture and release it at varying rates. However they generally are not able to absorb much so using a very inorganic mix will mean you will be watering frequently. I would like to mention LECA especially as a suggested ingredient. LECA stands for "light expanded clay aggregate." In Japan, the medium of choice is akadama. Which is essentially granules of clay. This clay is available in different hardnesses, as some are slightly fired. LECA is not akadama although it is made from clay, the key difference being that the LECA structure does not change over time. LECA absorbs some moisture but it's key advantage from my personal experience is that it provides excellent drainage and aeration to the growing medium. This results in very good root development. And it therefore stands to reason that you will have healthy foliage development too.

Olive bonsai roots using LECA

Image: LECA is a very good base to use for your growing medium as it promotes drainage and provides lots of oxygen for healthy roots.

A very basic, all round mix is generally 50:50 of organic and inorganic material. However so many factors determine this ratio including your ability to water, your local climate, the species of tree and the stage of development of that tree. So experiment with different ratios and different ingredients to see what works best for you and your lifestyle.

Remember, we offer ready mixed bonsai growing mediums for those of you who do not wish to buy the individual mediums and mix them. The ready mixes include our general mix and our professional mix.

In the next post I will use photographs to walk you step by step through the repotting process of a Chinese or Trident Maple bonsai tree.....

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Terry Erasmus
Terry Erasmus

Author


6 Responses

Malcolm
Malcolm

October 06, 2016

Afternoon Paul. Realize your above mail is now over a year old and maybe you won’t see my mail but I was reading backwards thro’ Terry’s bolgs the other day.
I live in Jo’burg and have 5 baobabs grown from seed – the oldest is about 10 and the youngest hails from last summer.
I use a very porous mix of about 25% compost and 75% course river sand, or sand and leca mixed, and I rarely water them at all.
They live in a very sheltered, sunny corner and when it stops raining up here in March/April they go into that corner and I don’t water them at all right thro’ to September/October when it starts to rain again.
When the green leaf tips appear, round about now, they might get one soaking and then they move out of their sheltered corner onto a patio table [not sure my wife likes that too much] and they then get watered when it rains.
If I feel it’s been raining too much they go back to their corner for a day or three.
My theory here is that’s how they survive above The Tropic of Capricorn and my trees, so far, are doing just fine.

Terry
Terry

October 15, 2015

Thanks for the question Des! The river sand I have seen in stores is too fine, you really shouldn’t be using anything smaller than about 2mm in your mix. In my “professional mix” and “general soil mix” the crush silica stone is used and it is graded 2 – 4mm. You could probably buy crushed silica stone at your local pool supply store as it is used in filters. Stores selling koi may also have it as it is also used in their filtration systems. You can almost use any inorganic component of sufficient size (>2mm) to promote drainage. I prefer using more LECA than stone as it does absorb and retain some moisture PLUS its much lighter when you are working with large trees. Hope I’ve answered your question.

Des
Des

October 15, 2015

We generally buy river sand from the local hardware. Is it sufficient for quick drainage and air flow or should it be sieved to collect the bigger sand particles. Is it fine to mix small crashed stone with the mix to encourage fast drainage. Thanks

Terry
Terry

October 03, 2015

Thanks for the question Paul. What is the condition of the roots of the baobab in your normal mix? If they are healthy then I see no reason to change. In my climate I would say that your mix might be a little too organic and I would use more LECA, however that’s me. I also don’t know what growing conditions baobabs prefer i.e. a drier or wetter mix, as I don’t grow baobabs. From what I know they prefer drier, faster draining mixes

Paul
Paul

October 03, 2015

What growing medium mix would you recommend for repotting a baobab tree. My normal mix is about 40% potting soil, 30% coarse river sand and 30% Leica. I live in Hillcrest KZN and water with a drip / spray irrigation system every second day.

Jan Vosloo
Jan Vosloo

August 15, 2015

Very knowledgeable

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