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