Why EARLY SPRING?
Well, firstly it needs to be slightly warm for new roots to grow. If you cut the roots in the Winter, it is still too cold. The root growth slows down and they will find it hard transporting energy to the branches and leaves. Now if you repotted in the middle of Summer this causes another problem. Because it is too hot, the tree needs plenty of water to maintain growth and if you have just cut the roots, they will be in high demand, putting too much stress on the roots that you did leave. The end result is a dried out dead tree.
|This particular tree is a Trident Maple. It is a very old and quite a large tree that I helped repot in Japan this Spring.|
The first step is to remove the tree from its pot. When you prepare to repot a tree try not to water it the day before. By not watering the tree, it becomes a little easier to take out of the pot and also to remove the old soil.
Have a look at the bottom of the pot and cut the wire that has been holding the tree in place to begin with. Take the tree out carefully as you do not want to damage it in any way. Some trees just pop out nice and easily but other, larger ones behave differently. For those larger trees you must use a root pruning hook and scrape away at the inside of the pot. Eventually, the tree will come out.
|A healthy looking tree. Quite a lot of new roots have grown over the past year.|
Tilt the tree carefully on its side and begin scraping the bottom of the soil away with a strong root pruning fork. As you scrape away the old soil the roots will begin to untangle. Once this happens, start cutting them and continue to scrape. I was surprised at how many roots you can cut and how close you can get to the base of the tree. The idea is to develop roots that grow out from the sides of the trunk as opposed to those that grow downwards.
|Quite a flat base. Roots that grew down have been cut and the tap root has also been cut into forming a slight concave shape. This can assist the tree to sit up a touch higher in the pot.|
|Again the same tree after it has been placed on its side. I wanted to show you how much soil and roots you are able to remove safely.|
Let's get back to the large Trident Maple.
After we scrape away the soil, cut the roots that are growing down, and remove those that are growing across and on top of one another, it is time to add new soil. We keep the same pot as before - sometimes you may want to vary the pot.
We then put some netting over the drainage holes to prevent the soil from coming out. Next we have to prepare to secure the tree in the pot with wire. The tree must be stable in the pot for the roots to grow properly. If the tree is not stable, there is a chance of the roots not taking. If you have really large trees it may not be necessary to hold the tree in place with wire, the tree's weight will do that job by itself. I plan on writing some more posts dedicated to tying trees in pots at a later stage.
Because this pot is quite a big size we have to add a layer of pumice (a lava rock which is extremely light) to aide drainage. The larger pots will hold more water in the bottom for longer periods of time. This is not great for the tree as it might be too wet for the roots to grow, resulting in root rot. If you have a smaller tree the bottom layer of pumice is not necessary as the smaller tree in pot will already dry out quite quickly.
|You can see the wire that will be used to tie the tree securely in the pot.|
Deciduous trees like holding more water than conifers. Because we know this we add a greater percentage of acadama to deciduous trees. The acadama will absorb more water and the river sand will allow the water to pass more freely. If on the other hand we were adding soil for a conifer we would add a greater % of river sand - maybe 30% or 40%. Conifers like to be on the drier side. The lower % of acadama will not have the same absorption rate and therefore the soil mix will be drier.
Having said all that, it also depends on what development stage your tree is at. If you already have a refined tree and you do not want it growing too fast anymore, add more acadama. A TREE WILL ALWAYS GROW FASTER IN SOIL THAT IS A LITTLE DRIER COMPARED TO WETTER. So if we have a tree, a tree that is quite refined, add more acadama - this will be a wetter mix because the acadama will absorb a lot of water, and it turn keep the soil mix wetter.
Now say you want your tree to grow quicker because we are still developing it, add a drier soil mix - more sand and less acadama.
Acadama in Japan is cheap. Acadama outside Japan is expensive. If you can't afford acadama then all is not lost. However, what you must do is find something that has the ability to absorb water but isn't too compact.
|Beginning to add the acadama, river sand, and charcoal.|
|As this tree is tilting slightly towards the left we place the tree off center on the right side. We do this for balance and aesthetics.|
|The next step is to secure the tree by tying the wires together. Look for a large root as an anchor point where the two wires can twist up against - that way, the tree will be nice and stable.|
|This is known as a 4 point tie.|
HOW OFTEN DO YOU REPOT?
It DEPENDS on the development stage of your trees. Firstly, repotting is stressful for your trees so it is important to fertilize well during Autumn in preparation for the transplant.
The year after you repot the tree will grow well but it will grow a lot stronger the following year. Normally it is the second year where the tree will flourish. If you have a tree that is refined and you do not want it to grow much bigger it is a good idea to repot your tree every year. That way, you can slow down some of its growth.
If you want your tree to grow quickly, plant it in dry soil and repot every two years. The larger trees that you have, consider repotting them every 3 years.
Conifer trees do not need to be repotted as often as deciduous trees as they generally grow slower.
Also if you have smaller bonsai you will probably have to repot them more often than the larger trees as their pot will be filled with roots at a quicker pace. An easy thing to do is lift the pot and see if the roots are growing out of the drainage holes. If they are, then it is best to root prune. The other thing you can do is feel the soil. If it is rock hard then repot. If the soil is rock hard this means that water penetration to the roots will be difficult resulting in an unhealthy and weak tree.
One final point. After repotting do not fertilize your tree until after about 4 weeks. The reason being that the fertilizer can burn the root tips and kill them. As you reintroduce fertilizer, do it gradually.
Definitely the final point...root prune conifers a little before deciduous trees. Conifers can be root pruned in late Winter - they are nice and tough and have green foliage to keep them alive until the new roots grow.
Hope this information helps you out there.