In previous blogs I wrote about the background of repotting bonsai trees. In this post we will go through the practical steps.
Image Caption. The tree I will repot and use as reference is a chinese maple in need of repotting. This maples roots are pushing the tree out of its pot. Its best to repot the tree before the situation gets worse.
Before starting you should prepare your chosen container, your growing medium and have all the tools you are likely to need at hand so that once the trees roots have been exposed you are able to work rapidly and get them covered again - dry roots are dead roots. I would also suggest that you allow the tree to dry out a little more than usual as this makes removing the old soil from the tree easier, and with less potential damage to the roots.
You will need:
Some growers like to submerge their newly repotted trees in a tub of water with something like Superthrive for a few minutes. This makes sure that there are no air pockets in the mix however I find this is unnecessary unless using heavier soil type mixes. Most modern growing mediums are lighter than water and will float, besides it is unnecessary to soak them as they readily absorb moisture.
Image Caption. The pot we are planting into, prepared with drainage mesh to prevent loss of medium through the drainage holes, and wire for securing the tree into the container. I use aluminium wire for this but you could use galvanized wire, however the latter will only be functional for one season as it will rust rather quickly - although by then the tree should be securely anchored by its own roots in the pot.
If it's a sunny day it is best to work in the shade as this will slow down the drying of the roots after they become exposed. After you have prepared all the tools described above, cut the wire keeping the tree in the pot from the last time it was repotted.
Carefully lift the tree from the pot. Use the sickle or an old knife to cut away the roots at the edge of the pot if necessary. Pots with an undercut, from a lip, present the biggest challenge and you may need to spend considerable time in cutting away the roots which will prevent the tree from being lifted.
Image Caption. Lots of roots! This amount of root growth is typical of strong growing species such as maples, planted in a suitable growing medium.
Place the tree onto a flat surface and begin to comb out the roots at the edge of the root ball. Perform this gently so as to limit the damage done to the roots. Trim the root ball by around 20 - 30%, cutting back any thicker roots you encounter with a sharp pair of scissors to minimize crushing of the root rather than a sharp cut.
Tilt the tree so you can repeat the process under the root ball. It is very important to remove any thick downward growing roots also as you want to encourage lateral growing roots as much as possible.
Image Caption. Trim the roots after you have gently combed them out. Eliminate any strong downward growing roots using a root cutter so the bottom of the root ball is level as depicted in the image.
Using the pick push gently into the root ball to loosen it. Start from the edge of the root ball and work your way closer to the trunk. Never forcefully jab the pick into the root ball as you may pierce a root. This step is very important because as the root ball becomes denser upon each repot the area next to the trunk will become more and more solid, preventing water penetration as well as oxygen. Slowly the roots in this area will die and only roots in the more free draining are will remain - which is no good as then all the healthy roots are at the extent of the root ball with the inner area being devoid of roots.
Image Caption. After completing this pruning the roots throw some growing medium into your pot to create a slight mound in the middle.
Now is a great time to do some root grafts if your roots or "nebari" needs improvement (which it almost always does). I always use the occasion of repotting to improve the roots at the same time as its the only time when they are exposed like this.
As this is a Trident Maple, they tend to like a more water retentive mix, and with an old tree like this where you don't want a lot of growth a wet mix will slow growth down. So you will notice that the particles are rather on the fine side. If this were a juniper or pine for instance I would not want any of those fines as this will impede drainage.
Place the tree onto the mounded growing medium and gently move backwards and forwards to make sure there are no gaps underneath the root ball. The ideal position for a tree in a pot is roughly behind the centerline when viewed from the side, and then slightly off centre from the front.
Image Caption. Secure your tree into the pot using the wire which you prepared earlier.
The style of the tree will influence this position of course as the completed planting should be visually balanced. It might be necessary to stand back repeatedly to check and make small adjustments before wiring the tree into the pot. I cannot stress enough the importance of wiring the tree into the pot. When the tree has just been repotted it will begin issuing tender new roots. If the tree is disturbed by the wind or other force, there is a chance these very fragile roots will simply break. By then the tree might have exhausted its reserves and it will either be set back seriously or even worse, it may die. If the tree has a reasonably large canopy already, the first winds will simply pop it out the pot. Unless you notice this in time the roots might have dried out completely by the time you are able to get them covered and moistened again.
The final planting height should also be such that the base of the tree is slightly above the rim of the pot, but never below it. Bear in mind also that the whole planting is likely to sink a little once watered and given time to settle. Throw more growing medium into the container and gently work into the root ball and around it using a blunt implement such as a chopstick. Keep adding more growing medium until you have filled the pot.
If using a tub with water you can now submerge the tree into it. However if you are using LECA, Groperl or other lightweight mediums you will not be able to do this, you will need to use an overhead spray. Water softly until water runs clear out the drainage holes.
Image Caption. The example maple repotted into an old Japanese Shuho pot. The surface is covered in New Zealand sphagnum moss which will keep the new surface roots in the shallow pot from drying out in summer.
Aftercare of recently potted trees should also be considered. It is advisable not to place the tree into direct sun just yet. Dappled shade is best for the first couple weeks, moving the tree into increasingly more sunlight as the weeks pass. Possibly more important than sheltering the tree from too much sun is protecting it from wind, which can rapidly dry the tree and the fresh and weak spring leaves out as there are insufficient roots to transport water lost through evaporation fast enough.
When the tree begins showing signs of growth you can begin applying light doses of fertilizer. If using BonsaiBoost you can begin with one bag or so, depending on the size of the tree and increase this gradually.
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