After last month's program on repotting, I thought that this would be a great time to review what was covered in that session, both to reinforce and amplify techniques for repotting. Next to watering, repotting is the most difficult aspect of bonsai to learn. I for one am still working at it. As such, it's not something that you can just put down on paper, especially without any pictures. So we are not going to try and cover everything, but hit some of the basic concepts and details that might be different than what you might already know.
First of all, know why you are repotting. There are just a few basics reasons:
1. Aesthetics — you want to change the pot size, style, etc... or change the planting position.
2. Loss of percolation — the soil is compacted and no water gets into the rootball.
3. Soil breakdown — the soil particles have broken down so far that they retain too much moisture for the tree.
Knowing the reason you are repotting will guide you in many decisions. Don't do it just because the calendar says so. A conifer can go six to seven years without repotting, and deciduous trees a shorter time. Before you start, have all of your pots and supplies ready. Have your soil sifted and mixed and ready to go. A good basic mix is one third each of Akadama, Pumice, and Lava, or 1-1-1. You can shift to 2-1-1 for more of a boost to the tree, and deciduous trees can even go to all Akadama. Sift all of the particles down to between one quarter and one sixteenth of an inch. For shohin, just use from one eighth down to one sixteenth. I find the easiest way to mix the components is to dump it back and forth a few times between two bins. That way you don't crush it.
Now that you are ready, free the tree from the pot. First, clip the wires at the bottom and clear the drainage holes. Delicately use a chopstick or sickle to free the edge of the tree from the pot. You only need to do three sides to get it out. Usually I find a piece ofjin or something firm at the top of the tree to tip the tree out of the pot. Once it is out, make sure that you know where the base of the tree is and where the new soil line will be. Then start working the bottom into a flat plane that can sit flush on the bottom of the new pot. Once that is done, work in the sides the desired amount and don't get too greedy to get it into that new pot you bought.
Take care of the health of the tree and don't do too much at once. You can really do a lot when you have the proper technique and aftercare. Know your limits, as each tree will have a different challenge. If you are working with a collected tree, especially one due for it' s first potting, be very cautious and do not handle the roots too much. Make sure that you are spraying the roots with water before they start drying out. Double check your pot dimensions and how much root you need to remove.
Once you are ready to go, spray the root ball again and prepare your pot with tie down wires. Galvanized Steel is great as it does not stretch like copper or aluminum and it is cheaper. 16 gauge is great for heavier trees and 18 works for medium trees. For shohin I prefer aluminum because it doesn't take much to hold them in. I found that Orchard Supply or hardware stores have a great selection of sizes of rolls to choose from. Home Depot is a little short in this department. Be sure to pre-bend your wires before inserting them into which ever holes you use. A ninety degree angle, nice and sharp. Make sure the portion between the two holes is convex so that it pulls up against the pot. And speaking of holes, if your pot does not have holes in the right place to anchor the tree, drill some new ones. Use a tile bit in an electric drill and a spray bottle with water to cool the bit. Mark your hole and begin drilling. Once you start penetrating through and hear the sound change, stop and drill from the other side to finish. This will keep the bottom from blowing out.
Add in a single layer of drainage material of the same proportions as regular soil, but at one quarter inch size. Then add in some regular soil on top of that, with a mound in the middle. Place your tree on the mound and settle it in well. Use pointed bamboo chopsticks pounded into the side of the root ball to secure the tree. Wrap your tie wires around the bamboo and tighten while pushing down with your thumb. That should get things nice and snug. Now that the tree is secure, add in plenty of soil.
Use your beveled chopsticks on the sides of the pot to begin working the soil in. Plunge the chopstick all the way to the bottom, then move it up and down lightly to allow soil to fill in the space behind it. Repeat as you careful work your way around the tree. Feel for the roots with your chopstick so that you do not damage them. Use a fine chopstick around fine roots and check for voids under the root mass. Are you still using lots of soil? Heaps of it? Hold the soil firmly with your left hand and you chopstick with the right. That will keep all of those small roots in place. When everything is firm, brush off the excess soil and tamp it down. Be sure to leave a lip at pot's edge so that the water does not run off quickly.
Then sprinkle some shredded New Zealand sphagnum moss on top. This holds the soil particles in place and keeps moisture in the upper levels of the root mass. If you want to hasten the growth of new moss, shred some dried green moss into your sphagnum mix. You can find this moss, which is much cleaner than our domestic moss, at pet stores or places that sell orchids. Be sure to spray the moss with a fine mist before watering in or you will wash it all off. When you water, be sure run the water until it runs clear out of the bottom. Be sure to protect the tree from freezing and if it is particularly tender, it can go on a heat bed for a few weeks to give it a head start. I know this was rather quick and brief, but hopefully you have picked up something new to make the transition easier for your trees.