Changing Soil


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Mooresville, NC - USA
I have some trees that need to be changed to a different type of soil. They look like they were put in some kind of top soil or something. I want to change them to a decent bonsai mix. They are all collected trees that will probably start being worked on next year. One is a bald cypress, a barberry, and a trident maple. Should i just try and get through the winter, maybe aerate it, or chance changing the soil after dormant? If anyone could explain whats happening to the trees energy and roots, and why or why not you should do it, i would appreciate it.
I'd recommend a book called "Botany for Bonsai...the science behind the art", by Enrique Castano de la Serna, which is fascinating as he explains a lot of the botany behind the bonsai techniques we apply. Castano has a Phd in biochemistry and biophysics, and is a bonsai artist, so it's quite a perspective for an avowed "bonsai idiot" like me!

Long-Short to your question: Your goal is always to preserve the tree's energy. Energy is manufactured by leaves and ultimately stored in roots & heartwood. In the fall, all of the energy (starches and sugars) moves from the top of the tree down into the root system and the top becomes dormant. Since the energy isn't in the top now, it's safe to prune and wire the branches in the winter. In the spring, all of that energy stored in the roots makes a big push up into the remaining buds, and leaves are manufactured. Since the energy isn't in the roots anymore, it's safe to prune them and repot.

Look at a tree like a half-inflated balloon that you have gripped in your fist like an hourglass, and the energy is the air inside the balloon. When you force all the air into the top half of the balloon (simulating spring time growth), you can cut the bottom of the balloon and still have air (energy) in the top half; that's like springtime root pruning. The metaphor is applicable in all other scenarios; some of which result in the balloon popping (tree dies).

If you prune roots now, you remove much of that stored energy, and the tree has less energy to push up into the buds in the springtime. Assume it has enough to make some attempt at growing, and then you start pinching growth, you're again removing it's capacity to generate sugars/starches, and less energy is stored in the root system...downward death spiral for trees.
Does the soil they are in now drain OK? The best thing to do would be careful with how wet they get this winter, and repot at the proper time in the spring. But if that isn't possible, or the soil is really, really bad, you might consider removing them from the pots, disturbing the roots as little as possible (but you could remove any excess soil with no roots in it), and place in the ground. If you need to keep in a pot, use some well draining mix around the rootball. Then do a proper repotting in the spring.
The bald cypress won't care what soil it's in. Ditto the trident -- at least for over winter. I've not worked with barberry, but I doubt that it would be stressed, either considering that they're hedge plants.

Here in NC you just will have to keep an eye on the soil moisture. If you feel the soil is not getting we through, jam a chopstick deep into the soil a few dozen times, then water. You won't need much water in the winter, anyway.
The problem with messing with roots and soil at the beginning of winter is the roots are mostly inactive (this activity is a sliding scale, depending on species and local conditions, though). If you change out the soil, you will invariably damage roots which won't begin healing until springtime, also root growth is drawing to a close now, so they wont' really settle into the new soil as well as they would in springtime.

BC actually NEED sloppier soil than other bonsai. They do better with more organic in the mix, so don't be too anxious to get them into faster draining soil. It could be TOO fast draining.

Since you're in NC and you really don't get dramatic, long term-frigid weather, I'd wait til early March or so--possibly sooner (ask experienced local growers) to repot anything.
Clarification -Heartwood vs Sapwood

Hey Brian,
Just a technical note on your use of heartwood in the second paragraph –
“Sapwood is the younger, outermost wood; in the growing tree it is living wood, and its principal functions are to conduct water from the roots to the leaves and to store up and give back according to the season the reserves prepared in the leaves”.
Cheers G.
When were these collected? You will also need to be carefull on the work on top that you do if you will be repotting in spring, especially if they were recently collected.
Good point Bill. The barberries were collected in november of 09. One of them was given an initial pruning to start a semi cascade style, but the others have been growing with no pruning since, and have just finished loosing all their leaves. I cant tell the unpruned ones grew any better than the one we chopped. He used a mix of water retention top soil, lava rock, and pea gravel. It all just looks like top soil to me. The BC was collected in spring 2008, but not sure what all has been done to it. Ive had it since may or so of 2010. What would you suggest that BC grow in for a finished soil. Is a mix of 40% haydite, 40% crushed granite, and 20% pine bark good, or would you use more organics, or even akadama? It is ready to be root pruned, chopped again because the top is dead, and put into a bonsai pot if the roots can be reduced that much at a single time. Ill see when its time. Its been in a plastic tub 18" x 24" since collecting. The tridents were field grown, and collected at the end of feb 2010.

If part of the soil was replaced where the roots arent pruned, but just washed off. Does that still damage the roots. Maybe the smallest roots still get damaged. Im thinking ill just wait, but wanting to learn more. Ill check on that book Brian, Thanks alot. Your explanation is great. Thanks to everyone for the input.
"Is a mix of 40% haydite, 40% crushed granite, and 20% pine bark good, or would you use more organics, or even akadama"

This is a recipe for a disaster. It is far too fast draining for BC. It will dry far too fast and could leave the tree without moisture around the roots. Akadama is also not ideal for BC development.

FWIW, I use a 50 percent potting soil (yeah, icky sticky potting soil), 25 percent haydite and 25 percent swimming pool filter sand for my BC. I've been using that mix for going on 10 years. I used to use regular old bonsai soil, but found that it dried far too quickly--especially in windy weather. BCs like moist soil-have no issues with wet feet, so the risk of root rot isn't as great with them as with other species.
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