First of all, my apologies for missing this column last month. The deadline passed, and as such, most of your repotting season. But I wanted to cover a few things that came up during the February presentation and following workshops. I hope that this will help fill in some gaps and set you up even better for next year.
The subject that I wish to grapple with is bare rooting. A bit of clarification is needed in terminology. Some folks see bare rooting as merely removing just the soil from the roots. Other folks will see it as removing all of the soil AND washing off the roots. So here is the analogy. The first is like stripping down to your underwear. A bit cool but bearable for a moment. The second leaves you stripped naked. All protection is gone, and that is the way we need to look at it. When you wash the roots, you strip away any bit of security the roots have, any mycorrhizal mycelia that may be present. This may be fine for some trees, and death to others. In general, any evergreen tree, whether a conifer, or broad leaved tree, does not like to be bare rooted, and much less have their roots thoroughly washed. There are of course, exceptions, but it would be rare for an evergreen. Last year I bare rooted and washed almost all of my deciduous trees. I was fighting a still unknown disease that I suspected was soil born, and spread through the reuse of soil (dried, resifted, etc…) It was a ton of work, especially since I had to dig about thirty trees in growing beds and do that same process in order to move to a new house. A monumental task. But everything survived and is busting out this year, with kind of growth that I have not had for some time. I attribute this to the complete renewing of the soil, a spraying program during the year, and aggressive fertilization in late summer and early fall. But this would not be possible on a conifer. I would have to come up with a different strategy.
With conifers, if you need to change out the soil because percolation is non-existent, or because you just have some sort of mountain muck for soil, you can remove up to half of the soil all the way to the base on one side of the tree, or split it up in wedges. Be sure to do the WEAK parts first. If you do the strong roots first, then the entire root system is weakened. When the tree recovers in a year or two from that, you do the other half. If you have a fairly newly collected tree, limit yourself to one third of the total volume. Another technique I use is to lightly trim a bit of the outer roots and put it into a generous sized pot and let it grow some new roots. In the next repotting, I try to leave the new, strong, outer roots alone and go after the black gunk up under the base of the tree. It gets cleaned out, but the process is buffered so that the impact is minimal.
So, you may ask, why the difference between conifers and deciduous trees when it comes to bare rooting? I have no definitive answer, but I like to think of balancing the roots with the foliage load. When a tree has lost all of its leaves, it has no load. The roots are not required to supply moisture to the leaves. This can happen naturally as the leaves depart their branches in the fall, or induced by our cutting them off (defoliation) for some other purpose. That means that spring is the best time to repot – no load on the roots, so we can cut, bare root, whatever, andthe treehas time and temperature to recover as the leaves emerge. On evergreens, we must be much more careful. The tree ALWAYS has a load on it, so anything we do to the roots can induce stress. We need to limit root work as much as possible to provide as smooth a transition as we can. Anytime we work on roots, no matter what kind of tree, we are impacting its health. We may not notice the effects, as the tree marches on. Heck, we may want to purposely slow it down. But be careful. Repotting any tree may cause the needles or leaves to grow longer and larger as it seeks to recover.
Respect your elders. Deciduous trees get old, just like conifers. It’s just that the relative range is shorter. So if you have any kind of older tree, treat it more gently than you would a new seedling. Younger trees may need to have their roots worked on every year to help establish a solid foundation. As the tree ages, it can be repotted less often. On a deciduous tree, you are still going to want to really clean it out every couple of years to inspect and makes sure that no single root is getting too strong, but in general things will begin to slow down and even out. Less drastic work is needed. On conifers, even on young trees, you want to let them go a couple of years between repotting. Mature trees can easily go five or six years. If you have an older tree, or any bonsai approaching a mature state, you really want it to slow down and produce smaller growth. That’s what all that nice Akadama in your mix is for. The roots are able to penetrate the particles, drilling right through them. This increases the available space for roots to grow through multiple seasons, stabilizing the tree. That is when the fine ramification starts to occur. That is what we are striving for in the end. So next time you get the garden hose out to blast away on your trees, proceed cautiously.