Root Pruning Bare Root Seedlings

by Brent Walston

Introduction

This article was an Internet Bonsai discussion between Brent Walston and Rick Choate. Rick is asking follow up questions about a previous article Brent had written on root pruning field grown bare root seedlings.
Brent had described his practice of removing the long tap root that these seedlings have and placing them in small pots about three inches deep. The tops were left unpruned even though some were almost three feet tall. The object was to get a good shallow root system for bonsai very quickly. The seedlings were then placed in light shade for the rest of the season.
Rick
I remember this good information you previously posted but don't remember why I didn't ask some questions at the time. Before going on with my questions, I think it appropriate to emphasize this is dealing, at least initially, with trees during their dormant period, isn't that right?
Brent
Yes, that is correct, fully dormant deciduous material only. Attempts to do this with conifers has had mixed results.
Rick
[You previously wrote] I cut off all the tap root leaving only 3 inches below the crown (that part where the trunk of the tree met the earth). I do this even if it leaves only a stub with no side roots. I plant these whacked trees in 3 inch pots with high quality well drained soilless mix. I leave all of the top.
I understand the small pot, you want to fill the pot with roots as soon as possible. However, I'm not clear on the advantages of leaving all of the top.
Brent:
The advantage of leaving all the top is that you let the roots determine how many buds can be sustained rather than just guessing and whacking some top material off at random. If you guess wrong and remove too much top growth you are robbing the plant of potential photosynthesizing leaves. Replacing these leaves would come at the cost of growing new shoots and buds. This would be a net loss to the plant at the start. By leaving all of the top, the roots will only open as many buds (which contain their own food) as can be sustained, probably through how much food and water can be pumped up the xylem by the diminished root system. The portion of the stem at the top that cannot receive food and water dies. Leaves open at the other bud sites, no new shoot growth is necessary to start food production. The cost to the diminished roots is minimal. Usually no new shoot growth will begin until the roots are regenerated.
Rick
[You also previously wrote] The reduced root system will limit the amount of leaves that will bud out and the size of the leaves, but you will get the maximum leaf surface possible and thus the fastest recovery and new root growth possible. Let them grow undisturbed until the roots completely fill the pot, it may take the entire season. If it does, then they may be pruned down to the first desired curve in the trunk the following winter when you normally do your winter pruning. I cut mine down to 4 to 6 inches. The following spring you will have a beautiful fat trunked little tree that can be grown out or treated as small bonsai. If the roots fill the pot by July, you may cut it then and you will get the same result a season earlier, although this a a little riskier, and depends on your zone and over wintering care.
I know you generally style your trunks by clip and grow, but have you ever wired one at the time of potting. I try to get my bare root stock in during the fall so (at least according to Andy [Walsh]) that would be a good time to wire, if I chose to do so. Also, let me make sure I understand something. I would suspect that in my climate the roots would fill this pot within a few months. I am assuming that when you say pruning the trunk in the summer is a little riskier it is because in more northern climates the new growth may not harden before winter, is that correct?
Brent
With smaller material, wiring the trunk should be no problem. There is no physiological problem of which I am aware. Much of the material I get is quite large, three quarter to one inch trunk, two to three feet tall and quite stiff, so mechanical manipulation would usually be difficult, especially with no soil as an anchor. Of course as you have pointed out my trees are cut down to four to six inches anyhow so there is little that wire would do. Wiring would be more useful if you were not going to cut your trees as far back as I do.
Pruning the tops down in summer is riskier for two reasons. One is that if you live in a cold winter area there may be insufficient time for the new growth to harden off. That is not a problem here in Northern California. The second reason is that by pruning them back to sticks again (the foliage is almost always above the point at which I prune) you ask the roots to use their stored food that you just worked so hard to achieve to move back up again and fuel a new burst of foliage. This depletes the root system again. There must be sufficient time before the end of the season for new growth to once again to pump up the roots.
I find that this manipulation of root (tissue) stored food and food making leaves to be extremely valuable in planning the growth of bonsai. It is actually a very simple concept with considerable ramifications, but few people seem to think in these terms.
I just finished 'topping' the field grown seedlings that went through this process this spring. The roots were a little lighter that I would have liked. I think a little heavier fertilizer and more light once they started growing would have been in order. The losses were surprisingly low, less than five percent. Most of the buds had opened almost to the tops of all the stems with small leaves and no shoot growth, so this strategy really seems to have worked. If I had pruned almost any amount of material off the top when potting them, I would have taxed the roots with making new shoot and leaf growth or robbed them of potential leaves from existing buds.
Rick:
[You also wrote] I have compared this method to the normal practice of pruning the top and the roots and have found it to be superior, producing a better root system in a shorter period of time.
Since, from previous posts, I know you care abut developing a good nebari, as do I, when do you start working on developing it, the next repotting season?
Brent
That is correct. After root pruning the long tap root, there is usually nothing left but a stub, no side roots whatsoever. The following spring there will be many large fleshy roots radiating from the cut end of the tap root. And they do radiate beautifully. Since I jam the end of the cut tap root into the bottom of the pot to get as much soil covered root tissue as possible, it is easy to pop them out of the pot and look at the new root growth. There is almost always a perfect ring of radiating thick roots. They are still a little fragile the following spring so you must proceed with caution, and it probably best to straighten them out but leave them all until the following season.
Rick
And lastly, do you know why this method seems to work better than the technique of pruning the top as the same time as the roots? Do you think it allows the tree to focus its energy toward 'healing' one area at a time (as Andy [Walsh] suggested in his post on heated trees)?
Brent
Pruning the top will result in a net energy (food) loss to the roots in the short run by either forcing roots to fuel expansion of new shoots and buds to increase production or by indiscriminately limiting the leaf surface area without regard to how many leaves the remaining roots can sustain.