I grow a lot of deciduous material for bonsai, including Malus. My normal practice is to grow them for 2 years, root prune in the fall, lift the plant in spring before budswell, chop the trunk back hard (to induce taper) and finish the root pruning with a focus on the fact I'm going to want to be able to put the plant in a shallow pot when it's ready. I usually rotate the tree 180* each time I lift it. It takes 6-8 years to grow an apple from a cutting to where it has the kind of visible root system (nebari) and taper that a good tree needs and it's at the point it's ready to be moved to a training pot.

I'm very close to being in the same zone as you, so there should be no reason to expect problems.

Al

I've done it in the fall for transplant in spring...smaller trees, it worked but only done it so I cut back my work load come spring.
Preferably, I dig in spring, better for my cold zone and probably for yours also, I don't think any advantage,..it can be the opposite.

What is the diameter of the trunk? Root pruning before transplanting crabs and other trees is something written about but seldom actually done. For crabs, I don't see the point, as they can withstand a lot of root loss and still survive, although the leaves will look like hell for a growing season if the tree is fairly mature.

When root pruning is done you are supposed to trench beyond the severed roots and fill with a loamy rich mix to encourage lots of fine roots there that can be moved with the tree. Otherwise it is a complete waste of time with any tree, as far as I know.

I suggest you move the tree bare root once it drops most of its leaves this fall taking as much root with the tree as possible. You might want to hire some help if it is a big tree and look for a you-tube video of proper procedure.

the tree is about 4 inches in diameter...

The valuable roots occur in the undisturbed soil proximal to root truncation, and go with the tree when it's lifted. Obviously none will appear distal to the point of truncation, so filling the trench with foreign soil would be of little value. The polar flow of auxin, which stimulates root division will be concentrated in the distal area of the truncated roots, which stimulates root development in that area and provides fine rootage to help the tree better tolerate the stress of transplanting. It's not filling a trench with loam that stimulates root development, it's the level of auxin that builds in the area immediately proximal to the root truncation.

Al

Tapla, the method I described was what I was taught 25 years ago when I was in hort school- I never actually took it as a useful technique, if only because it's too much work. What you are describing is not something that I've ever heard of so a link to supportive research would be helpful.

Here is a link to research that, in part, supports the method I describe, although no improvement of soil was done (beyond aeration, which is certainly something), but the trees were dug up beyond the point they were root pruned when transplanted and had much more functioning root than the control trees because of root growth stimulated outside of the pruning.

I spent a half hour trying to find research that supports your contention- it does seem logical (but far from obvious). I just couldn't bring anything up.

Here is a link that might be useful: root pruning

I never thought about having to look up any research. Many years ago, I reasoned through the physiology part, then looking at the results of the thousands of plants I've root pruned in containers and in the ground confirmed how it all works.

This is on a smaller scale, but here's a plant I initially air layered off a trident maple:
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Getting into the root pruning here
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Root pruning complete
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Top quality trees for bonsai require a perfect root base. We either have to graft roots onto the plant or create a perfect root base some other way or the tree is just 'eh'. Note the tourniquet, which blocks the downward movement of auxin and photosynthate through phloem tissues where it accumulates in the tissue above the tourniquet. I drilled 7 holes through the phloem and vascular cambium, filled the holes with rooting gel, repotted the plant. You can see the result.

Trees in the ground are no different than trees in pots. I manipulate them in the same fashion, but rather than using a root cutter I'll use a lopper or very sharp spade to sever roots in fall & lift the tree in the spring. Of course, there is an advantage in the timing I use, seldom going longer than 2 years w/o root pruning trees in the ground, which helps to keep fine rootage close to the trunk.

I also collect (from the wild) yamadori, which are specimens taken to be trained as bonsai. Often, you'll find a specimen with only 1 primary root, and no fine roots for many feet from the trunk, especially in rocky terrain. If you want to collect those trees, you need to be inventive in forcing the tree to produce roots close enough to the trunk that you can collect the tree. Sometimes it's a process that takes up to 3 years to get the tree to where it's collectable. I don't want to jinx myself by saying it, but I haven't lost a wild tree yet. I guess the whole point is, this in-ground root pruning stuff is a valuable aid when it comes to easing the plant's transition to a transplant site.

An interesting aside: I at a dinner 2 weeks ago with a past president of the Michigan Nurseryman's Association. We were talking about "something new", I don't even remember what it was. He said, "You know, Al, we don't do ANYTHING (his emphasis) the same way that we did 30 years ago. Everything has changed, and we had to relearn everything we thought we knew along the way". The man is almost 90 years old, and I was surprised that what he attributes the largest change of all to is the advent of plastic. Anyway, the point is, what was considered a good and reasonable practice in 1987 has probably been improved upon considerably since.

Take care.

Al

Thanks to everyone for their in-depth consideration, but I have to say, I'm more confused than ever! It sounds like some folks are saying that root pruning itself is of limited use. Is that really so? If so, can I dig it out now and transplant right away??

Thanks!

Well, ..that potted bonsai is a different scenario all together.
Neil,
If you had a frost or two then transplant now but better in spring for zone 5 and colder.

okay! Will root prune now and move in spring

No advice here suggests there is an advantage to root pruning in fall and moving in spring. The only conflict is about helping difficult to transplant trees without fibrous root systems SURVIVE transplant and it is about allowing the pruned roots at least a growing season to store compounds inside the cuts that will help a tree establish roots quickly (or about establishing a flush of fine roots that can be moved with the rootball during transplant).

A 4" caliber tree is very difficult to move, depending in large part on the nature of the soil. What you should be concerned about is moving as much root as you can with the top of the tree- this is what all researched guidelines emphasize.

Are you planning to move a root ball with soil or to move the tree bare root? If you are doing it bare root you should certainly just do so in one operation because you don't prune roots when BR transplanting, you bring them with the tree, unless they are too difficult to dig up

If you are planning to move a root ball for a 4" caliber tree you are going to need equipment to move the root ball and are going to have to ball and burlap it unless you have a really big back hoe to use. It takes two men just to handle a properly balled 2.5" diameter tree.

I have a fruit tree nursery and move up to 3" caliber apple trees bare root on a regular basis, I would not generally want to move any tree larger than that bare root or B+B'd. It is more work than the worth of a tree to me.

By the International Society of Arborists standards, a 4" caliber tree requires moving a 38 inch diameter root ball and keeping it intact during the transplant. Good luck!