I wouldn't hesitate to reduce the ht by as much as 3/4 by cutting the top off immediately above any branch that tends toward the vertical. If you look at the picture, you'll see a number of branches coming off the main trunk at about the ht of the middle horizontal fence board. Pick any one of those as your new leader, preferable a branch that has only one opposing branch on the other side. Cut the opposing branch back to a stub. You'll get a new chance to learn how to restrain the top. ;-)

What you can do if you like, is leave 12-18 inches of the trunk above your new leader. Then, you can tie the new leader to the stub so it's growing vertically. If your tree is leaning, you should be able to correct that when you repot, or by carefully selecting the branch that will be the new leader so when you tie it to the stub, it is actually vertical.

It looks like you have some branches occurring low on the tree. If you're daring, you can cut all the way back to the lowest branch and the tree will be fine. Lol - usually, the tree will tolerate the treatment better than the grower, who will usually be a nonbeliever and certain the tree will die if you cut it back. If you were to donate that tree to me, and the graft union was something tolerable for bonsai, I'd cut it back to about 6" from the soil line with no concern about whether or not it would tolerate the operation. It will.

Questions?

Best luck.

Al

Here is an oriental hornbeam that is about the same trunk caliper as your tree. You can see how I chopped it back, then chopped off the branch that is quartering away from you. The tree is fine - leafing out nicely. I'll leave it in the ground until the wound has healed, by which time I'll have chopped it back several more times to induce taper - so when it's a bonsai it will look old. Building trees that taper very quickly is one of the tricks we use to give the tree the illusion of great age. Though the change in taper is very radical as things are now, within 2 years the transition to the branch above the cut will look entirely natural.
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Al

I had no idea a maple could get that tall in a container that size..wow

This is going to be very interesting.........:-)

MIke

Al--thanks so much for responding. I'd hoped you'd do so! :) Thanks for including the picture of the hornbeam. Do you happen to have an example of a bonsai further along in the tapering process?

I'm going to trust you on this and cut off over half the tree. Can I go ahead and do this now? After I cut back the trunk and pick a new leader, are there any next steps?

Also, I have a half whiskey barrel that I was going to repot it into. Once I've reduced the height of the tree, could I keep it in it's current planter? I'd rather not have to try and move around something the size and weight of the half barrel.

Mike--I'm surprised, too!

While I'm not Al, I do have a series of photos of a tree that shows how taper is beginning to form due to a series of heavy pruning and regrowth.

This is an Acer palmatum Coralinum. It isn't as large caliper as your Orange Dream or Al's Hornbeam and I didn't originally cut as low as Al's Hornbeam (different end objective), but maybe it helps to see the progression.

Ball and Burlap from nursery:
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Initial pruning cuts made based on Al's guidance:
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At the time Al suggested even heavier cuts. Not being as experienced as he, I was afraid to make those heavier cuts and let the tree grow. Here's how it looked by mid summer last year.

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So I made additional heavy mid summer pruning cuts:
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By fall it had filled back in:
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And here it is early this spring:
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And after spring pruning:
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If you compare last years spring pruning to this years spring pruning, you can see the taper beginning to form by cutting back hard, letting shoots grow, then cutting hard again and letting shoots grow, and cutting hard again.

Hope this helps,

Blake

You can cut it back now if you wish. After you cut off the top, there will be a branch opposite your new leader choice. Cut that branch back to a stub. You're going to get a lot of back-budding, so plan on learning how to manage the energy - I'll help with that. Basically, it includes pruning progressively harder toward the top of the tree.

If the plant is very root bound now, you should at least pot up. Then, next spring you can repot it and go back down (considerably) in container size.

Here are some plants I chopped back. They are all prebonsai, so you can see the scar and how it's healing. Some of these cuts were very significant, given the trunk caliper and how far they were cut back.

mulberry:
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Aeonium:
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Ficus:
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The following pics were taken a few minutes ago. Virtually all my trees, except those grown in literati style, go through several taper-building chops, so 95% of my trees have been or are waiting to be chopped. It's an every day occurrence, so you can see how I can be so nonchalant about it. ;-)

little leaf linden:
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Cotoneaster:
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Acer P (Jap maple) chopped twice so far:
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Acer buergerianum (trident maple) - the next chop is already planned - just above the really thin branch:
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another Acer b - this one will eventually have the center of the 3 trunks carved out of it (I think ...... it may well have other ideas about what it wants to be when it grows up. It will let me know & I'll act according to what it says. ;-)
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For you guys that like Japanese maples, and don't mind green foliage, I highly recommend trident maples, as well as A. ginnala (amur maple). The later is extremely hardy, and both are nearly care free, other than regular root work if it's in a container - not as fussy as most A palmatums.

Al

Thanks for the great examples, Blake and Al!

I'll tackle the cutting back this weekend and post some pictures of how it goes.