Take the soil off the roots??? Where did you read that?
The only place you want to remove the soil is from the root flare. This is where the trunk flares out from the trunk to the top-most root. You want the root flare exposed and at or a little above grade. Leave the rest of the soil in place though. The rest of the roots need the soil to retain moisture. Washing the soil off the roots is known as bare root planting, something you don't need to do.

I just had another choice field-grown tree planted with an intact field soil root-ball die this year. Same pattern with all of the several more recent failures: tree looks more or less normal the first growing season after planting, second year top is obviously stunted, and downhill from there. Clay-like field soil forming a bowling ball of markedly different texture from soil on planting site. I've found dust inside the outer shell of dampness sometimes years after planting. Kind of like a malted milk ball.

"The Bottom Line
- Balled and burlapped plant materials usually contain soil significantly different than that of the
transplant site.
- Differences in soil texture will impede both water movement and root establishment.
- Root defects can only be found and corrected if root ball soil is removed.
- Proper root preparation combined with best practices for installation will greatly improve tree
establishment and survival in any landscape"

Here is a link that might be useful: The Myth of Collapsing Root Balls

EJR - I've advocated for the bare-rooting of trees at repot and transplant time for years, for the same reasons that Dr Scott mentions. I think though, that there are some broader considerations to take into account, aside from simply removing the soil from roots and transplanting as we please.

Timing is an important consideration. It's best to repot and transplant temperate deciduous plants when they are not in leaf. Removing the soil from roots is often rather traumatic to fine, feeder roots. Many break, others, if you're not careful, quickly die from dessication, which takes only minutes on dry windy days, especially if you're working in the sun. The stress of summer transplants often sees trees shedding parts the roots cannot support, a survival mechanism in response to drought stress. While the idea you should prune the top in proportion with the roots at transplant/repot time is passe, it may actually be required if you attempt bare-root transplanting when conditions aren't favorable. You see, if WE don't select the branches/foliage to be 'shed', the tree will select them for us, and they use a different standard than the one we would choose - which is generally one based on appearance/symmetry.

To minimize stress, I would wait until the tree loses its foliage; then I would lift, bare-root and transplant at an appropriate depth, back-filling with native soil.

Al

Ok! Now I've heard everything.

Actually, I forgot to mention the very important part of the process .... correcting root defects. Roots growing back toward the center axis, circling, hooking and potential girdling roots should be corrected during full repots. Full repots differ from potting up, in that repots include removing all or a large fraction of the soil and root pruning.

This attention to roots is what allows bonsai trees to live for hundreds of years, healthy and full of vitality, while trees that are only potted up, with little or no attention to roots are certain to decline. Generally speaking, once trees in containers have been allowed to reach the point where the roots and soil can be lifted from the container intact, growth and vitality are permanently affected, unless corrective measures are undertaken to correct root defects - (see 'Plant Production in Containers II', by C. Whitcomb, PhD).

Al

Top pruning at planting interferes with root growth. Correction of root deformities more important than temporary loss of foliage. But if you read all of the page linked to method described keeps roots moist throughout process. Fall planting (of fully hardy stock) preferable anyway. Would also be preferable not to cut roots in fall so that full amount of elongation is taken advantage of, but if roots deformed by apparently careless container culture (an almost universal problem it seems) then correcting this at planting trumps other concerns.

Existing roots elongate the most of the whole year when winter buds are set at stem ends. This can be as early as August on species affected by summer drought or maturing early for other reasons.

New roots are formed mostly in spring, when winter buds open.

In both cases hormones sent from winter buds to root ends are what cause the roots to grow. Top pruning at planting time reduces the amount of winter buds. It also reduces the amount of stored energy in stem tissue that is used to fuel root growth.

I fully understand all that (reducing the canopy reduces photosynthate production, so less food to fuel root growth .....), as well as the auxin/cytokinin balancing act, but repotting or transplanting a bare-root tree while it's in leaf, especially in summer comes with its obstacles. I'm not advocating a canopy trimming on trees repotted or transplanted as a matter of course; I'm saying that in certain cases, especially in summer, it may be necessary to prune the canopy hard if you bare-root; otherwise, the plant will be likely to shed the weakest and highest branches indiscriminately; whereas, if WE select the branches to be "shed" the tree is less likely to dispense with branches you might view as an important part of the trees structure. Another way to read my comments is bare-rooting in the summer is not such a hot idea; and if it's required, there are ways to save your tree from total collapse, chief among them - reducing the canopy.

I do full repots of some 100-125 temperate deciduous plants each spring - close to half being maples. The repots include bare-rooting and root pruning. Except with plants I'm intentionally trying to slow down or improve ramification, I do the repot and wait until the first hard push of growth is beginning to mature before I prune the canopy.

Al