If you're mauling the roots late winter, not long before bud break would probably be best. New root growth from cut ends is prompted by dormant buds at shoot tips opening in spring.

panserbjorn,

This is what I have noticed over the last few years in TX. I have always repotted my container maples in mid feb, a couple of weeks or so before the break dormancy in TX. Like you, I always pretty much bareroot everything the first repot that I do after I buy them, just to make sure the root system is all spread out ( I have had tree in 3 gallon sizes whose rootballs were tangled up into the shape and size of a 4" container. Had I not repotted them, I would have never caught the tangled/wound up roots.
I have not lost any tree due to repotting in early spring, but have always noticed that the repotted trees put on minimum growth the first year of repot.
This year I have repotted 3 trees in the last couple of weeks, and want to see if there is any difference between the time of repot. But because of the warm weather we are having in TX this year most of my maples are just starting to show fall colors, making any repot now a little more risky especially if bareroot repotting.

An article that I found regarding fall repotting.

xman

Here is a link that might be useful: fall repotting

Need to move on to something else so without reading that last link first I will just say: Bare-rooting always produces a setback unless most of the roots are somehow not damaged. Doing it in spring (end of winter) you at least get some new tips coming out immediately after. Doing it in fall the tree will sit there all winter, in a damaged condition with little response and repair occurring. Top growth will not pick up again until new tips have elongated enough to start replacing lost root area.

If the specimen can be re-potted in fall, with the roots undisturbed then fall potting has the advantage of existing root tips elongating into the new potting medium. This would coincide with potting on into a substantially larger pot.

Pruning of roots when re-potting is used to dwarf tops in bonsai culture. The roots are everything with a tree or shrub, if something significant happens to them the top is dramatically affected.

This topic is very interesting to me. I've been following a similar discussion at UBC acer forum.

I re-potted and root pruned about 1/3 of my maples in sept and oct as an experiment. Fall re-potting/pruning never occurred to me before now! Looking forward to results in spring

Here is a link that might be useful: Repot Fall or Spring?

I pot anytime. Since I'm not in commercial production I don't care if one time is a bit better than another. If the rootball is not rootbound I don't even tease the roots apart. One small advantage of potting up in the Fall in colder climates is the added insulating value of the new soil around the rootball. Spacing the pots a bit and adding woodchips around the pots helps also.
To protect a pot in the hot summer sun I have sometimes put the smaller potted up plant in a larger empty pot.
If I purchase a plant I repot asap into my own potting soil I make up.

I've been following the discussion on the UBC site, too, and also never thought about repotting in the fall. I did most of my Japanese maples in containers, no root pruning, but I'm in zone 5 and they had about a month before being put into my unheated garage, which stays above 32 degrees because it's under the house. I must say the article kind of freaked me out. I hope I didn't mess up by doing that. I have noticed the setback from repotting in spring, even though it is before they break dormancy and even though I didn't do any root pruning. I guess we will have to wait until spring to see the results. Another reason I wanted to try repotting in fall is because I could tell the chips in my mix had broken down quite a bit and I was worried the mix would stay too moist over the winter. I do have some that I did not get to, so I will be able to compare the spring transplanted to the fall.

Kay,
Instead of root pruning do you always pot up to larger containers?

How long do you usually go between potting? This year i used a lighter mix and the pot was full, but not bound, in fall after sprung root pruning. I root pruned a few of those to see how they take off in Spring.

herman
The only reason I potted up to larger containers is because the maples I have aren't very old. Eventually, I will have to root prune (dread it) but so far they are not pot bound.

As far as repotting, I was thinking I read every two years, but seems like everyone else is saying each year. I don't know...what is the concensus? Right now I am just trying to get the mix right. Not too heavy, not too light. I am not really good about watering, so I worry about small pots and light mixes. On the other hand, in my part of the country, we seem to get water "feast or famine" and if my mix is too heavy, they suffer (drop leaves, etc.). I feel like I am getting close to having a good mix, but I was surprised how quickly it seemed to break down. I was using pine mulch instead of pine bark, the bark just seems too open. Maybe I'll have to use the bark in the future.

Let us know how the root pruning seems to go. Did you root prune just to experiment or did they need it?
Kay

Kay,

Definitely a good idea to make the soil mix complement your watering schedule.

The root pruning is definitely required. There will be a huge tangle completely filling the bottom 1/3 of the pot. I always take at least 1/3 of the roots and sometimes 2/3. But it does definitely result in a setback. Hoping the autumn pruning might diminish the setback.

I find that the lighter mix encourages quicker root growth which requires more root pruning. I prune the smaller ones

I don't know if you all have already visited this particular website, but I found this site, and it was helpful because I didn't understand root pruning.

http://www.gardengatemagazine.com/extras/72video-rootpruning.php

I have two JM in containers, and I know they're ready to be repotted, but I want them to grow taller. Does anyone have any pix of their container Japanese Maples they would like to share? I'm really interested in any large maples someone may have in a container.

Thanks.

Hey Mum,

I'm also in Z7. I viewed the root pruning vid and his root pruning technique is like mine. I do not use a peat based medium to repot, and i trim maybe about 25% more than shown. I have a few trees that have been in pots for 10 yrs or more. I have not figured out how to post here but the link below is a Matsugae potted for about 5 yrs in Cape May NJ. It stays outside year unprotected year round, it is a robust grower. Source: Forest Farm.

Here is a link that might be useful: Post #3 -- 10gal 'Matsugae'

Thanks Herman for the link. That website has a wealth of information! The pix of the Matsugae in the container was nice. I really like how it has been incorporated into the garden.

I don't want to restrict my trees' growth, so I don't know whether to root-prune or repot now, or just wait till fall. There are so many different opinions about this topic. I love my maples and I don't want to do anything to damage them.

Mum, if your trees' roots are as crowded as shown in the video and your soil needs replenishment I would suggest potting it up in Spring. If you plan on using a larger pot just prune away tangles and encircling roots, comb through the roots and spread them as you place into fresh medium.You won't lose much momentum and you'll avoid set backs that may result from stunted roots and exhausted soil.

In Philly and NJ I do spring root pruning as the trees are coming out of dormancy and are just beginning to swell their buds. This fall I root pruned a test batch to see how they fare in contrast to spring pruned trees.

Thanks again Herman. The pots are definitely too small. I feel confident now moving forward with repotting.

I sure would love to see your container-grown trees.