Hmm,I see no-one's answered this yet,maybe because the art of Bonsai couldn't be explained in a simple forum reply.Perhaps you should read some literature/Bonsai forums,or if you're serious maybe join a Bonsai class.However,there's nothing to stop you root pruning your maples to keep them in the same pots and it'll also limit their growth,until you've learnt or decided what to do,though obviously the ultimate size will vary with cultivars.There's 2 schools of thought as to when is the best time to root prune,either autumn or spring,but hopefully you should be able to keep those maples on the porch for a while yet...good luck.

Well, I actually guess I wasn't clear, too.

I really don't want to bonsai my trees. I just want to be able to keep someon pots, healthy, for a while. I always read about something being "good for a pot".

Mine become rootbound, so their pots keep getting bigger and bigger, whereas the tree remains its slow-growing self.

by root prune, do you mean getting in there and removing all the brown, hair-ish roots? I've actually done that in January when we've had the 2-3 days of thaw before we get hit really hard again with Winter. It's been successful for me with most all my plants.

Thanks,
Christine

Well not exactly, the root pruning I do is less selective,the idea for me is simply to reduce the size of the root mass to allow it to grow another season in the same pot.It should also add some vitality to the plant.
I'm sure if you search you can find many explanations how and why to do it,and how to bareroot and give the roots a good 'overhaul'.It's good to read up on these things to decide how in-depth you want to go and what's best for you..As I've said,I've only root pruned purely because I like and want to keep the present container,and have only performed a basic 'root chop'.
All I do is simply remove 1-2" all round the circumference,and the same off the bottom(depending on pot size)You can either snip away with secateurs,reducing the RADIUS by an inch or two and also off the bottom,or you can take a sharp carving knife(not serrated)and chop the sides off resulting in a hexagonal/octagonal looking root mass(I think you can picture what I mean)also chopping off the bottom...(use secateurs if needed to clip through any thick roots you come across)The idea is simply to make the root mass complete with soil smaller, allowing you to refill with new soil just as you do when repotting.I'll leave you to decide how much to remove...don't go mad...just enough to allow another season of growth,you can always do it again the following year.You may wish to seek further advice as this is just how I've done it to suit my needs and no more.I don't know if it's the correct and probably not the best way of doing it(knowing myself), but you've got plenty of time to gather more information before proceeding...good luck:)

That is a pretty good description of the process. I have been studying bonsai for a few years and happen to choose JM as one of my favorite species to study and work on. While you do not need to "bonsai" the tree per se, understanding the techniques used will allow you to keep your maples in pots without feeling like you are fighting them. As houzi described you simply remove the outer edges of the roots, forcing them to regrow a new. Trees in the ground grow long roots for anchoring and only have new growth at their tips. This is the only place they really feed and take up much of their water. By cutting back a tree's roots, it will regrow new, finer roots which will do the water/nutrient absorption. As long as you let the tree recover between sessions, you can do this many times to chase the root ball back to a very fine, dense mass of feeder roots to support quite a substantial tree. You can even "bare-root" a maple by washing away all of the dirt and replacing it every few years as it will break down over time. The extremes of bonsai are some very thick, heavy trees living happily in mere inches of "soil". But that takes great understanding and daily care. I myself choose a deeper pot. Adding grit to the soil will cause the roots to split more and produce a finer structure. Something that goes well with living in a pot. The other aspect of keeping them to size is selectively trimming them back. Most people thing of cutting the top half of a tree off as near murder. In bonsai a trunk chop is just part of annual working. Most deciduous trees will bud back to a bare stem. While it will take some time and studying to get used to the idea of cutting part of your tree off, the only way to keep the height down is to do so. Whether you cut a lot and re-direct a new branch upward to become the new leader or you just constantly trim the tips of the branches, pruning will be your friend.

Also choosing a smaller cultivar helps but you already have the tree so too late for that one.

Most work on maples is done in late winter/early spring. Major pruning and chops are done in full dormancy. Root work and light pruning is done at early spring at the onset of bud swell. Do not do major work after bud break or the tree will not appreciate it.

Sorry, that's the best I can do without typing pages. I don't stop in often but will try to help with further questions. I think Amazon has a preview of "Bonsai with Japanese Maples" by Peter Adams. While a more advance book the illustrations may help you understand some of what I have described.

sfhellwig,I never considered reducing the rootball further than a seasons growth but it makes perfect sense what you're saying.I assume it's a gradual process but thanks for that info,it could be very useful:)

Actually, this is very helpful, Sfhellwig.
I have trees anywhere from 12" tall at 2year grafts to some now that I've had for going on 7 years.
My criteria is that they 1) turn red at some point, and now 2) can take full sun. You'd be shocked at what has survived the demise of my shade, living in extreme, well over 90/100F in full sun for weeks on end. I was.

Anyway, I'll look for the book! Thanks again.
Christine

And there is no reason to go past a seasons growth, for the stated purposes. But if you ever had a craving to fit it into a pot that was just a little smaller, you would be fine to get away with it. I believe maples are pretty resilient to root work considering their deciduous nature, but every species is different. JM you can work fairly hard and not worry too much. Trident maple I have read can remove half of the root mass or more and barely blink. Many of the "native" maples (red, silver) can be worked but will just drop branches for no reason, ruining a design. So at least JMs are in the right arena for being worked. To the OP, just take a little off the bottom, a little off the top and keep reading. Over time you will learn what works and what doesn't. It sounds like you have enough experience to not do the usual "kill a few trees to learn" that full on bonsai tends to bring with it. Good luck.

Great advice!

I've been keeping maples in pots for several years now.
In the Spring, before bud movement, I root-prune, re-pot with new media, and prune back the top.

Here's a link to a Thread on containerized maples (on the Container Forum):

A Journal for following the development of Containerized Maples

Josh