You're over-thinking this. Keep the plants in the same containers they're in now and plant in the fall.
Keep them in the shade and water them twice daily.
Do not add fertilizer or miracle die products.

Do not try to overwinter them. not indoors, not outdoors. If you cant plant them in the fall see if you can return them.

JMs do best with dappled shade. Some tougher ones tolerate full sun but dont prefer it.

Hi Indybrad, jms are very forgiving, but it's to be commended that you're trying to get things right.

Here's my opinion, based on growing jms in pots for about four years:

1&2) It's okay to have MG in a mix. I wouldn't plant a jm in straight MG but your recipe should be okay. Mushrooms probably do indicate a bit too much water. It's better to error on the side of not enough water than too much. If you want, you can repot and replace a fourth to a third of the soil with a small, sharp gravel like that found in VoleBlock aka Soil Perfecter. (Do not use pea gravel, which is rounded.) This makes for a very fast draining soil, the plus side of which being that you wouldn't have to worry about over-watering. The downside is that you would need to water frequently, especially in high temps. Even with this type of mix, though, I personally only water once a day in the summer, and frequently every other day. If you do repot, also add more holes to the bottom of the containers.

Do not fertilize, especially with MG in the mix. No need to worry about the acidity of the soil in the pots, nor (probably) when you plant in the ground. You can get a second opinion at another nursery, but I think you'd only need to do something if the soil in your area was alkaline.

3) Here, full sun all day would be too much. All my jms are all in shade to mostly sunny. Also the more sun, the more watering you'll need to do.

4) Definitely don't bring indoors. Someone in your zone would have better advice as to how to get them through the winter in pots. I don't do anything special with mine, but they are in a small yard with a high fence, so somewhat sheltered. Don't let too much snow accumulate on the branches (but snow in and on the pots is fine, it's an insulator).

Re gardening in general, there's a book called 'The Perennial Gardener's Design Guide' that I found to be a fun read and very helpful re plant choices as well as landscape design. I wish I'd had it when I was starting out.

Also here's a recent post in the Landscape Design forum re contorted filberts.
http://forums2.gardenweb.com/forums/load/design/msg0715194426256.html

Good luck!

Here is a link that might be useful: Perennial Gardeners Design Guide

So I think what my immediate plan will be is to simply drill more holes in the bottoms and make sure the containers are set up on some small blocks so the bottom isn't in direct contact (thereby blocking the drain holes). Perhaps I should even drill maybe 4 modest size holes (1/4 or 3/8 inch or so?) in the sides right at the bottom edge (like some of the really thin plastic containers at the nursery have)...

If that doesn't do the trick I'll re-pot and add some gravel (not pea) to the mixture to aid drainage.

I'm still not quite sure what to do about over-wintering them though... returning isn't a plan for me... that defeats the whole purpose of trying to container grow them before use in the final landscaping...

It sounds like insulating the roots/container itself is key? Perhaps I could purchase insulation rolls and make a simple "wrap" around the containers to protect the roots? Then I'd just need to do as suggested and ensure there isn't too buch build-up of snow on the branches/leaves themselves...

If you're potting and repotting in the summer and monkeying with the soil and additives you may not have anything left to plant before long.

I think in zone 5 your only realistic option for overwintering your Japanese maples in containers is in an unheated garage or outbuilding etc. Plenty of people use this method successfully. Even for in ground planting you are in the coldest zone they will tolerate. The threshold for root death in these plants is -10C or so. I don't know the amount of bubblewrap needed to insulate a container to -10°C when it is -25°C outside, but I don't see it as being a practical solution.

See the link below for some advice from a Japanese maple nursery located in a cold zone.

Also Tamukeyama is one of the better dissectums for handling direct sun.

Thanks for the link to www.davidsansjapanesemaples.com - it was quite helpfull.

So perhaps a better option would be to go ahead with in-ground planting even if I have to relocate them at a later time? Say within 1 to 2 years...

Also - I'm assuming the problems with wintering them in containers would apply to the other tree types as well... not just the JM's... depending on their relative ability to withstand cold (i.e. they have same hardniess zone rating)?

Good plan, holes along the bottom edge are probably easiest and should work fine. It's not ideal to repot in the summer. However I've done it a number of times. I keep the roots wet throughout and make sure they get plenty of water afterwards. So far the worst I've had happen is a little leaf scorch.

What it boils down to is you will have to make it part of your daily routine to water these every day in summer. More than once a day too if you made the soil very fast draining. And that's if they survive the transplant.
If you stick it in your garage in the winter, it won't leaf out in the spring. If you keep it outdoors in the winter, it probably won't take the cold in an exposed container. What you might try doing is burying the container in the ground or in mulch.

If you don't mind the headache of doing this for a couple of years or whenever you plan to plant your trees, then go right ahead but you will have a much healthier plant at planting time if you just not buy things until you're ready to plant them. Not only that, if you received a warranty with your plants it will not be valid at time of planting if you wait too long to plant them.

You are going to spend far more on time fighting to keep these barely alive for two years that it's worth to buy a j maple 10 times the size of the one you started out with that was freshly field dug from a proper nursery. Not one sitting in a costco.

If they still have leaves, I'd return them now. I know this isn't what you want to hear. You want me to tell you they'll do fine and it will be a breeze and they'll grow into magnificent specimens living in a container for 2 years but that isn't reality. You'll thank me later.

Use a large container, or a container within a container, and insulate well with bark.
Burying the container in a bed, and mulching, is also a good idea.

Josh

The containers are giant plastic 23/25 gallon containers... the 2 Garnets came in fairly small pots and the 2 Tamuks came in fairly small pots as well but slightly wider...

From further reading, if I do re-pot - it sounds like I should remove the small pumice layer. Previously I read that it doesn't really help all that much - but I believe I've read more now and that it can actually be detrimental in that it can raise the "PWT" - I won't profess to fully understand the PWT concept but I think I get it at a basic level...

I dont know much of anything about pumice. My guess is as long as it allows drainage, then if the pots are big and the pumice layer is thin, I wouldnt think itd raise the PWT (believe that stands for perched water table) to the point of being a problem. I would skip it if you repot.

I live in zone 5a, probably a little too cold for JMs. I never had luck with JMs. I notice that people in my town are successful with 'Bloodgood', but here is no older disectum in site. In respect to 'Bloodgood', there are several strains and not all are cold hardy for zone 5, one I had once did not make it. I would be very careful with JMs in zone 5. I try to stay away from gardening problems.

I am always upset seeing JMs being sold in big box stores in a town north from here, zone 4, though the label says zone 5, and zone 6 for disectum. Here in zone 5 big box stores have no problems selling JMs which are good for -10F only.

Good luck!
Bernd