If you do it, do it soon. Before the tree is fully budded or leafed out will present the least amount of stress, although I have repotted containerized JM's at pretty much all times of the year. You just need to take more care at less ideal times.

The gritty mix is great but there are other appropriate bagged mixes you can access in this area if you prefer. Repotting is not necessarily a yearly activity. I have had an 'Autumn Moon' in container for at least 5 years and it has only been repotted (and root pruned) once during that period, although it may be due for a repeat performance soon :-) If the container is sufficiently roomy for the size of the tree and you use a very durable potting mix - like the gritty mix - repotting should only be necessary every third or fourth year, maybe longer for larger containers.

Thanks Gardengal! Do you have a preference on the bagged mixes if I decide to go that way? I can certainly make a trip to Molbaks if that would get me a better mix. I have been having mixed results with the gritty and my indoor plants. I'm a little concerned about putting my JM in it because I feel like I might not be making the mix correctly and I don't want to lose the tree. I also don't have any experience yet with the 5:1:1 and think experimenting with the JM probably isn't so smart. I plan on making 5:1:1 up for my container veggies this year and should get a feel for it after that.

Kellogg Garden products distributes bagged mixes under a couple of different labels -- look for Gardner&Bloome or Master Nursery. I use the acid planting mix. This is formulated specifically for use as a soil amendment or potting mix for acid loving plants and I use it for all my containerized conifers and Japanese maples. It is a composted bark based product, so quite durable - I've had it hold up for at least 5 growing seasons before I feel I need to replace. It is the closest premixed bagged product I have found to the gritty mix.

Gardner&Bloome products are carried by all the larger Seattle area nurseries - Swanson's, Sky, Molbak's, Emery's, etc. The Master Nursery label (same exact product) is often carried by the smaller and more rural independent nurseries.

A word of caution: last year I bought a JM and potted it in a combination of bagged steer manure and good grade potting soil. It sulked all year, lost its leaves, not happy. I would stick with the mixes that Gardengal recommends and not experiment like I did.

Thank you Gardengal and Dotty! I will look for the Master Nursery label....may be able to find it nearer to me than Molbaks