Either compost or manure you want to give them good soil to grow into. It depends on the root systems what size container you use. If they are seedlings and not transplants you should be able to plant them in 1 gallon possibly 2 gallon. Ask a local garden center if they have any used ones. I'd go with the 1'-2' size because generally the roots will not be any more established in the larger size, but you'll pay more. When you plant them in the fall you might want to spray them with flourescent paint so you can find them the next year.

Here is a link that might be useful: Propagating Perennials

I think your plants will be o.k. in pots if babied a little. If the pots will be in a sunny location you might benefit from piling up some wood chips around the pots. Also mulch your plants after the final planting and do not plant during the hottest part of the day. I would not amend the soil in the final planting and I would not transfer much of the potting soil to the planting hole.

I'm sorry, but I really disagree with bullthistle in your particular situation. If you *have* soil from the area they will end up in, then I would use that to start. If not, then I would get just plain ordinary soil from the area where you are. Amend it a little bit with composted manure or other compost, or some topsoil if the dirt you have locally is not great. You do want to get the root systems developed. But if these trees are going to have to make it on their own later, I would not use potting soil or heavily composted soil. It's a really lousy analogy, but this would be 'spoiling them', and 'stunting their growth'. Root systems usually will develop very differently in 'rich easy soil' compared with how they develop in 'ordinary dirt'. You want them to be able to be moved to 'ordinary dirt', and make it on their own. Stick with plain old soil, a little enriched but not too much, and don't even be afraid to let them dry down some occasionally after some months in pots, if they are otherwise doing ok. Plant them pretty much in ordinary soil, baby them early, then gradually wean them off the babying, and then let them toughen up as they get really established. *Then*, they will be ready to be planted up north, and have a good chance on their own. Otherwise, you'll be taking overfed, pampered, underdeveloped-root-system garden trees, and sticking them out in the wilderness. If you make it possible for them to develop in pots in a similar way to what they would have to work for in a more real environment, they'll develop in ways that will let them settle in fine when planted, and make it on their own.

The problem with just using naturally-occurring soil in pots is drainage. Pots aren't natural, and that needs to be accounted for. You can easily get over-compaction, where water just stands on the surface, or it can stay so porous that water just runs through. potting soil, with the righ mix of vermiculite or equivalent, is designed to avoid that.

That's the nice thing about opinions - there are plenty to pick from! :)

I can't see the idea that the plants will be "spoiled" by a single season in good potting soil before that rootball, potting soil and all, gets put in the ground. "Babying" them after they get dug and have their roots bared, probably killing off most of the fine root hairs, seems like a GOOD thing, to me. Not for years, but long enough to recover and get a good base of vibrant, functional roots in the core before sticking in the ground to fend for themselves.

Potting a tree in good soil seems wise to me, especially if your native soil isn't all that great. For me, I'm dealing with an alkaline soil that I need to amend every single time I plant (and am trying to amend in the long term as well). So a $5 tree goes into a $20 hole when I plant. But as dbc3 appropriately points out, there are many opinions, and more than one may be right!