I also know nothing about trimming roots back for containers and would prefer not to, so I want to build the box large enough for longtime growth or choose a tree that won't need it.

You are building boxes you don't intend to ever move, why not leave off the bottom and have a raised bed instead? Your trees would root into your soil and do away with all the restrains of container growing. Calistoga Al

If you are trying to get the wide part of the tree right at 6-8' off the ground for maximum privacy, I can see the logic of 3' tall planters for Star & Ann. Not for a Southern Magnolia as they get huge. There is a dwarf Southern but they also tend to grow UP much faster than out.

Geneyus,
Of all the trees in the world to NOT plant in a container, a magnolia is one of them.
Please understand that a magnolia is an ancient tree that spreads its roots OUT, close to the soil, which is exactly what you don't want.
If you want it for privacy, why not just plant an evergreen like an arborvitae or a green giant?
I know you can trim arborvitaes, and they usually aren't that expensive to get a nice sized one.
They would take to raised beds, or even jsut planted in the ground if you can find one big enough.
You would have privacy all winter too.
Just a thought.

I visited a nursery yesterday, and I came away with some new prospects. Now, I'm thinking about planting "Tea Olive" or "Wax Ligustrum" in the boxes. Both of these are bushy and spread out. I'm leaning more towards the Tea Olive.
To answer calistoga, I was going to have the boxes off the ground because I have a gas line under my fence, so I'm not supposed to plant anything taller than 6'. I also figured boxes raised off the ground would be better for drainage.
Does anyone have comments on the two plants I mentioned above?

This post was edited by Geneyus on Sun, Sep 28, 14 at 11:51

Tea Olive is a very fragrant bloomer. Make sure you like the scent of Jello filling the air in late spring and early fall.