Plant it in the ground for a few years, let it grow.

I agree with what _geoff said. I would add that having the tree in the ground thickens the trunk up. What I would do is each spring root prune by driving the a shovel into the ground about 12" to 15" away from the trunk. But one year I would do it on the N and S sides then the next on the E and W side. This will keep the growing tree roots from running way outta bounds and causing a big pain when it's time to take it out of the ground and put in a pot. HTH.