I have an alberta spruce topiary 5' tall in a 12" tall and 18"diameter pot. Last fall there was about 1/2" of allowance in the pot before the roots grew to the edge of the pot. My son thinks I am going to need to plant it into the ground at some point, because even if I keep repotting into larger pots, it will soon outgrow even the largest pot and will not survive for more than a couple years out of the ground. Any thoughts on that?