Answer: I would definitely remove the plastic pots prior to planting! If you leave the pots on, even if you cut out the bottom, the present root system will grow around and around in a circle until it fills the pot and breaks it, before the roots will be able to escape and grow out, in a natural fashion. You'll end up with a knot of gnarled roots which can actually girdle the crown of the plant and cause early death. Take your spruces out of their pots, gently loosen the roots, then set them in your new containers so they will be growing at the same soil level as they were before (you'll see evidence of a soil line on the trunk).
Best wishes with your new spruces!
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