I have used a 20 gallon pot sunk into the ground to plant one in to keep the color for my blue es from changing from blue to pink . Is that what your trying to do? If so it helps but you may still need to add soil amendnents. I would use the biggest pot you can get your hands on and put a few inchs of rock in the bottom so it can drain well. I have also used thick black plastic in the same way making a really big hole and lining all the way around it (not the bottom ) with the plastic to keep color stable .

Yes, that's what I am trying to do. My favorite nursery has a recycle drop off area for black plastic pots. Next time I go, I'll try to find a large one. I really only want to block the concrete sides of this area, so I'll cut it up. Thanks.