I had my 18L and 30L pots buried so that the rim of the pots were level with the ground. I just dug another 2 feet out under the 18L pots today and put a layer of rocks under them to keep the roots from growing into the native soil. Don't be afraid of putting them deep into the soil. It was 34 degrees and sunny here yesterday, I don't want to imagine what kind of heat shock the roots would've gotten had the pots been standing above ground.