Board Thread:Questions and Answers/@comment-8327572-20130628113019/@comment-8327572-20130628221731

Yeah, that would indeed take care of the planting issue. However, then the tree would not grow, as that needs a 3x3 dirt area. I've tried using other materials as well (stone, wood) and while that stopped the planter from planting, it also stopped the tree from growing.

So thanks for the idea, but I'm afraid that won't work.