Your guide lines are not directly on the face your trying to erase. move them down to the face and redraw the rectangle. Whala =D

Very cool indeed.

Kinda reminds me of the vertical forest in Milan. You may want to rethink the plant material. I don't think you will be able to sustain a full size tree with that much soil. You may get 4-5 years before it dies. Where I live we require 30 cubic meters of raw loam per tree and 15 for shared plantings. I know it sounds like allot. I think you could get away with a large shrub or perennials and have better luck.