I use multiple boxes (2 this year, moving to 4 next year). I do plant them on the bottom layer, then actually once the leaves are coming up, I add more mulch/compost/soil mixture (about equal parts), and allow it to grow upwards again before repeating. I will be re-making some more this winter with tutorials on how to do it (photos if @mwoodall will take them while I work). Drainage is not an issue at all, as the bottoms are open. You can have removable planks, but If you do not seal them, the wood will swell so once you remove them, you will be hard pressed to get it back on. Sealing them is key I find