Perhaps you could treat the box with Cedar Shield. That might help preserve it from water. And I would think about bracing it - it will be very heavy once filled with soil. I learned a trick once, for filling large pots that you don't want to be too heavy -- place broken up pieces of styrofoam in the bottom and the soil on top of the styrofoam -- the roots will probably not grow that deep, but if they do, there will still be soil down there, in between the pieces of styrofoam. This would lighten the box considerable, and also keep the soil from compacting at the bottom. I have had very good luck doing this. Keep us posted!
