I am going to do a small square foot garden consisting of two 4x4 boxes. Because of where the sunlight is available, the boxes will have to both be along the side edge of my backyard, next to the fence. I'm going to have to put one of them directly north of the other one. How far apart do I need to put them so that the tall crops in box A (the one to the south) don't cast shade on box B?