Thanks, paperhouse, but part of the problem has been that with as much water as these boxes require in the summer's heat, things like my petunias end up with powdery mildew and whiteflies.
Does that have more to do with poor air circulation, or something? Or do I just plant annuals, and spray the heck out of them with herbicides and pesticides?
Maybe I want too much. I had hopes of some pretty window boxes, like I see in the gardening books and magazines.

This has been an on-going challenge for the past three summers, and I have yet to find a solution, short of calling a carpenter/handyman to come remove the silly things.
The exposure is generally East, with two of the boxes facing northeast, so you would think I'd have a plethora of choices.