Sustainable Seattle has been posting (Facebook) some great ideas for gardening in small spaces.
The key to the success of this kind of project on the side of your house is to make sure you have an air gap between the box to allow water to flow behind as well as air flow to dry out your siding. You also want to keep water from getting behind the siding – a trick is to use a ton of caulk as you screw the boxes to the siding.
Although hanging gutter or attaching these innovative solutions to a fence (as commented on the Facebook posts) reduces the risk all together. Enjoy the bounty!