How Deep Should My Raised Bed Be?

Q: How deep does my raised bed need to be? 

A: Given perfect soil, different vegetables send roots to different depths.
Lettuce, radish, basil and spinach grow fine in soil only 4″ deep. Beans,
pepper, eggplant, and carrots need 8″ of soil or more. Tomatoes, okra, corn
and summer squash can use a full 12″ of soil. My advice? Use 8″ wide boards
to build the frame, then thoroughly shovel the soil under the frame 6″ deep.
Add three 2″ layers of bagged soil designed for outdoor raised beds,
thoroughly mixing each layer with the one(s) beneath. That should give you
12″ of soil depth, which is adequate for all vegetables.