I am planning a raised bed vegetable garden for my Southern California backyard. my current design was going to use 3'x6' cedar boxes, 7-1/4 inches high, until I read that that this shallow dirt would not accommodate many deep rooted vegetables. I read that Tomatoes can have 7 foot roots. A raised bed box for that would look like a skyscraper! What depth is required for a raised bed vegetable in Southern California?