My bed is only 6 inches deep no problems growing anything so far, carrots fat and long, large onions, lots of cabbage, lettuce, parsnips coming on awesome along with a few other bits and bobs cant think of any issues but mine was also put on top of double dug soil that i had took the turf off.
I want to build another two beds by next year but im also having issues with pricing :O lol
You will be surprised how much it takes to fill them as well :/ lol
Good luck
