If you are putting them in over lawn or former gardens, 12 inches is plenty. The roots can go down as they want.

I agree, a foot or so should be plenty for raised beds. But you shouldn't need to put anything down to keep the grass from growing back up through that ... once you get the topsoil for the raised bed in place, anything that you have rototilled in that's below it won't be able to grow anyway.

But you say you used a grass killer? How long ago was that? I'm not so sure that THAT won't have some effect on whatever you try to grow ... you might be surprise to learn that carrots, for instance, can send roots down as much as three feet! Most of the others you mentioned will have roots that will go a foot or more, thus putting them in range of whatever grass killer you used.

In any case, good luck with it all ...

Here is another thought, lay in news papers about 6 layers thick then fill in the 12 inches of soil. the paper will kill the grass but will rot in time for your veg. roots to grow through

I put down several layers of cardboard before I built my beds. Worked great. I would not go higher than 10" with the beds. I have three 10" high beds and one 8" high bed. The 10" high beds require constant watering to maintain moisture. Also, it's a good idea to install a central 1" pipe down the middle of each bed with 1/2" vertical pipes 2' high every four feet with sprinkler heads on top. I won't tell you how I learned that it's easier to install first rather than add later.

Thanks everyone for the ideas! I decided to use weed stop plus the newspaper method and my beds are 12" deep but the soil doesn't reach the top so it's probably only 10" deep. I've already got some radish, beets and turnip sprouting up! I'm so excited!

~Rose

If you added soil from a soil place as I did, you may want to be prepared to add several more inches next year or the year after. The new soil is pretty fluffed up and will settle with winter rains, stepping on the soil (I try not to)etc.. HTH