It would be actually a CONTAINER GARDENING. Depending on the vegetables,I would say one foot depth should do, even less, if you space the plants adequately. So they grow roots side ways.

I would say with a "closed bottom", poor drainage will be more of a problem than depth. If you solve the drainage problem, then I agree; 1 foot would work with MOST veggies. Asparagus, carrots and other root veggies may not like it.

Agree with the other posters.

One foot is what I've built my boxes around. Of course, things such as tomatoes, corn, and peppers may need deeper if you plant your veggies close together. They'll then be forced to sprawl along the bottom and potentially rob plants around them of nutrients.

I would not recommend putting particle board at the bottom. You will find out that if you put a bottom under it using something like that, it will fill apart within the year.

Here is a link that might be useful: My Garden Blog

I garden in large closed bottom boxes. The only difference is that mine are on concrete. I personally like them 2 ft or more deep so that I can plant closer together and the roots can reach down into the potting soil/mix. I plant my herbs and onions in 1 ft deep boxes. Of course, mine are not the size of the average raised beds so if you have plenty of room for roots to go sideways, you can do less depth.

Mine are made from redwood or cedar. Cedar is probably the most rot resistant of the woods you can use.

I also do not use regular soil but use potting soil or potting mix as drainage can be a big issue in large containers.

Why are you putting a bottom on your raised beds? Is it a pest issue?

I've also grown in a kiddie pool that was about a foot deep. I had 4 tomatoes, about 5 peppers, 2 tomatillos, 3 eggplants and some herbs in there and everything was happy and the roots grew sideways.

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