If you plan for 6 "square" inches of space for each full sized plant you won't go too far wrong. You can definitely over plant and then thin/eat young plants as things get crowded to make more room for the rest.

I put the word square in quotes, because you don't really need the inches to be square ;) If your pot diameter is 12" then 4 plants would fit nicely and be grown to full size. Again, more can be planted, but the above soil portion will get seriously crowded so you will likely want to pull some before full size for a small salad or two, leaving more room for the rest to develop further.

Something I have not tried with Romaine types, but you may wish to experiment with, is harvesting the outermost leaves from each plant as they approach full size and leaving the rest of each plant. This approach should result in the plant continuing to grow more leaves and extending the harvest period/volume.

Thank you for the info I'll keep it in mind for reference.

I know that since lettuce has shallow roots, it doesn't need as much depth with the soil compared to root veggies, which need lots of soil depth (the deeper the better from what I've heard) since they grow underground.

What would the soil depth have to be in a 12"
container for romaine lettuce? What would you recommend as the minimum soil depth and what would be a good soil depth for romaine lettuce to grow in a 12" container? Please specify this in inches.

I typically grow stuff like greens/snow peas etc in containers with 6" of depth. Actual mix depth is 4-5" You will need to water more frequently than if the container were deeper as a higher percentage of the water is lost to evaporation, but depending on your perspective that may be a good thing or a bad thing.

Anyway, 6" of depth is adequate for *almost* everything in terms of root depth. With the exception of plants grown for their below soil parts overall volume is more important than depth. Even volume isn't all that important unless the roots systems will tangle each other massively and this won't happen with lettuce. Greater volume just means more water holding capacity.

If you are skeptical that 6" of depth is adequate for nearly everything consider that commercial self watering pots like Earthboxes hold about 6" of mix and clearly lots of veggies have been successfully grown in them.

Justaguy, I'm so glad you said that. :) I've been wondering soil depth for a really long time, after being utterly confused with the success of people growing tomatoes in commercial Earthboxes, and then reading stories about how tomato roots like to go deep, and can extend for several feet.

When I ordered 2 of my raised beds, the guy who sold them to me said that he'd seen a friend of his sucessfully grow tomatoes in the boxes, whose walls are only around 7 1/4", but I was skeptical because I knew he was pushing the sale, so I didn't dare trying any maters in them.

Maybe next year I'll give it a shot, since your reasoning really does make sense.