The title should have read 6" (inches) not 6' (feet)...

You'll be fine. Carrots would be shorter, especially if you have a barrier at the bottom, but otherwise you should be fine.

Good luck.

Here is a link that might be useful: Sinfonian's garden adventure!

Hmmm....I've pondered this same question, myself. My SFG box is 8 inches and the same vegies in pots are out growing the sfg veggies 3 to 1. However, this could be because they also get more sunlight.

The depth, however, is a concern I've had myself. I decided to do the 8 inch just in case, but I think I'll even upgrade my next one to taller. Again, just in case.

i don't know about going deeper, just make sure that when it settles, fill it more, IMO, keep in mind that i havent gotten my vege box done this year, only flowers. LOL
but from what i've noticed in the flowers deep rooted and not, that is what i'd do. ~Medo

I'm doing what medo suggests my boxes are all 8" to 10" because of settling and angles, etc...I just refill em as they get low. I have had zero problems with crop size. I agree with ribbit, I think sunlight is the biggest issue.

The only thing that I have grown that I think that 6 inches would have been a problem with, would have been carrots. Even with 10 inch boxes I get stumpy carrots.

I have a bunch of carrots in 6 inch boxes but have not pulled any yet.

If you're truly concerned about the 6" depth you could alter Mel's suggestions a bit without building a taller box. Instead of using a plastic weed barrier at the bottom of the box, you could use a thick layer of newspaper. It'll suppress the weeds and eventually decompose so root crops, such as carrots, may be able to grow down a little deeper. In theory it sounds like it would work.

I've wondered about this too, I don't understand how SFG differs from planting in pots, and 6" deep pots at that. Tomatoes and other plants have roots that go a lot deeper than that, and I must not understand how the plants don't get rootbound in the boxes, especially when planted so densely.

I double-dug all my beds and generally follow the methods outlined by John Jeavons. I want to try a 4x4 SFG but I am so sceptical, of the whole thing and the book never really talked about roots very much, so I'm curious about this.

Perhaps since the vermiculite and peat moss help to conserve moisture the roots don't need to grow as deep. Since 1/3 of the mix is compost the roots seen pretty well fed so they may not need to seek out nutrients as much. Just a theory. I'm starting my first SFG in the Spring. It's a 8" tall box. I'm still going to fill it up as far as a can as long as water doesn't cause the soil to run over. From what I've read 6" must work though.

I garden in six inch deep boxes and have had no problems with anything not having enough root space. I have noticed that the roots are wider than plants I've pulled from "regular" garden areas, but that's not a problem so far.

I'm adding composted soil to my beds as I pull plants and have added some to other areas where I felt the soil had settled, but everything seems to be growing fine in only six inches. I did compromise and plant a carrot that is supposed to be rather short, but haven't harvested any yet, so no results on that one.

If you go to the SFG website, Bartholomew has an excellent article on how he grew everything in a 3 1/2" box that he took around in his car for demonstrations.

my beds are all 2 foot deep except the tomato beds which are 3 foot deep .

sassy, Are your beds filled with Mel's Mix or with something else? Mel's mix would be expensive! for a 2 or 3 foot deep bed. :)