Well, the plants will grow into the existing soil as well. 14" is a lot to fill. But not so much more than 10 and you can always leave lots of room to top off and mulch.

Personally, I would be less concerned with the height than the width. 4' is just a tad much for me. I tend to garden on my knees and tho' I can and do reach halfway into a 4 foot bed, next time I think I will go with 3 feet.

Good luck with your build! Kay.

One idea that comes to mind is to run 2X4s parallel to each other, say 4 of them 18 inches apart to form 3 rows each 18 inches wide. Use the 2X8s to form a bench like brace shaped like an upside down U on each end so that the 2X4s are suported on the ends by the strong 2X8s. If you run the 2X4s setting on the 2 inch side (on the narrow side) you could drape heavy plastic tarp over the 2X4s to form three 12 inch deep troughs that you can fill up with potting mix. The end braces might actually be set at six foot spacing so that the 2X4s extend out 2 foot on each end. You can make the legs 4 feet long and bury 1 foot in the ground to make the over-all height 3 foot high, an easy height to work with. To make the end braces, I would cut the legs 4 foot long each and the cross piece would need to be 3X18= 54" long. Actually, the legs could be 4X4s, or even landscape tembers which are a lot cheaper. (return the expensive 2X8s for credit) Use a stapeler to attach the plastic tarp to the 2X4s that form the troughs. Use some heavy duty clear plastic over the plastic tarp material, and the fill with the potting mis. Well, if you are having any trouble invisioning the idea, I just happen to have some pictures that might help because I built a simular setup several years ago and had good success with it. It you care to, just click on the link below and see if you care to give it a try. Good luck and have fun, chuck

Here is a link that might be useful: trough garden

Mine are 16 inches high, but I think I went overboard. 10 inches is fine for most things. And I recommend putting landscape fabric or at least newsprint or cardboard under then to stop weeds from growing into your fresh wonderful dirt, so the extra depth is good (10 inches that is, vs. a standard 2x6). Good luck!

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