Hi Glad to see some one posting. We are still deep in snow here but I can not help but think of my new pottager I hope to get started.

I do not have an answer for you. I am using old stock water tanks and they are 2 to 2 1/2 foot deep. I do not have to fill them all the way to the top with compost and soil.

Chris

My former garden beds were about 8" deep. The new ones will be about 4" deep, simply because I'm going from a double layer of landscape timbers to a single layer of bricks. The beds will be slightly deeper toward the center away from the edges.

I'm making lasagna beds (sheet compost) for veggies and if I remember correctly they rec making the layers around 18" to 24", but as it breaks down it sinks. But the lovely thing about it is that it is very cheap to make as I try to scrounge/beg for the stuff. (linky below)

(In the square foot gardening forum, they say 6 inches is enough for most veggies, but it must be very rich -- basically home-made potting mix -- compost + peat moss + vermiculite)

Here is a link that might be useful: lasagna gardening 101

I was thinking dreaming about this last night after posting. I have a slew of the large black plastic nursery buckets. I had a flash of maybe filling them with good compost potting material and putting them in the big tanks. Then when the soil gets yuck which container soil tends to do I can switch it out easily. These are large containers 16 or more inches deep. The reason for the tanks and not in the ground is the ground where the potager will be is awful. A year ago I did have morning glories growing in the nursery buckets around the large tank and they grew up the arches so I know it will work. This tends to make it feel more possible to get done than hauling the soil for the whole tank. I have four of them.

The picture is from our last house and I moved the tank here. I know it does not look the best in the picture and I do plan on painting it. I was really ill that year and felt good just to get the morning glories planted.

Here is a link that might be useful: {{gwi:1151076}}

You really should try to stick to the 12 inches if you can...for the long term. Of course...it depends on what you will be planting. Some plants have deep roots while others have shallow roots. Some roots can and will spread width wise if they can't go downwards.

Also, depending on your setup...you'll want to amend the soil over time and over that time the soil will get higher and higher.

That being said, if you can dig into the base soil you have...so that the roots can grow into that..you may be able to get away with less than 12 inches.

I have a raised bed potager..the entire area is fenced in, however the raised beds are not framed. They are just piles of soil/compost 6-8 inches high..and I dug well into the ground below it.

So I think depending on what you have going...you could get away with less than 12 inches..it just depends on what you have underneath and what you plan to plant.

Hope that helps!

I agree with Ruth's comments. Although the actual raised area of my beds will be 4"(was 8"), I'll dig (dug) about 8" into the ground below the beds when turning the soil.

Yes, I do intend to dig down first before building up. If the soil will just dry out a bit, I am ready to get started. I have drawn up my plan which consists of 9 beds (one for center) eight feet long and four feet wide. After alot of thinking, planning, and pricing, I have decided to go with concrete retaining wall blocks (have no idea if that's what they're called. The decorative kind.), simply because my husband is so busy with work that I need to be able to build this on my own. Since the beds will be on a slope, I expect the high side to be four inches high and the low side about eight inches. We'll see. I am very excited about this project. I haven't grown vegetables in a good decade, but have about a dozen large flower borders. I have ordered some apple trees too, so I intend to do this in a big way. :)

Donnabaskets,
I like the idea of using the decorative retaining wall blocks. First of all they will last forever and not rot away as wood will. Secondly they will be very attractive when put together. They'll also provide a nice space to sit and work from. They'll also be amenable to making into archs or circles which can really give you some great design alternatives. I would think that 8" is a safe goal to aim for as far as depth goes. Good Luck!

MeMo

If you don't have a bottom on your bed the compost and other good things you fill it with will serve to make the soil below improve over time. I can't imagine why you'd need 12" beds unless you were starting with foot-long carrots! Most plants roots won't go down more than 6" and if they need 12 or 14 or 18 even, they'll go right thru the soil below the raised bed. 12" seems to be more a matter of style than function.

And I LOVE the idea of making it of retaining wall blocks! It should look wonderful!

I am into the project and for now, I am making the beds eight inches. As I am building beds, I am digging, tilling, and amending the soil about a foot deep under the bed, then going eight inches high. As I was working, it occurred to me that a great advantage to these blocks is how easy it will be to add more height as time goes by. As I get older, I may very well want the sides higher just to make maintainance easier. I can add more rows as time and money is available.