Well composted organic matter and/or coir. One foot depth is adequate, and adding anything such as sand below does not help much. The top layer will saturate before any drainage occurs below, so only something very coarse, such as crushed rock on the bottom, is needed to drain away the excess water. If your irrigation periods are accurate, little waste drainage should occur anyway. Most commercial greenhouse vegetables are grown in 5 gallon sized plastic bags, either upright or lay-flat, perforated on the bottom , with only a small drainage layer or pipe below. The same principles apply to raised beds. In some places, vegetables are grown in long beds, about one foot high and wide and lined with plastic, with a growing medium of burned rice hulls. (burning makes the medium somewhat more dense, thus allowing for more water and nutrient absorption time)

We are just starting construction of our new (still in the boxes) Riga greenhouse and I am also wanting to put in a raised bed on one side of the greenhouse. I was thinking it would need to be 18" deep so I am very interested in this thread. Are you saying that 12" is deep enough, and the base of that can be coir or other organic matter? In our case, the ground is mostly gravel and dirt so it will drain well, maybe too well. Perhaps a good reason to start with compost or something organic that holds moisture. I just never thought that 12" would be deep enough for vegies. I wonder if it would be worth patiently filling the bottom with leaves, compost, kelp from the beach, etc. all summer long and then topping it off with a foot of "paid for" dirt in time for planting winter greens. In the meantime I could use containers for tomatoes etc. this summer and then dump that soil on top in the fall. I'm also wondering if it's a good idea to add soil from the garden to get some worms, bacteria, nematodes, etc incorporated. I think I read here about someone who said it's a good idea to exchange some soil from the greenhouse to the garden every year.
I'm so excited, I can't believe I am finally getting my greenhouse! It might happen this week, we have the sod cut out of the footprint and the four foundation pieces are starting to look square. Don't ever let anyone tell you your dreams can't come true, I have been wanting this for a long long time. We are in our 60's, it's never too late!

My tomatoes, peppers and cucumbers are grown in one cubic foot plastic bags, filled with well composted organic matter. No soil ("dirt") is used. The bags sit on an 8 inch bed of crushed rock.

Thank you for the variety of ideas. My veggies in deep outdoor beds only utilize the top 10-12" for their roots, for the most part. However, one of my reasons for growing in raised beds is for my personal comfort. I want to be able to easily reach the soil surface, and want an easy up and down to sit on the edge of the bed in which I am working. I am not getting any younger, and my 60's are not that far off! So as long as I provide for adequate drainage in my beds, is there any good reason not to build them taller (2 1/2 feet)? I also plan to include a modification of the SHCS which will circulate through my growing beds. Thanks!

The concept of raised beds saving on your back may be a bit misleading. It would depend on the crop you are growing but for the crops Cuestaroble mentioned (tomatoes & cucunbers) I find that I'm only bending for planting and the first few weeks of growth, possibly a little for some lower hanging "First fruits". The harder chores involve climbing on a ladder and lowering the supporting stringline to keep plants in the waist to shoulder working range.

The framed raised beds may look impressive but they add far more work than they eliminate IMO. Think about how you are going to till, fertilize, renovate and perform other chores including picking from a 5' plant that is raised to 7' because of the raised bed. Today I added peat moss to my 30x96' greenhouse soil, spread lime and fertilizer, then cultivated the entire surface with a tractor mounted cultivator. Tomorrow I'll rototill again with tractor mounted tiller, lay drip tape lines, cover the entire surface with black plastic, lay boards over the plastic for walking paths and plant. Without the boards I'll admit I would sink into the well amended soil since this is the 10th year that I've repeated this routine. A few of the longer boards are harder to handle each year and next year I may cut a few but I've only been retired for 7 years and so far I can still manage them.

The concept of raised beds saving on your back may be a bit misleading. It would depend on the crop you are growing but for the crops Cuestaroble mentioned (tomatoes & cucunbers) I find that I'm only bending for planting and the first few weeks of growth, possibly a little for some lower hanging "First fruits". The harder chores involve climbing on a ladder and lowering the supporting stringline to keep plants in the waist to shoulder working range.

The framed raised beds may look impressive but they add far more work than they eliminate IMO. Think about how you are going to till, fertilize, renovate and perform other chores including picking from a 5' plant that is raised to 7' because of the raised bed. Today I added peat moss to my 30x96' greenhouse soil, spread lime and fertilizer, then cultivated the entire surface with a tractor mounted cultivator. Tomorrow I'll rototill again with tractor mounted tiller, lay drip tape lines, cover the entire surface with black plastic, lay boards over the plastic for walking paths and plant. Without the boards I'll admit I would sink into the well amended soil since this is the 10th year that I've repeated this routine. A few of the longer boards are harder to handle each year and next year I may cut a few but I've only been retired for 7 years and so far I can still manage them.

I have found the best garden soil recipe is 1/3 peat moss,1/3 vermiculite, 1/3 compost. My veggies are growing like crazy........steve

Thanks for all of the great ideas. What about containers in the greenhouse? I would like to elevate strawberries into "gutters" or other linear planters. Best soil for these, and for growing gorgeous hanging flower baskets? Same advice, use well rotted compost?

I just received a beautiful Oregon cedar 8x10 greenhouse as a retirement gift from my husband in May. I have been an avid gardener in Butte, Montana for 25 years. So, I thought a greenhouse would be a nice addition to adding longevity for my tomatoes, peppers and cucumbers. There are 3 raised beds in the greenhouse. At the bottom of each bed I put a layer of small stones. Then black landscape fabric, followed by 12 inches of soil. In one bed I planted 3 tomato plants, the second 2 cucumbers, the third 4 pepper plants. After 3 weeks all the plants were dying. The tomatoes had leaf curl, end blossom rot, and yellowing leaves. The peppers and cucumbers never did "take off". I went to the nursery that sold me the soil for the beds and asked for a hard copy of their soil analysis. The pH in the soil was 9, and all the other soil components were low. I removed all the sad looking plants. Then I removed about 1/3 the soil in all the beds, and I patiently re-added manure, peat moss and store bought topsoil to each bed. After rototilling all the soil mixtures together in the beds, I replanted the same things. And the same thing happened with the plants. My next step was to have a soil analysis done with the county extension office. The pH level was now 6.5, however there was a "trace" of Nitrogen in the soil...potassium levels were very high. Today I removed all the soil from the beds right down to the small stones...hosed down the beds and am starting over. While I will probably wait until spring to add any medium to the beds, any input would be greatly appreciated.

Hi there, I am not far from you, I'm in Bozeman.

I wonder about the drainage in your beds. Perhaps the soil is not draining well? That can cause the leaf curl. In our GH we don't have any lining below the beds, they are just open on the bottom to the natural ground. We dug out about a 2' depth of crappy subsoil, and replaced it with a combo of peat moss, sand, some nice topsoil from elsewhere in our yard, organic goat manure, and organic pelleted fertilizer (Dr. Earth brand, 5-5-5).

So I'd start by taking out the landscape fabric for sure. I can't think of any benefit to that.
Can you use some decent native soil and supplement from there? I'd rather start with native soil than the bagged stuff. If you are seeking excellent supplements, check out Planet Natural in Bozeman, they are a terrific resource if you ever come out this way.

Also, in the current issue of Zone 4 garden magazine, there is an article I wrote that is all about getting the most from an unheated greenhouse in our cold climate. It might be a handy starting point for you.

I hope you get this worked out. It is so nice to have a greenhouse around here and that is a perfect retirement gift!