Raised garden bed on concrete. bad?

tcon83March 5, 2014

Im new to gardening, and i have a 6' x 10' concrete slab in my backyard from an old shed that has been since demolished. I would like to build a raised bed garden on this but have the below questions. any advice / tips would be greatly appreciated.
1. how about drainage? do i just put the soil right on top on the concrete? or do i put stone down first to allow drainage?
2. Do i have to worry about anything harmful leaching up into the soil from the concrete slab?
3. do i have to put a plastic membrane down on the concrete slab and on the wood walls of the bed ?
4. how deep of a bed should it be? (will be growing tomatoes, peppers, cucumbers etc)

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Plants put down 90% of their root system in the top 12" of soil. I have found no benefit from raised beds deeper than 15" (of growing medium/soil mix, exclusive of any drainage underlayment). 12" to 14" of actual growing medium will be fine. You want it to be 12" deep AFTER settling - my 15" usually settles down to around 12" to 14" actual depth by planting time.

If paint and other yard chemicals (gas, lawnmower, oil, etc) were stored in the now-demolished shed, there is always the chance that some of that was spilled on the concrete slab. I wouldn't feel comfortable growing on that slab or near it without some soil testing.

A gentleman who was very active in the development of solar stills used to build self-watering raised beds for use in dry climates such as Arizona/Nevada. I have plans for his beds, which were published in Countryside Magazine in 1998. The text of that article is available online, but no pictures. If you can find a copy of the magazine in the library or get a copy of it via interlibrary loan, that would have the pictures in it. Or it has been published in Countryside's 1998 anthology, available online for around $20. I just bought a copy of that anthology because my copy of the magazine in which it was originally published was lost.

He uses an impermeable barrier to contain the pea gravel and maintain the water level - commercial pond liner. You are unlikely to find the stuff at Lowe's or Home Depot. That would protect you from the lingering effects of any remaining chemical contaminants.

What he uses for "sand" is unclear - he states builder's sand is not suitable. I'm not sure what the stuff that blows around in the Arizona high desert that he uses instead is, but I'm planning on using mason's sand - this is finer than builder's sand. It is not the same stuff used to make concrete.

What you have here is essentially a large container. You will only be able to fit one 10'x4' wide bed on the space you have. I would suggest mulching as deeply as possible, especially if you have droughty summers, to keep the soil evenly moist.

This is for a self-watering system, and it will work as well as you did when following the plans.

You can build much simpler raised beds (with no self-watering) if you're planning on building them at ground level - but I wouldn't put ANYTHING directly on that slab unless I had evidence that it was clean of any contaminants.

If you are very short (like me) or have impaired mobility/reach (like me) you may not be able to reach all the way across a 4' wide bed. If you build with concrete blocks, however, it would be fairly simple to sling a 4' long piece of 2x12 (or wider, if you can find it) across the top of the bed (being careful not to pinch or crush any plants). Like a walkway, to get you in a little deeper without stepping on the bed and compacting it.

However, because of my mobility issues, I prefer a 3' wide row. I square-foot garden in spaces such as this. Since you need at least 3' between rows for access, you could still really only fit one 10' row on that 6x10 pad, regardless of whether it is 3' wide or 4' wide.

Here is a link that might be useful: Horace McCracken's self-watering raised bed plans

This post was edited by zensojourner on Wed, Dec 17, 14 at 20:12

    Bookmark   March 7, 2014 at 7:05PM
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Look for the book "Gardening on Pavement, Tables and Hard Surfaces" by George Schenk at your library. This book gave me the courage to create 2 long, narrow raised beds covering a too-long driveway. Mine is just planting soil piled directly on the concrete, with a path running down the middle. No problem with drainage, and I keep the soil at about 18" deep, I grow a variety of flowering plants, grasses, and small shrubs, and have had no problems in the 5 years since I started this project. Everyone thought I was crazy, but it was a fun experiment, and it worked!

    Bookmark   April 11, 2014 at 4:05PM
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I built my garden using discarded refrigerators/freezers. With pier blocks underneath, they are waist-high, and can be built on any surface. A good source for them is Craigslist. The one drawback is the cost of the amended soil I fill them with, but they provide ample square footage to grow whatever you want!

    Bookmark   December 2, 2014 at 7:38AM
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