raised beds

[MountainWood]

cme4dk said:

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The site is over a concrete slab...about 2 feet high out of cedar.

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Concrete?

That MAY be problematic.

At 2 ft high, the soil will be likely 1.5 to 1.75 deep. I'm not sure that's deep enough for the rooting system of many vegetables. According to my grad school research, roots of small seedlings can go 3 ft deep in 20 days. Plus the potential drainage issues.

Is there another site available?

[chips ahoy]

would be really short carrots as well.

Mel

[Rodneywt1180b]

cme4dk said:

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I have been asked to oversee construction of raised beds for a community garden, the proceeds will go to a food bank. The site is over a concrete slab and I wanted to make the raised beds about three feet by nine feet, about 2 feet high out of cedar. To keep the costs down, would 3/4 inch planks work?

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It can be done but I think you would be better off using thicker lumber or better yet, concrete or brick. 3/4" cedar will bow out without additional support and will rot out fairly quickly. I have a small raised bed made of 2x6 lumber and it has only lasted about 7 years. The boards now need to be replaced.
I would also seriously consider getting rid of the concrete under the beds. The plants will thank you.
I understand about wanting to keep costs down but not spending enough at the beginning will only mean spending even more later.
Rodney

[vernonator]

MountainWood said:

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[blockquote]cme4dk said:

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The site is over a concrete slab...about 2 feet high out of cedar.

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Concrete?

That MAY be problematic.

At 2 ft high, the soil will be likely 1.5 to 1.75 deep. I'm not sure that's deep enough for the rooting system of many vegetables. According to my grad school research, roots of small seedlings can go 3 ft deep in 20 days. Plus the potential drainage issues.

Is there another site available?

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[/blockquote]

The concrete will be more than just problematic - it will lead to failure unless you make the beds REALLY tall - like 36" to 48". You will also have to allow for drainage and filtering at the drain holes, unless you want the soil to all run out. You will also need some sort of automated irrigation as the shallow beds (on cement) will dry out VERY quickly.

All in all a bad idea putting beds on a concrete slab.

[AHill]

It's really all about drainage. That's the challenge here. Depth of soil isn't as much of a big deal as you may think. Think about container gardens, where soil height is at most 24". It does limit the kinds of vegetables grown (root vegetables, mostly), but it still allows quite a few to grow just fine. You may need to space them apart from each other than a normal garden, since the roots will now spread sideways more than normal. Others have already provided suggestions on tackling the drainage part. I recommend finding a good book on container gardens.

[tnff]

Two feet deep should be a gracious plenty for vegetable gardening. I'd use at least 2x PT material or maybe even timbers since a community garden would take more abuse than a homeowner garden. Line the inside with landscape fabric to prevent it washing out and bust a few holes in the concrete for drainage. But at two feet deep, unless you make the joints excessively water tight, it should drain ok.

[BillN]

I rented a demo saw and cut out some concrete in an unused driveway, then installed a couple of beds (I was younger then). It worked out great if that's an option.

[PhilJoines]

Check out a book called Square Foot Garden at the library. Tells all about it.

[sroxberg]

PhilJoines said:

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Check out a book called Square Foot Garden at the library. Tells all about it.

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I agree, or Half Price Books has a used book section that is pretty darn cheap.