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I have a bunch of raised garden bed frames, and I'm considering putting them on a slab of concrete that I have in my back yard. I wanted to avoid putting them on the grass, as when I've done this previously the grass has grown up through the soil in the raised bed.

I plan on growing vegetables, however I'm concerned that there won't be adequate drainage. I also don't know what sort of soil layering I should use since this is on concrete.

I'd like some advice on how I should proceed.

concrete with garden bed frames

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It appears your beds are pretty deep. At least a foot deep by my estimation. Most edibles you'll plant won't need to be that deep. 4-6" is enough for a lot of plants except for root vegetables but you can always add an extension in those areas. You don't have to make the whole bed deep just for a few potatoes or carrots.

Since you have that much depth what I think might work is to do what people do with plants in pots. Provide drainage at the bottom of the beds and poke some holes on the sides, along the bottom to let the water escape.

Maybe 1-2" of gravel, landscape fabric, then your soil mix on top of that? That's what I would try if I absolutely had to put the beds on concrete.

As for what to fill the beds with. There's an active community of gardeners that practice and modify Mel Bartholomew's Square Foot Gardening which recommends equal parts of compost, vermiculite and peat moss.

But you said you're doing this mainly because the grass grows up through your beds. Have you tried solarizing the area to kill off vegetation and seeds, putting down some landscape fabric then your beds and growing medium?

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  • I'd definitely echo the drainage point. Without some gravel at the bottom and good amount of holes to let water out your almost certainly going to have problems
    – dkackman
    Jul 13, 2013 at 19:15
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Its pretty well known how plowpan reduces crop yields and is why many farmers subsoil their fields. Plowpan is the hard layer of soil 6-7 inches down caused by the repeated plowing at the same depth. Subsoiling breaks up that layer http://www.youtube.com/watch?v=8VKsK3u_JWE

Roots

Image taken from: http://www.soilandhealth.org/03sov/0302hsted/030201/03020100frame.html

Note that the units are in feet.

Roots can go deeper than you would think. Plants can do ok in pots, but do better when their roots are allowed plenty of space to grow.

See here and click on the crop of your choice: http://www.soilandhealth.org/01aglibrary/010137veg.roots/010137toc.html

It is fairly difficult to find a vegetable with roots only occupying the top 6 inches of soil. Most can easily go down 2ft.

But, as far as drainage goes... If you dug a hole in the yard as big and deep as those frames are and filled the hole with water, I bet the frames on the concrete would drain faster than the hole in the yard. So drainage on the concrete is probably not a big deal unless the frames make a fairly water-tight seal. If in doubt, stick a 1/4 inch shim under each corner of the boxes, or drill big holes. One good thing about the concrete, it will never become water-logged. It will drain the same no matter where your water table is and no matter how long it rains.

As a test, fill as many buckets as you can and then dump all the buckets in the frames at the same time and see how long it takes to drain.

Vermiculite is good stuff. Compost and moss will decay and disappear, so you will have to add it as the level drops. Alternatively, there could be someone in your area who sells a nice garden soil. The price here is $20-$35 a pickup truck load for some nice black soil. I'd check craigslist. As an illustration, here's an ad in my area http://chattanooga.craigslist.org/grd/3924301730.html

It reads:

Vegetable Garden Soil - $26

We sell Blk Gld known as Windwood's Premium Planting Mix at Windwood Bulk Center on 5342 Battlefield Pkwy in Ringgold. Windwood's Premium Planting Mix is great for vegetables, flowers and shrubs. Just put down 6 sheets of white newspaper then 8 inches of Windwood's Premium Planting Mix and you have a flower garden right on top of your grass, no digging required. For a vegetable garden just put down 6 sheets of white newspaper then 12 inches of Windwood's Premium Planting Mix and plant your vegetables. Windwood's Premium Planting Mix is available with 6 months slow relaease fertilizer or organic with no fertilizer it is a weed free planting mix We can load your pick up or trailer or deliver.

There are many ways to go about this, it just depends what you want to accomplish. My only issue with the concrete is root depth. If you're ok with that, then go for it. The worst that will happen is you'll decide to drag the frames onto the grass later.

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  • Great answer, you raise some interesting points! Thanks for the links, I'll check them out.
    – bradley.ayers
    Jul 15, 2013 at 4:22
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    From the same site (and from what many have experienced themselves) it shows plants can and do adapt their root structures to their environment. The article on lettuce roots shows lettuce will not grow deep roots in compacted soil, instead they grew denser near the surface. The article didn't say whether one root system provided more or less yield than the other. The only mention of yield was if the shallow rooted plants' soil was disturbed by hoeing to remove weeds yield dropped. Chop the roots of course yield drops. soilandhealth.org/01aglibrary/010137veg.roots/010137ch34.html
    – OrganicLawnDIY
    Jul 15, 2013 at 10:19
  • Whether that particular article mentions yield is inconsequential to the fact that subsoiling produces more yield. Why else would farmers go through the trouble and expense of the machine and the fuel? Watch some of the youtube vids and you'll hear farmers talking about till vs no-till yields. The tilled land yields more than no-till and the subsoiled yields more than the tilled. Sure, a plant can adapt to plowpan, but its not optimal for the plant to have a barrier to root growth. If one wants to limit root growth for other reasons, that's ok. It will be fine, just not the best.
    – Randy
    Jul 16, 2013 at 3:18
  • Raised garden beds vs production farming is an apples to oranges comparison. In a raised bed you'll add a much nicer growing medium than the soil on most farms. Yield would have been important to mention in those linked papers. We can see that plants roots adapt to soil conditions. How do shallow roots compare to deeper roots in terms of plant health and yield? This is similar to the question you asked on lawn watering. Though I agree OP would be better off not putting the bed on concrete. Some people may not have a choice. Rooftop gardeners for example.
    – OrganicLawnDIY
    Jul 17, 2013 at 16:39
  • So, you're advocating shallow roots for veggies and deep roots for grass? I'm just saying if you put a concrete barrier in the way of potential growth, I don't see how that is in the best interest of yield. Will a tomato plant produce more in a 6" deep pot than one in a 12" pot? Better soil means better aeration, which would make roots grow even deeper because the O2 level is higher deeper in the soil. We lay tomato plants on their sides in trenches for more roots. Luckily, tomato plants can root out of their stems. But with other plants, deeper aerated soil = more roots = more yield.
    – Randy
    Jul 18, 2013 at 4:19
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I wanted to avoid putting them on the grass, as when I've done this previously the grass has grown up through the soil in the raised bed.

I forgot to add one more thing. I think it's unlikely the grass is growing up through the beds given how high they are.

More likely you're seeds are already in the medium you use to fill the beds or are being blown in from other sources.

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Be aware that concrete is alkaline and so over time the pH of your garden soil is going to be less acidic than it would be otherwise. You can think of cement as man-made limestone, calcium carbonate. Crushed limestone is often added to soil to raise the pH.

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there's something appealing about raised garden beds isn't there? Not only are they practical but they look great too. I have two in my back garden, mostly to grow veggies (but also some herbs).

If you're set on placing them on concrete, definitely drill some holes in the bottom and place a thick layer of stones/gravel on the bottom of the beds for aeration and circulation. Also, invest in top notch soil that will provide your plants with the necessary nutrients to compensate for any deep roots.

However, in my experience, grass very rarely grows through garden beds so if that's your only concern I'd consider just raking and removing the grass over your desired area. I'd say this is your best bet. You can always turn that concrete patch into the centrepiece of your backyard with some innovative garden designs.

Best of luck!

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After asking so many questions and throwing around so many ideas we have decided to lay concrete footings under our new 40cm high corrugated raised beds to restrict eucalyptus roots invading my food crops! I believe its the only way. I live in country australia and didn't want to chop the established trees down. We're also making sure the beds aren't under the drip line of the trees.

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