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What are the pros and cons of using cement blocks for a raised bed?

I am thinking of using something similar to these cement blocks and building them 3 blocks high. That would give me a good 2ft deep bed. I am also thinking (but unsure) if I should pour cement down the slots to help make it solid and connected or how I should go about keeping them together.

Once all the bricks are lined I will also be getting some nice treated wood to build around them. This will be mainly just for aesthetics but I guess could also help hold the blocks in place.

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I've seen it done with one row of blocks laid right on the ground -- gives you the same height that a single 2x10 plank would. Two rows might work ok without cementing them together if you do some prep work for a foundation (coarse sand or stone, 4-6" deep). At three rows, you'll definitely want to do foundation work and tie them together. If I were doing it three rows high, I'd mortar the joints instead of just dry stacking them (it will look nicer and you won't need to pour cement down the holes). On a solid foundation, this won't need any external support (I have a 10' wall built from 6x16x8" blocks, built on a concrete foundation, that is not going anywhere.)

As a pro, if you put in the up-front work and do it right, it will last just about forever.

As cons, the up-front work is time consuming, requires some skill to get it right, and once you're done, it will last just about forever, whether you want it to or not. ;)

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Cement blocks will work but are likely to make your soil alkaline. By adding cement you are definitely going to adding a chemically active compound to your bed. Why not use rebar or T-Bars to stabilize your wall?

Also, from a structural point of view, unless you prepare a bed of crushed stone to a depth of 6" or more that has been compacted cement will not hold the blocks together if the soil heaves due to frost or cold.

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    Will the alkalinity from the blocks really travel very far through the soil? I'd agree if you're talking about pouring cement in while the soil is in the bed, but I wonder how much effect it will have once the cement is dry.
    – bstpierre
    May 10, 2012 at 19:45
  • @bstpierre Good point, it depends on the composition of the soil, the amount of water added to the bed and probably a few other things I don't know about.
    – kevinskio ♦
    May 10, 2012 at 20:00
  • With my geological hat, I think would would need some water acidity to get significant mobility - theoretically useful if blossom end rot (Ca deficiency) is a problem! However I do agree about re-bar: From my experience with timber raised beds, I think 2ft high cemented concrete blocks might tend to spread with the weight of wet soil - especially if narrow. Barring frosts, wide breeze-block (cinder block) sized blocks should be fine? Brick widths not so much.
    – winwaed
    May 11, 2012 at 2:25
  • Great answers! I just want to add that yes, the concrete blocks all by themselves WILL raise the pH. No big deal, just make sure you do a test and if it is too high, 7.0+ you can add sulfur to lower. Don't need wood to make look great. Functional and uniform are very aesthetic. I have seen these blocks coated with a thin layer of Portland cement that was beautiful. Just make sure as in any circumstance to test your soil. Make sure there isn't a bottom and dig a few holes at least 2' deep to check for impervious rock or clay layers. Do not put rocks in the bottom of these planters!
    – stormy
    Jul 16, 2014 at 23:20
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I think those blocks would be very unsuitable for the purpose: there will be no way to get them to hold together without a great deal of extra effort and expense. Besides that, they are ugly. I would advise you to look into masonry retaining wall blocks, which are designed to be dry-stacked with good structural stability. They will tolerate a good deal of ground movement/settling with no issue, and they are far more attractive.

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  • attraction is not an issue because I will be putting wood around them. You will not even know they are there.
    – corymathews
    May 11, 2012 at 13:25
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The blocks you are choosing are too narrow they will collapse in a short period of time. You need to use a full cement block with a crushed and compressed bed with the first course buryed or as the above poster stated use a decorative landscape block. Regardless the 6 inch base of crushed and compressed stone should be in place as well as burying the the first course of block. Good luck

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Just built a great raised herb garden with cinder blocks, stacked 2 high/tall. You can use construction adhesive for securing them if you want, but I didn't need to use anything with just 2 high, you go too high and leaning over would be a problem. My husband leveld ground, put down constuction cloth (like a wire cloth) because we have moles, then that black mat'l, to help keep the weeds down. We did a U shape. sand then good mix of potting soil. Planted 33 plants, put tomato plants in the holes of the block (every other hole) down one side. Have seen this done and works great. Used 60 blocks. will use a food safe paint to decorate the walls, maybe stencil a design.

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