Hello everyone,
does anyone have any experience using concrete block raised beds? Im considering building one that is three blocks high but I’m concerned about the blocks moving. Will rebar down each block be enough to secure them? I don’t want to use mortar because the bed may need to be moved in a couple of years.
Thanks!
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I’m not sure your vegetables will enjoy the aesthetics and the blocks would seem to cause a huge waste of space. I’ve seen sheets of corrugated metal used for a similar utilitarian purpose to hold up beds and that doesn’t waste room and looks a lot better to my eyes. But beauty is entirely in the eye of the beholder and nothing shows strength like concrete blocks.
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I agree that it’s not a good use of space, and I think it likely that they’d move some over time. I doubt rebar would help - maybe try one bed before you make too large a commitment? Who knows? It might work just fine.
(I packed a few block a couple of days ago - I’d forgotten how heavy those things are! So maybe it would work.)
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For some reason I am imagining the square 8 type blocks as I read your query, although you don’y actually specify that. They are on the heavy side, so unlikely to move around once you have the beds filled. You can also fill the holes with soil that will minimize potential even more. If bears decide to climb on them, that’s a different story. I had these three high at a former rental. Nothing ever moved them.
What I should be imagining is the more solid concrete blocks made specifically for walls and borders and raised beds, because I have several such constructs in my yard. Many of these come with a little lip on one side to help lock them into place. Those lips rub off almost as easily as freshly opened buds on a rootstock, so they only really help with alighnment. Mine are from one to three high. The only time I saw any shift, even with me waking on them, it when I shoved a couple with the riding lawn mower backing up.
The trick with both is to stagge the courses, so that each brick above are sitting on two below. You also need to level the ground they’ll sit on, or at least made the transition halfway smooth.
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I’m going to disagree on the use of space concerns. If you’re using retaining wall blocks, you can use the edge as a place to sit while gardening or for guests. If you’re using cinder blocks with the holes, you can either fill with soil and plant in there as well, or cap them and use them for seating.
As for movement, a three-block high one (assuming cinder blocks) might move on you if you don’t mortar it. Two blocks high would probably be ok, at least for something you’re planning on moving in a few years.
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My friend uses concrete blocks and fills the holes and plants flowers or strawberries in the holes
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Drawing on my experience (link), I wouldn’t recommend going any higher than two blocks. Using vertical rebar, filling with compacted stone, and building on top of stone footers will make a big difference in terms of long-term stability. The walls will still move ever so slightly over time but can be realigned with a quick tap from a rubber mallet.
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I’ve read concerns about leaching from cinder blocks if they are truly “cinder” blocks (i.e., if they use coal ash as part of their aggregate, which some still do since it’s cheap). Also, freshly manufactured cement probably will leach lime and impact pH at least at the edges of the bed.
This page addresses both issues (as well as aesthetics), but seems to conclude that other than for really acid-loving plants (blueberries, etc), you’re probably fine with actual concrete blocks:
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I’ve been considering making some forms for raised bed sides and pouring my own. I’m tired of dealing with my poor soil.
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I use cinder blocks for my veggie bed. Mine is only 1 block high, though, so I can’t give any input re: having the bed 3 blocks high.
I think it’s perfect for my purposes. My veggie beds are 9 blocks long by 3 blocks wide. I usually do “big” veggies in the middle (tomatoes, zucchini, peppers, etc.), and then I fill the individual cinder-block holes with herbs (parsley, thyme, basil, etc.) and “little” veggies (arugula, kale, etc.) I also add some marigolds and nasturtiums interspersed around the individual holes to attract pollenators, and cause they’re pretty. The bed is low enough that it’s relatively easy to fill (I top it up with compost every spring), but high enough that the bunnies tend to ignore my veggies.
I was not aware that there may be potential leaching issues or PH issues, but my veggies have always grown large and healthily, so 
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It’s possible the lime leaching could actually improve pH for some soils, so it’s not necessarily a bad thing!
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totally agree with that article about “cinder blocks” possibly raising soil pH, especially if soil volume held is relatively small
but totally disagree with the article categorically saying cinder block is ugly. Aesthetics is subjective, Actually think it is the most aesthetically pleasing, and is the only readily available material that imparts a sense of timelessness.
Something to do during dreary winters, or year round - General Gardening - Growing Fruit
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I’m with you on that! Especially once the concrete starts to grow moss and lichens (at least here in the PNW that happens pretty fast).
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know exactly what you mean, as i used to live in a warm and humid tropics 
The living, green patina is hard to beat !
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There was an extensive raised bed topic on this forum several years ago, many pictures posted, do a search for that.
I have had a 2-layer cinder block bed 21x3 feet for many years. The main problem with cinder block gardening is the extremely sharp and abrasive edges. I tapped pieces of plastic hybrid deck wood into the voids of the top layer of blocks, flush with the top edge, and then attached 2x8 wood to the inserted plastic pieces, creating a continuous wood cap. That keeps the blocks in place and eliminates scratched hands and arms. For this to work, the whole project needs to be very level. With 3 courses of blocks, the levelness of the bottom course is critical.
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If it is just temporary, consider buying the concrete blocks with the slits on each side to fit the 2x boards vertically. I know lumber is expensive right now but so is the rebar you plan on using so I think the price will be about even. I was considering using the concrete underlayment cut lengthwise in half just to take up less space, but I haven’t priced it in a while…
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I saw a display of those slotted blocks in front of the local box store and have been considering it for some of my short term experiments. I probably already have functional lumber. I also keep eyeballing the logs I recently cut down for a similar purpose. It’s mostly maple and pear, so not a lot of potential to stunt the growth of anything, and, if I don’t pull it up and move it, it will just end up topsoil. I’m toying with growing some mushroom plugs in the next year or two, so … eventual dual purposing. There are a couple more trunks coming down in the same time frame.
Continuing on the concept of cinder blocks. The area I had was against a house. It was three high, and the freeze/thaw cycle did push the top out a bit after a few years. But not enough to stop me from being able to jump around on it, use it for leverage weilding a shovel, and regardless of what I was carrying onto it. It was not mortared and the soil around it likely covered the bottom inch on the outside to set the course. It drained well even after the building owners let their handyman allow a gutter to drain into it instead of into the original drainage pipe. It was not competing with tree roots, but it was stuffed with Nandina, hosta, ferns, trillium, and a variety of other stuff built for competing in a forest. I used the holes in the top course to start all kinds of things. Not the easiest holes to get things out of if you leave them there more than a season. I’m pretty sure whoever put them there had probably filled the bottom course or two with gravel, and maybe the whole bed started that way. Given the history of the property, it is quite likely that the bed has been there approaching 40 years.
The ones I’ve assembled out in the yard were the more decorative types, and the smoother versions will move with only a little encouragement, but none of the flat, rough styles, set on a level base, seem to want to go anywhere without significant help.
That same house currently has an unmortared “retaining wall” alongside it’s driveway comprised of the larger style designed to create circles and serpentine shapes. It is an essentially straight run of four courses for most of it’s length. It starts with a set of three and ends with a set of five. The heavy rains it gets do not put pressure on it to buckle, but the facade being comprised of a lot of open wedges underneath the capstones means there are veritable waterfalls coming out of the wall when it rains heavy, which is regularly. It’s been there around five years now.
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My raised beds are cinderblocks, just one course is all you need. I lay down landscape fabric, then put the soil on that. Works terrific, been doing this for two decades now.
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Do you lay the fabric underneath the blocks or line the bed with it?
I run the landscape fabric inside the blocks. Otherwise weeds just creep in from the side.
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