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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Depends on the garden design. I have 4 raised beds next to each other. Cement blocks are 8 inches wide so for me 4 raised beds would lose me 64 inches of space (16 x 4). Not a big deal if you have the space but in a small area it adds up.
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I definitely understand the math. What I’m saying is don’t discount seating as a worthwhile use of space. Now, if you’re planning on doing a bunch of raised beds, block is probably not the right material for a lot of reasons, including that you don’t need that much seating. But for one or two beds, why not?
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Find a day your cinder blocks are dry…very dry…
and use
‘Great Stuff’ formaldehiyde foam from the cans as your adhesive …use a small bead sparingly…3 blocks high is not a problem.
You can sit on your blocks too.
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You can plant inside of the holes of cement blocks. You really don’t lose anything but the outside inch or so.
I had a cinderblock bed for a few years before the shutdown left my handy husband with more freetime than usual and he did a garden makeover. I tried to grow within the block holes as well as the interior of the bed. That was fun to see what grew well in those smaller spots and what died quickly. It was hard to keep the plants in the holes watered well enough because they dried out so quickly.
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I’ve got several of these beds and while they move some, it isn’t too bad. I’ve only gone 2 blocks high though.
This one uses over-sized blocks (wider than most). It has worked well as a garden bed, mostly to grow potatoes. Note that it doesn’t really have ends, as I used tree trunk sections there and they have mostly decayed. In fact, to the upper-left, you can see a bed which is entirely made of tree trunk sections which are getting pretty soft after 6-7 years.
There are a few rows of radishes planted now and the slate acts as a place I can step in the middle.
I’ve got figs in this one, which is in a wet area. Over the years, at least half of the bottom block has sunken into the ground. I can’t say that these figs have done all that well, but I’m not sure if the bed is to blame. The one at the top of the pic is RDB and has produced a handful of figs each year for the last 2-3 years.
This one has probably been the most unstable, as I have it near the top of a hill. I think the mix I used to fill it drains a bit too well, as the strawberries I originally planted there gradually died out. I’ve used it for other stuff in recent years, and just mixed some compost in the other day and tried planting more strawberries.
I’ve also used a single-high row of blocks as edging. It worked OK, but probably wasn’t worth the work. I’ve been taking apart one of the edges (probably give up on weeding beds and just mow up to the trees) and re-purposing the blocks.
I just made this bed with the leftovers from the edge:
You can tell I didn’t spend a ton of time getting everything perfectly level and square. In fact, the ground slopes, so I tried to hug the slope, rather than building up (or digging into) the ground.
So, at the end, the last block didn’t fit perfectly. My solution- knock off part of it with a hammer and fill in the space with some left-over cement.
There was one part which looked particularly precarious, based on the slope of the ground, so I added some supports. (5’ galvanized rigid conduit (1/2") which was cut in half to get 2 stakes out of it).
I wouldn’t argue it is beautiful, but I was looking for functional. I should have checked into corrugated metal, as it sounds like it could have been easier. Moving a lot of cement blocks is certainly a workout. After the first trip where I moved 4, I settled into a more sensible plan of only bringing 2 of them up the hill at a time…
I’ve always squeezed things in too tight, so I don’t think it has been bad having the blocks enforce some separation. But, I admit that I am thinking about planting some of my extra day neutral strawberries in the holes. Particularly the block which has a double-wide hole (already filled with dirt on the upper-right of a few pics up).
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