While winding down for the night, I’ve been watching some videos, including this new one from MI Gardener; a resource I have recommended before.

This is an interesting video, since I fairly regularly see these concrete blocks being recommended to make raised beds. He lists the reasons he believes they should not be used.

I found it really interesting, and wanted to talk about some of them.

One of the first things he brought up was the weight. The ones he uses in the video are 8″x8″x16″ This is the size of most of the blocks we’ve been finding all over the place, though I think I’ve found some that are 10″x8″x16″

He also adds that they are cheap, and says they cost about $2 each.

So, of course, I had to look it up!

It turns out, one of these concrete blocks weighs about 30-35 pounds. The ones he’s using has flat ends. Some have shaped ends, and from what I found on the Canadian Home Depot page, they weight 42 pounds (19kg). The 10″ ones weigh 45 pounds (20.4kg).

I’d never really thought about how much they weighed. I found them heavy to carry around, but not that bad, really.

As for being cheap, interestingly, the 8″ ones are listed as just under $5 each – and are more expensive than the 10″ ones, which are just over $4 each. The last time I looked at their prices, at an actual store, not online, was a few years ago, and one 8″ block was priced at over $8 each, which shocked me. This was before inflation went nuts, too. Very odd.

This got me wondering, since we are using concrete chimney blocks as planters. So I had to look those up, too. I found plenty of links, but not one of them included a price. The closest I could find was an old image with a price on it, but the link was no longer active. Other than that, I found some on FB Marketplace were someone lists a whole bunch of things, not just chimney blocks, with a $4 each price listed. I don’t think that price was the same for all the wide variety of items listed, so I still don’t know how much they sell for here.

The chimney blocks we have are the standard square size of 16.75″x16.75″x7.62″

That measurement is so odd, I thought it would make more sense in metric, but not really. That works out to 42.5cmx42.5cmx19.35cm

What caught my attention was the weight.

Each one of those chimney blocks weighs 68 pounds (30.8kg)

So… I was lugging almost 70 pound blocks around the yard when I built the retaining wall and the chain link fence planters.

Okay, so weight was one of his reasons not to use them, but the other was height. They’re only 8″ high (slightly higher than our chimney blocks), making only 8″ of growing space.

I found that curious. His sample concrete block raised bed is on top of soil. Granted, it looks like the area was just leveled and has been driven over, so it might be compacted, but it’s still open soil, not a concrete pad or something. Any plants could send it’s roots lower. We have maybe 8″ of top soil – 10″ in a really good spot – before we hit gravel, clay and rocks. An 8″ high bed gives us about 16″ of growing space. We have been using modified hügelkultur, so the first couple of years would have sticks and branches at the bottom, so we don’t plant root vegetables in them right away.

I would find 8″ too low for accessibility reasons, and when I see people doing raised beds with concrete blocks, they are often two, three or even four blocks high. He talks about adding a second level, and demonstrates how easily the block spins around, since the base row is not perfectly level. He mentioned that people have suggested rebar to hold them in place.

I agree with him on the rebar. It seems almost counterproductive.

I’d just use concrete glue. AKA construction adhesive. It’s really cheap, one tube goes a long way, and can be applied using a caulking gun. Granted, that makes it permanent, but most raised beds don’t get moved around once they’re built, anyhow.

He makes a point about breakage being a problem, but not in the way he talks about. Where we are, we have temperature extremes. Pretty much all the concrete blocks we’ve been finding have been outdoors. Most of the chimney blocks we’ve used were stored outdoors since my parents bought this farm in 1964. Yeah, they have dings and chips, but they are still sound. The only blocks we’ve had actually crack and split where the two we put into the fire pit to hold old oven racks over the coals to cook on. It was exposure to high heat that lead to them cracking, not outdoor temperature fluctuations.

He mentioned that concrete is very porous, and that the edges of beds would need extra watering. True enough, though with our chimney blocks, there isn’t any centre space that would hold water longer than the edges. We still watered them as often as the other beds. We just have them more water. It’s been a few winters now for the retaining wall blocks long the old kitchen garden, and not one of them has cracked due to water absorption and the fresh thaw cycle. I’m not saying it can’t or won’t happen. Just that is hasn’t happened to us.

He then mentions that concrete absorbs a lot of heat. More so than even metal raised beds.

That may indeed be a problem in a lot of places.

Where we are, that’s a selling point! Even in the heat of our summers, it just makes them better for heat loving plants that might normally not grow well here.

There is another element to using concrete blocks he doesn’t mention, and that is the effect on soil pH. I’d read before that concrete can help lower the pH, which would be great for our alkaline soil.

As I was looking it up just now, though, I found one article that said concrete can lower the pH, but that was it. Other articles, however, said that concrete will increase the pH.

Which is the last thing we need!

Given the smallish growing space inside our chimney block planters, it wouldn’t take much to affect the pH in the soil there. Granted, I did amend the soil with sulphur this past year, as well as with peat, but with our pH already at 8 or higher, it’s going to take a long time to adjust the pH even slightly lower. Getting it to be slightly acidic, which most of the things we want to grow do better in, will take years. Looking up what vegetables grow well in alkaline soil, the list is pretty long, but most of them have an upper limit of 7.5. If we can get our soil at least down to that, it would be good, but it is harder to lower the pH than it is to increase it.

Well, the mint I transplanted into some of the retaining wall blocks seems to be handling the high pH quite well, as are the chives. Our biggest issue, however, is roots growing in from below. With the blocks at the chain link fence, it’s the nearby elms that took over the soil in the blocks. In the retaining wall, it’s some sort of invasive flower we haven’t been able to get rid of. It’s almost killing off the mint, in some blocks!

So when it comes down to it, whether using concrete blocks for raised beds is a good idea or not really depends on a lot of things. Stuff he saw as negatives, are positives for us, or non-issues.

Would I use concrete blocks for a raised bed?

Probably not.

We’ve got all those dead spruce trees that need to be cut down that can be used. For other raised beds, I’m liking the wattle weave idea and want to build more, though it’s surprisingly hard to gather enough suitable branches to use for the weaving. If we did happen to have enough concrete blocks to make raised beds, there are other things I’d much prefer to use them for.

When it comes to gardening, it is such an individual thing. We all just have to figure out what will work for our own gardens in our own regional area and microclimate, and use what materials we have access to.

Hearing what works or doesn’t work for others and why can still be a huge help. Which is why I like videos like this one. Not only did I find what he had to say very interesting, it got me checking things out and looking things up to learn more.

And now I know how much those chimney blocks weigh, too!

The Re-Farmer