How to Build a Concrete Block Raised Bed Garden

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Do you have lousy soil but want to start a garden? Consider building a concrete block raised bed garden!

I’m seeing people use these for backyard gardening more and more. Stacks of concrete blocks are usually associated with building construction sites, not gardens, so they grab your attention when you see tomato plants sticking out of them.

Placing a rectangular stack of concrete blocks in your yard sounds like an eyesore, not something that might actually complement your landscaping. Except… these beds look good!

People build their raised bed gardens in a variety of ways. If money is no object, you can use red cedar or stone. Some use pressure treated lumber while others refuse on toxicity grounds. It’s largely a personal choice. The lowest-cost option to build raised beds is actually free, you just pile the dirt into mounds. But I prefer concrete blocks!

Why Concrete Block Raised Beds?

Every style of gardening comes with its pros and cons. What works for one person in their location may not work for the next. I think concrete blocks are great for suburban, hobby gardeners.

Advantages

• Concrete blocks are easy to salvage from vacant lots, behind barns, the town dump, and wherever else.
• Sturdy!
• Rot proof.
• Made of natural ingredients.
• Covered with flat blocks, concrete block raised beds provide a nice, wide platform that you can sit on to plant, weed, and water.
• Easily adaptable to form hoop houses, screen plants, etc.
• They look good!

Those are advantages specific to concrete block raised beds, but raised beds in any form have advantages as well.

• Easier to weed (less bending over).
• No tilling required (less compaction from stepping on the soil).
• They’re easier to work (again, less bending over).
• You can build your own soil.
• The soil warms earlier in the spring.

Disadvantages

I said before that every gardening strategy has its pros and cons and raised beds made from concrete blocks are no exception.

• Concrete blocks are heavy and require more physical labor to construct.
• They can be expensive to construct if you have to buy many blocks.
• Concrete absorbs water that might otherwise go to the plants.

These disadvantages can be overcome, however. Well, you can’t do much about the weight of concrete blocks, but at least that heavy lifting will only have to be done once (if the bed is built correctly). You can ease some of the manual labor if you are buying them in bulk and can have them delivered to your house.

Buying blocks will assure that you are getting clean, sharp looking blocks; but if you keep your eye out, you might be surprised where you can salvage them. Start with your local dump or transfer station!

While concrete absorbs water, if you place a capstone along the tops of the walls as I suggest below, you can tuck clear plastic vapor barrier under that capstone and have it line the inside of the bed, creating a water-proof barrier between the blocks and the soil. More on that in a bit.

Are Concrete Blocks Safe for Vegetable Gardens?

Concrete blocks are widely considered to be safe for use in vegetable gardens. Confusion sometimes comes up because of fly ash. Fly ash is a byproduct of burning coal and it’s used in the construction of some blocks. Theoretically, if your blocks contain fly ash, heavy metals could leak into the soil.

Most people consider it safe, however. The University of Maryland Extension suggests using them is a personal choice based on your comfort level. The thing is, even when fly ash is used, it is supposed to have a chemical reaction with the Portland cement, making it safer. One study even found no difference between blocks with fly ash and those without.

I’m fine with using them. If you have concerns, you can seal the blocks (more work) or line the inside of the bed with the vapor barrier I mentioned earlier. Newer concrete blocks are going to be safer to use than older “antique” blocks you might find in some places.

How to Build a Concrete Block Raised Bed Garden

I began construction of the first bed in the late summer so it’d be ready for spring planting. That also gave weeds a chance to grow, which I then killed, reducing the next year’s total weed count.

Step 1 – Prepare the Base

Some people suggest setting lines on the ground to mark where your blocks will go. I find this to be an unnecessary step. Just set them on the ground and dig next to the side of the row so you can prepare the base layer of material that the blocks will sit on.

If you experience frost, as I do, you can’t just set the blocks on the ground and expect them to look nice and level the next year. You don’t have to dig a 4′ trench to get below the frost line, but a little extra effort in the beginning will make a big difference down.

Place several inches of sand or gravel under the first row of blocks. This will help allow for natural ground movement without disturbing the alignment and level of the walls too much. If you’re doing this on a budget, you can look for free sand the same as you look for free blocks.

I also dug into the ground slightly so that the first course of blocks sat halfway into the ground for added stability with frost. This was likely overkill, but it provided a finished look, and the bed had nice, level lines winter after winter.

If I had skipped setting the first row halfway into the ground I probably could have saved me an entire row of blocks as it would have been high enough with two rows.

Step 2 – Set the Blocks

It’s important to point out that concrete blocks come in different styles. Some have flush ends that are ideal for visible corners. See how I used them on the corners:

Stagger the blocks like bricks for added stability. The bed is loose fit (no mortar). There is no sense spending time, money, and effort cementing these in place. If the bed is going to move, better to have it move with loose blocks that you can just adjust.

Capping the wall with flat blocks (top left corner of the bed in the pic) is an optional step, but I like the look. Thinner blocks can be purchased than what I used, saving you money. Some people opt to fill the holes with more soil and plant marigolds or other flowers in the holes.

Step 3 – Add Soil

In the background of the above picture you’ll see four piles of dirt. The one on the far left is aged horse manure. In the center at the far back is screened loam. The smaller, darker pile to the right of that is homemade compost. And the pile in at the right, with the shovel sticking out of it, is the clay-heavy soil I dug out of the ground.

I mixed the aged horse manure, screened loam, and compost together in the wheelbarrow (1/3 of each). There wasn’t much science to it. You might opt to be more precise with your ingredients depending on what you want to grow.

You can see I had to fence the raised bed because of the deer problem I had. This was just a quick fix to the problem with longer-term plans of moving the fence further out around a larger garden space.

3 Optional Construction Ideas

Concrete block beds are versatile! You can add features to them for your situation and strategy.

Line the Blocks with Vapor Barrier

I mentioned the vapor barrier before as a means of preventing water absorption by the blocks and to prevent the off chance any unnatural materials would leach into the soil.

Cut the plastic so that it fits under the optional capstones along the wall and let it drape down the inside of the wall. Obviously this should be done before you fill the bed with soil. Make sure to cut the plastic at the foot of the interior wall. You do not want to run plastic all the way under the bed as it will prevent drainage.

Drip Irrigation System

Drip irrigation is a great, water conserving way to keep your garden growing when the rain isn’t cooperating. You can see in the very first image in this article a drip irrigation system in use. It specifically designed for raised beds.

You can conserve water even further by setting the drip irrigation system in place and then covering it with straw or some other natural weed barrier. It not only helps prevent weeds but prevents evaporation of water.

Build a Plastic Tunnel Cold Frame

You can extend your growing season on the early and the late end by building a plastic tunnel over your bed to protect your plants from colder temps. While I didn’t do it over my concrete block raised bed, it is easily accomplished.

Instead of screwing the flexible Pex tubing into boards, you can insert that tubing directly into the holes of the cinder blocks (without the flat concrete capstones on top, of course).

If you want keep the capstones, another option is to place rigid PVC cut to height inside the wall before you add the soil. Place them according to where you want the Pex tubing to stretch across to shape your hoop. The PVC will be held in place by pressure from the soil. Stick the Pex tubing inside it when you want a hoop house, remove it when you don’t.

Run some rigid PVC tubing lengthwise across the flexible tubing just like in the picture above. This will form the “ridge beam” of your hoop house.

Cinch the PVC to the Pex tubing with zip ties. Get some big spring clamps like in the picture above if you want to avoid buying zip ties each time you make changes.

Run vinyl gauge fence over the frame (again, just like in the picture) and cinch it with the same zip ties. You can buy the garden fencing here or at your local garden store.

Then cover it all with plastic, something like DeWitt Supreme Plant Frost Protection Blanket.

When the cold period has passed, keep your frame in place and then use insect netting over the frame to protect the plants from destructive bugs.

Raised Beds are Ideal for Square Foot Gardening

Your raised bed garden is going to be rectangular in shape with a consistent width. This makes them great for use with square foot gardening methods.

If you’re new to gardening, it’s a great way to get a grip on planting and spacing. It’s a technique that’s similar to the long-used French intensive gardening approach.

French intensive gardening, also known as biodynamic, raised bed, wide bed, or French market gardening, is a method of gardening in which plants are grown within a smaller space and with higher yields than other traditional gardening methods. The main principles for success are often listed as soil improvement, raised beds, close spacing, companion planting, succession planting and crop rotation. Originating in France, the practice is popular among urban gardeners and small for profit farming operations.

https://en.wikipedia.org/wiki/French_intensive_gardening

This process worked really well for me. If you want to try it, buy a copy of Mel Bartholomew’s best-selling book All New Square Foot Gardening.

Pictured here is some eggplant, peppers, and broccoli. Already harvested from the raised bed was radishes, carrots, two types of lettuce, and spinach.

Inch by Inch, Row by Row

You’ll get your garden to grow with concrete block raised beds. Then, when you’ve inevitably produced way more vegetables than you can eat in a season (a common beginner’s mistake), you should buy a copy of the USDA’s Complete Guide to Home Canning.

It’s the go-to source on canning, and it’s inexpensive. You can get a pressure/canner cooker to accompany canning supplies and Mason jars that run from the high-end All American 930 to the more reasonably-priced Presto 8-Quart cooker.

What’s your experience with raised beds and square foot gardening? Help others learn by adding to the comments below. Garden on!

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