Are cinder blocks OK for vegetable gardens? Answers to that & other soil safety questions

raised bed made with cinder blocks
When growing food, it’s important to protect your soil. Photo by Connie Oswald Stofko

by Connie Oswald Stofko

Is it safe to use cinder blocks in a raised bed, or might chemicals from the concrete blocks leach out of the blocks to contaminate your soil and food plants you grow there?

Can you use pressure treated lumber?

Can you grow food plants in the hellstrip, the area between the street and sidewalk?

John Farfaglia, extension educator with Cornell Cooperative Extension in Niagara County, responded to my questions on soil safety.

Cinder blocks in raised vegetable bed

I have seen warnings on the Internet that cinder blocks or concrete blocks used in a garden might leach harmful chemicals into the soil and ultimately into your food plants.

After doing some research, Farfaglia’s short answer was: “I doubt whether there is any issue to worry about.”

He found universities recommending the use of concrete blocks in container gardens or raised beds.

The concern cited in many warnings is specifically fly ash, the residue you get from burning coal, that might have been used in older cinder blocks, but he doubts whether that is commonly used anymore.

However, one thing you should take into consideration when using old bricks, old concrete blocks or other recycled material is where those materials came from, he said. The materials may have been resting in soil that was contaminated with chemicals. If you’re going to use bricks for pathways, pressure washing the materials should be sufficient. If you’re using materials for a vegetable garden, you would want to be more cautious.

If you’re unsure of the source of used materials, he recommends using new material.

UPDATE (8/13/2018): A reader left a comment, saying:

“Connie, you and Farfaglia are wrong – fly ash is still used in manufacturing ‘cinder’ blocks, sometimes. If you go to this product page at home depot for a 16 in. x 8 in. x 6 in. Concrete Block https://www.homedepot.com/p/16-in-x-8-in-x-6-in-Concrete-Block-30163601/100350201 and look at the question answer section, you will see that a customer asked: ‘Is there any fly ash in these concrete blocks? If so, how much?’

“The manufacturer responded with: ‘It may sometimes be included in the mixture. Fly ash is a recycled green product and is requested from some builders because they have green criteria they would like to meet – LEED certification, reduce CO2 emissions, etc.’

“I take that to mean that sometimes they have to put fly ash in a batch because it is requested by a builder, and to keep costs down, assuming the builder does not want to buy the whole batch, they have to sell the remainder to home depot and other retailers so the block you buy at a retailer like home depot might have fly ash in it.

“So yes, modern day ‘cinder’ block might have fly ash in it.”

John Farfaglia looked into this more and said that there probably needs to be more research on this. There aren’t any studies to show whether the heavy metals that may be contained in cinder blocks or concrete blocks can leach into the soil. If you are concerned, you could get your soil tested for heavy metals.

Naturally rot-resistant wood like cedar or redwood is the best choice for raised bed construction for gardeners that have concerns regarding any possibility of exposure of chemicals in the building materials.

Farfaglia also sent along information from the University of Maryland Extension: Cement block, cinder block and concrete block all are made with cement and fine aggregates such as sand or small stones. Fly ash is also often included. Fly ash is a byproduct of burning coal and so contains heavy metals and other hazardous waste. Labels do not give specific information on exactly what aggregate is used in the manufacture of the block. There is also little research data on this topic. Ultimately, this becomes a personal choice based on your comfort level. If you plan to use block as a raised bed material — and many people do – and you are concerned about potential risks, you could seal the blocks with polymer paint. Or you can choose to use another material.

(Polymer paint is latex paint or acrylic paint.)

Pressure treated lumber in raised vegetable bed

Farfaglia said people often ask about using treated lumber for raised beds. At one time, arsenic was used in treated lumber, but isn’t any longer, he said. The risk of using new treated lumber is low, but he still recommends using natural wood such as cedar to be safe.

Line a raised bed to protect against potential leaching

As added protection, when growing food in a raised bed, you can line the bed with plastic to act as a barrier from any chemicals that might leach into the soil from the building materials. Use a thick gauge plastic, like 6 mil, Farfaglia said.

Hellstrip not best option for food plants

Awhile back, we told you about one local gardener who plants herbs in her hellstrip, the area between the road and sidewalk, but a reader commented that he would be wary of eating food planted there.

I asked Farfaglia about it, and he cautions against it.

“In a lot of cases the risk is not high, but as a general practice, save that strip for ornamental plants,” he said.

That area can contain residues from salt and other chemicals used on the road, and there may be a higher concentration of lead still there from auto exhaust.

You should also be wary of beds near the foundation of an older home that may be contaminated with lead from paint that flaked off and accumulated in the soil, he noted.

If your soil is contaminated, rinsing your herbs or vegetables might not be enough to get rid of the contamination. How big the risk is depends on many factors, including how high the concentration of the contaminant is, how often you eat the food and how you cook it.

You can get soil tested

If you’re concerned about your soil being contaminated, you can get your soil tested at the Cornell Nutrient Analysis Laboratory. It costs between $50 and $150, with some tests cost extra.

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18 Comments on “Are cinder blocks OK for vegetable gardens? Answers to that & other soil safety questions

  1. I wanted to use cedar for my raised beds – but when I called the lumber yard for prices, the man said to me “Even if we had what you want in cedar, you couldn’t afford it!” I used treated lumber and 15 years everything is great.
    If you line with plastic they will dry out very fast – I did not line mine and they draw water from underneath themselves. That is the reason I put in beds in the first place, as my garden was always very wet and hard to get plowed in the spring. It made my growing season very short, now the ground is easy to work early.

  2. Paula, that’s a good point about drainage. If you push the plastic up against the sides but leave the bottom open, you could create a barrier against any leaching and still have good drainage.

  3. What about tar? We always lined our planter boxes with tarpaper, but now my daughter is questioning whether tar or tarpaper can contaminate food plants.

  4. Ruth – I would hold off on using tar paper when growing food plants. It contains asphalt, which is a by-product of petroleum refining and, therefore, potentially toxic. It also contains fiberglass. If you are attempting to waterproof your boxes, try painting the interiors with melted beeswax. Once dried, it will last quite a long while.

  5. What about the BPA etc. leaching out of the plastic liner? I would use just plain wood, then you wouldn’t have to worry about chemicals.

  6. I did a cinder block garden-2 blocks high fairly large garden. Third year in my garden vegetables didn’t grow and didn’t produce. They were beautifully green but that’s it. Had soil tested and PH o
    Is too high. Suggested tear down garden and rebuild with wood because the blocks put too much lime in soil. I’m at a loss as to what I really need to do. Any suggestions? Thanks

  7. Concrete is alkaline, which has a high pH. If the pH in soil is too high, lime could be added to the soil to bring down the pH. (Concrete doesn’t leech lime.) I haven’t heard of problems with cinder blocks raising the pH of the soil to a level that it harms plants, but I’m not an expert. If you had a problem with your plants, it could be you fertilized too much or didn’t have enough sun or had any of a number of other issues. I would go back to the Master Gardeners in your area and see if they can help you puzzle through this problem. I hope that helps.

  8. Just because there’s no research to determine if cinderblocks allow harmful chemicals to leach into the soil, doesn’t mean it won’t happen. Lining your blocks or boards with plastic will simply change the potential leachate. Most plastic has BPA, and we don’t really know what other plastics will do over the long term. It makes sense to use untreated, natural wood, or try organically grown straw bales! You may have to replace the wood in 5- 10 years, but at least it will be safe for you and your family. When the straw rots, it adds nutrients to the soil. Incorporate it and get some more! You can also plant in the bales if you like!
    My 2¢.

  9. Where can I purchase some Bee’s wax to line my Cinder Block Garden with?? And Is there a specific type of Bee’s wax that is used for gardens?

  10. My raised beds made from wood rot too fast so I had some men come and make concrete raised beds. Now I’m wondering if that’s even safe to grow food in as one man said at the garden shop that it would be okay and it takes a long time to leech anything bad. So what do you know about this? I use all organic materials otherwise

  11. Kat, since there isn’t a problem using new cinder blocks made of concrete, there shouldn’t be a problem using concrete for raised beds. The concrete shouldn’t have anything bad in it. The concern with cinder blocks was that they used to contain fly ash, but they don’t use that anymore. Even the pH of the concrete shouldn’t be a problem. Concrete, when it’s new, is alkaline. Its pH can be around 13. However, as it ages, its pH can go down to 8, which is almost neutral. If you’re concerned, have the pH of your soil tested. It’s a quick and simple test. If the pH is off, you can take steps to get your soil where you want it to be.

  12. Connie, you and Farfaglia are wrong – fly ash is still used in manufacturing “cinder” blocks, sometimes. If you go to this product page at home depot for a 16 in. x 8 in. x 6 in. Concrete Block https://www.homedepot.com/p/16-in-x-8-in-x-6-in-Concrete-Block-30163601/100350201 and look at the question answer section, you will see that a customer asked: “Is there any fly ash in these concrete blocks? If so, how much?”
    The manufacturer responded with: “It may sometimes be included in the mixture. Fly ash is a recycled green product and is requested from some builders because they have green criteria they would like to meet – LEED certification, reduce CO2 emissions, etc.”
    I take that to mean that sometimes they have to put fly ash in a batch because it is requested by a builder, and to keep costs down, assuming the builder does not want to buy the whole batch, they have to sell the remainder to home depot and other retailers so the block you buy at a retailer like home depot might have fly ash in it.

    So yes, modern day “cinder” block might have fly ash in it.

  13. Chuckers, thanks for your information. John Farfaglia looked into this more and said, “There probably needs to be more research on this topic. Naturally rot-resistant wood like cedar or redwood is the best choice for raised bed construction for gardeners that have concerns regarding any possibility of exposure of chemicals in the building materials.”

    Farfaglia also sent along this information from the University of Maryland Extension: Cement block, cinder block and concrete block, all are made with cement and fine aggregates such as sand or small stones. Fly ash is also often included. Fly ash is a byproduct of burning coal and so contains heavy metals and other hazardous waste. Labels do not give specific information on exactly what aggregate is used in the manufacture of the block. There is also little research data on this topic. Ultimately, this becomes a personal choice based on your comfort level. If you plan to use block as a raised bed material — and many people do – and you are concerned about potential risks, you could seal the blocks with polymer paint. Or you can choose to use another material.

    (Polymer paint is acrylic paint or latex paint.)

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