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.

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.

11 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?

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