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.