Q: I have read that an inexpensive method of building raised beds is by using cinder blocks. However, when I researched the contents of cinder blocks I discovered that they contain by-products of coal combustion that are added to the mixture to increase its volume and thus cut production costs by using what would otherwise be considered “waste material” as an ingredient. Are such cinder blocks safe for vegetable gardening? Will these chemicals begin to leach from the blocks and find their way onto edible leaf and crop surfaces or find their way into lakes/rivers/streams and groundwater? Also, I have read that a distinction exists between “cinder” blocks and “concrete” blocks in that the “pure” concrete blocks do not contain any of these waste by-products. Any information or advice that you can offer on this will be greatly appreciated. — Justin in New Jersey
First of all, there are concerns when bringing anything into our gardens. Even organic compost could harbor some noxious weeds or seeds of invasives. But before we start talking about cinder blocks specifically, let’s talk about ways to create raised beds. They are a great way to grow, the beds warm up early, but on the down side, they dry out more quickly.
The safest way to build one is by just piling up compost and leveling off the top. In my garden each bed is the result of years of annual additions of compost. Now the beds are a couple feet above the original soil level.
Of course there are other situations that might call for the soil to be added into an enclosed frame. My first choice would be rot-resistant wood like locust, cedar, or redwood but never pressure-treated woods because they could leach chemicals into the soil.
Stone or bales of straw or hay would also be an option. Even untreated pine will last for years.
As far as cinder blocks go, it’s hard to know what they’re made of. Through the years, there have been different recipes and each factory can use a slightly different formulation. It often depends on what raw materials are available. I do know that some low-density cinder blocks contain fly ash, an industrial waste from coal-fired plants. It’s something I would never want in my garden. The porous nature of cinder block assures something will be leached into the garden. But we don’t know what; it might be fly ash or just some lime.
High-density concrete block would certainly be safer; they are often made of a combination of Portland cement and aggregate. But there’s lots of concrete made with fly ash too.
Just as we want to know where our food comes from, in today’s world we need to find out where just about any product originates, how it’s made and with what. Ask questions, find out the ingredients. Sometimes it comes down to what you feel in your gut. Would you be better off with some rough cut lumber from nature or something coming out of a factory?