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@ Beware: Cinder Blocks contain Fly Ash – a toxic by product of coal burning power plants. The EPA is and has for the last year been doing a study to decide whether or not to label Fly Ash as a Hazardous Waste due to the high levels of mercury, arsenic, and lead; leaving some "Industry Folk" to refer to concrete as the "New Asbestos" or the "New Lead Paint". It may be unsafe to plant any edible plants in or near Cinder blocks.
We've used cinder blocks to build raised beds for years now. Have the best garden around with plenty enough to share. Hope I'm not poisoning anyone!
Are you sure they are cinder blocks? New blocks same shape are made of cement with no cinder used. I was told this in the garden shop at Lowes when I was asking for cinder blocks.
We made a retaining wall out of these last summer. I think next summer I will fill the holes with wave petunias that would look nicer than the weeds haha!
One caution here: I also use these as edgers in front of my tomato garden, planting them with marigolds. Since the hole is pretty small, the roots of anything planted in them get very hot. Those little marigolds like a lot more water than the tomatoes!