Q: I was told that sunflowers produce toxins in the soil they grow in. Will these gentle giants destroy my surrounding plants? Do the toxins build up over time?
A: When one plant reduces other plant’s growth, it’s called allelopathy. Many people believe black walnut trees inhibit the growth of some kinds of plants around them but research on if (or why) this happens is inconclusive. It may be just a garden myth. There are a couple of studies that concluded sunflowers inhibit broadleaf plants nearby. The conditions of the studies were not similar to garden situations, so I’m not sure the conclusions can be applied to your garden. In any case, the inhibition was mild. I’d say you have nothing to worry about unless you have lots and lots of sunflowers.