Indeed the substance produced by black walnut roots is allelopathic, or toxic, to many other plants…including tomatoes. A friend has the same problem at her house. So frustrating. I was interested to learn what other plants are affected negatively like the tomatoes, and it’s a list of various gardening favorites, including apple trees, potatoes, rhododendron, blueberries… Check out this Ohio State fact sheet on the topic for more details: http://ohioline.osu.edu/hyg-fact/1000/1148.html
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Welcome! I’m Margaret Roach, a leading garden writer for 25 years—at ‘Martha Stewart Living,’ ‘Newsday,’ and in three books. I host a public-radio podcast; I also lecture, plus hold tours at my 2.3-acre Hudson Valley (NY) Zone 5B garden, and always say no to chemicals and yes to great plants.