I checked this out when my dad told me to be sure that I did not use copper nails when I hung things on trees. I am a scientist, so I kind of trust experiments. I know that some folks feel differently, but his is my take on it. If you look around on Google Scholar there are several papers that explain why the toxicity of copper is too weak to kill plants by using metal objects. There are also experiments where researchers put copper nails around trees every 1.2" with no effect.
