Are sycamore trees poisonous

'''Yes''', the American sycamore tree (''Platanus occidentalis'') may be described as poisonous ... to certain plants. Specifically, the tree exerts an allelopathic effect on surrounding vegetation. That effect actually is a germination inhibitor, which keeps plants from maturing into attractive, healthy plants. It's the same effect that black walnut trees (''Juglans nigra'') exert on surrounding vegetation, to control competition.