green thumb
n. An extraordinary ability to make plants grow well.
Either its origin is linked to the reign of King Edward I of England, who enjoyed fresh green peas so much that he had half a dozen serfs working to keep him supplied, a prize going to the one with the greenest thumb, presumably from hours of shelling; or it comes from the fact that repeatedly handling clay pots encrusted with algae will stain a gardener's thumb (and probably fingers) green.
