Wikipedia is wrong to link Gringo to the popularity of Green Grow The Rushes-o. surely 'Green grows the laurel and soft falls the dew'?

Yes, it's wrong, but, in fairness, it does list this reference under "undocumented and folk etymologies" and not in a scholarly context (so far as anything in Wikipedia is scholarly), and notes that the use of the word predates the Mexican War. None of it's footnoted, though.