According to James Underwood Crockett, it comes from the fact that algae growing on the outside of earthenware pots will stain a person’s thumb (and fingers) if he or she handles enough pots. Hence, a person who is always working with flower pots has a green thumb.

Another theory is that it originated during the reign of King Edward I of England. He was fond of green peas and kept half a dozen serfs shelling them during the season. The serf who had the “greenest thumb” won a prize.