Why do so many Americans drive for miles each autumn to buy a vegetable that they are unlikely to eat? While most people around the world eat pumpkin throughout the year, North Americans reserve it for holiday pies and other desserts that celebrate the harvest season and the rural past. They decorate their houses with pumpkins every autumn and welcome Halloween trick-or-treaters with elaborately carved jack-o'-lanterns. Towns hold annual pumpkin festivals featuring giant pumpkins and carving contests, even though few have any historic ties to the crop.

In this fascinating cultural...