Kennett Square ~ Mushroom Capital of the World: Although 'mushroom capital of the world' might be a bit of promotional hyperbole, it is true that half of the US mushroom harvest comes from the countryside around Kennett Square, Pennsylvania, right outside of Philadelphia. In fact, as the industry was getting started in the 1800s, the horse manure on which the mushrooms grew would come out from center city. (No cars until the 20th century, remember!) It was perfect for agaricus bisporus, which isn't a green plant and needs to get all of its nutrients from the medium in which is grows. So, where would you go to see how deeply mushrooms have penetrated local culture? St. Patrick's Cemetery.