Corn, squash and beans have been traditionally planted in concentric formations in a hill: Corn grew in the center of the hill, beans around the corn, and squash around the beans. The tall corn is strong and provides stalks for the beans to cling to. The leafy squash provides shade for the beans and surrounding ground, which holds moisture and forms a living mulch that reduces the amount of weeds that can grow. Beans also have nitrogen-fixing bacteria on their roots that supply nitrogen-hungry corn with fertilizer.