The Three Sisters is a name given to the three basic crops by Native Americans: beans, corn, and squash.
They would plant them together in mounds and each plant would add a positive element to the group. The corn provided a structure for the beans to climb. The beans provide nitrogen to improve the soil. The squash would spread along the ground and prevent weeds, cool the soil, and reduce the need for water. The native Americans of the Northeast would also bury fish or eels in the mounds as fertilizer.
Nutritionally, they complement each other, too. Corn lacks the amino acids lysine and tryptophan which are needed by the human body to make complete proteins and niacin but beans contain both so the beans and corn together make a balanced diet.
Native Americans of the Southwest are known to add a forth sister: Cleome. Cleome or Rocky Mountain Bee Plant, attracts bees to help pollinate the beans and squash (corn is wind pollinated).
The Three Sisters planting method is featured on the reverse of the 2009 US Sacagawea Native American dollar coin.
Pumpkin: a winter squash.