<P> The Three Sisters are the three main agricultural crops of various Native American groups in North America: winter squash, maize (corn), and climbing beans (typically tepary beans or common beans). The Iroquois, among others, used these "Three Sisters" as trade goods . </P> <P> In a technique known as companion planting the three crops are planted close together . Flat - topped mounds of soil are built for each cluster of crops . Each mound is about 30 cm (12 in) high and 50 cm (20 in) wide, and several maize seeds are planted close together in the center of each mound . In parts of the Atlantic Northeast, rotten fish or eels are buried in the mound with the maize seeds, to act as additional fertilizer where the soil is poor . When the maize is 15 cm (6 inches) tall, beans and squash are planted around the maize, alternating between the two kinds of seeds . The process to develop this agricultural knowledge took place over 5,000--6,500 years . Squash was domesticated first, with maize second and then beans being domesticated . Squash was first domesticated 8,000--10,000 years ago . </P> <P> The three crops benefit from each other . The maize provides a structure for the beans to climb, eliminating the need for poles . The beans provide the nitrogen to the soil that the other plants use, and the squash spreads along the ground, blocking the sunlight, helping prevent the establishment of weeds . The squash leaves also act as a "living mulch", creating a microclimate to retain moisture in the soil, and the prickly hairs of the vine deter pests . Corn, beans, and squash contain complex carbohydrates, essential fatty acids and all eight essential amino acids, allowing most Native American tribes to thrive on a plant - based diet . </P> <P> Native Americans throughout North America are known for growing variations of Three Sisters gardens . The milpas of Mesoamerica are farms or gardens that employ companion planting on a larger scale . The Ancestral Puebloans are known for adopting this garden design in a drier environment . The Tewa and other peoples of the Southwestern United States often included a "fourth Sister", Rocky Mountain bee plant (Cleome serrulata), which attracts bees to help pollinate the beans and squash . </P>

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