In one 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.
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 utilize, and the squash spreads along the ground, blocking the sunlight, helping prevent 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. Maize lacks the amino acidslysine and tryptophan, which the human body needs to make proteins and niacin, but beans contain both and therefore maize and beans together provide a balanced diet.
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 Anasazi are known for adopting this garden design in a drier environment. The Tewa and other Southwestern United States tribes often included a "fourth sister" known as "Rocky Mountain bee plant" (Cleome serrulata), which attracts bees to help pollinate the beans and squash.
^Mt. Pleasant, Jane (2006). 38. In John E. Staller, Robert H. Tykot, and Bruce F. Benz. "The science behind the Three Sisters mound system: An agronomic assessment of an indigenous agricultural system in the northeast". Histories of Maize: Multidisciplinary approaches to the prehistory, linguistics, biogeography, domestication, and evolution of maize. Amsterdam: Academic Press. pp. 529–537. ISBN978-1-5987-4496-5.
^ abBushnell, G. H. S. (1976). "The Beginning and Growth of Agriculture in Mexico". Philosophical Transactions of the Royal Society of London (London: Royal Society of London) 275 (936): 117–120. doi:10.1098/rstb.1976.0074.