Native Americans developed a number of domesticated crop plants; corn, squash, beans, amaranth, and sunflowers are probably the best known today.
In the Eastern Woodland areas, corn was a major crop. Trees were cleared either by controlled burning or by girdling, a process in which one removes a strip of bark all the way around the trunk, which kills the tree. There is also some evidence that river floodplains were used for agriculture in the northeast. Intercropping was commonly practiced in the southeast. The "three sisters" planting tradition of growing corn, beans, and squash together is an example of this method.
In the southwest the staple crops were similar, but farmers concentrated on developing fast-maturing varieties that needed less water. Extensive networks of irrigation ditches were developed to make the most of the available water.
In the Pacific northwest we have less evidence of native agriculture. It seems likely that people living there got most of their sustenance from the sea and the coastal marshes and estuaries. The incredible size and density of the forest trees would have made land clearing a difficult activity there.