Around A.D. 1150, this mesa top supported fields of corn, beans, and squash, supplemented by wild plants such as amaranth, tubers, and sunflowers. Children watered corn with clay water jars, or ollas, and young men cut piñon and juniper trees to expand gardens for future years. The Ancestral Puebloans were drawn to the mesa top by fertile soil, a relatively long growing season, and enough precipitation for dryland farming. Mesa Verde averages about 17 inches of moisture each year in winter snow and summer rainstorms. Although there is no evidence that agricultural fields within Mesa Verde were ever irrigated, the Ancestral Puebloans built an extensive network of ditches, canals, and reservoirs to collect and store rainwater for domestic use. The introduction of smaller, more drought-resistant corn helped change the Ancestral Puebloans from nomadic hunter-gatherers to settled farmers. Corn required 70 frost-free days to mature, and the mesa top averaged 85 frost-free days. A probable system of collector ditches and a canal ditch carried water from the fields to Far View Reservoir.