Intensive rain-fed agricultural systems represented the foundation of the agricultural economies of the island of Hawai‘i and parts of Maui in the centuries before European contact. These systems largely were abandoned in the nineteenth century, and our understanding of how they functioned as productive systems is sparse. We established three experimental gardens within the ahupua‘a (traditional Hawaiian land division) of Puanui, in the Leeward Kohala Field System, where we have measured climate and soil properties and planted several Polynesian crops. We obtained relatively large yields of ‘uala (sweet potato, Ipomoea batatas) (from 1 to 4 kg of tubers per m²) from spring and summer plantings in two wetter, higher-elevation gardens; growth was slow there in the winter. In a drier, lower-elevation garden, only winter plantings provided reasonable yields (0.6 kg per m²). We suggest that Hawaiian farmers cultivated a winter crop of ‘uala in the lower, warmer, drier portion of the field system and grew spring-summer crops in the upper, wetter portion of the system. Ahupua‘a-level management in rain-fed agricultural systems could thus have functioned to integrate environmental variability and sustain yields through the year.