The Use of Leaves from Young Trees of Quillaja saponaria (Molina) Plantations as a New Source of Saponins. Quillaja saponaria is an endemic tree in Chile. It has been extensively exploited for the extraction of saponins, which are used in mining, agriculture, and in the pharmaceutical and food industries. The overexploitation of native adult Quillaja forests has necessitated the establishment of plantations. Until now, biomass had been obtained from wood and bark of native adult trees, excluding the use of leaves due to their low biomass contribution to the whole tree, and also because industrial production lines have been designed for wood and bark and not for leaves that contain different compounds (e.g., chlorophyll). In this study, we propose using complete trees from young plantations, including the leaves. Quantification of saponins in the leaves of 24 three-year-old Quillaja trees from a plantation was carried out during the four seasons of one year. Our results showed that leaves represent 27.8% of the weight of the whole young trees and that leaves contain on average 2.58% of saponins. Considering that this amount is similar to that found in wood, we argue that leaves could contribute up to one-third of the saponins of young trees, making its processing key to attain good yields. Significant differences were observed in saponin contents during the four seasons. On average, the lowest concentration was found during winter (July = 1.39% dry weight) and the highest in autumn (April = 4.49%), suggesting that abiotic factors may have an affect on saponin production. A comparison among the 24 trees showed significant differences in saponin concentration (0.90% to 4.71%), supporting the need to select consistently superior saponin-producing trees. These results contribute to the understanding of a more sustainable production of Quillaja extracts, showing a potential to use whole young trees from plantations rather than only the wood and bark from native Quillaja forests.