For vegetative propagation of sweetpotato, single or multi-node leafy cuttings are used as propagules. A quantitative understanding of leaf development and the effects of environment conditions on leaf emergence and expansion rates is important for predicting the number of propagules produced after a given production period. Single-node cuttings each with a fully expanded leaf were grown under two levels of photosynthetic photon flux (PPF, 160 and 250 μmol·m–2·s–1) and photoperiod (10 and 16 h/day). The time courses of the number of leaves larger than the standard leaf area (As) were obtained by analyzing the time courses of leaf blade length recorded every day on each leaf. The number of leaves larger than a given As increased almost linearly after the first leaf reached to the As. PPF and photoperiod affected both the duration until the appearance of the first leaf with As and the leaf development rates (leaves per day). The effects of PPF were more pronounced than photoperiod for the development rate of the leaves regardless of As. Results obtained in these experiments were incorporated into our previously developed model, and the number of propagules produced under different environment conditions was predicted. Such techniques need to be used effectively for planning and environment control of vegetative propagation.