In container nursery production, managing irrigation is critical for reducing agrichemical runoff. Boxwood (Buxus) and deutzia (Deutzia) plants grown in 1-gal and 3-gal containers respectively were conservatively irrigated by one of two substrate moisture sensor (SMS)-based regimes: 1) a daily water use (DWU) system that delivered the exact amount of water that had been lost in the previous 24 h and 2) an on-demand (OD) irrigation system based on a specific substrate moisture content derived from the relationship between substrate moisture and photosynthetic rate. Unfortunately, commercial nurseries have not readily adopted SMS-based irrigation systems partly due to expense and reliability of the sensors. The objective of the present study was to evaluate easily accessible weather variables [light integrals (LI) and vapor pressure deficit (VPD)] as a substitute for substrate moisture sensors to determine water volume applied for the DWU system and to predict timing of irrigation based on OD. Daily irrigation water volume was correlated with daily LI (correlation coefficient, R = 0.68 and 0.63 for boxwood and deutzia, respectively) and less so with accumulated vapor pressure deficit (R= 0.36 and 0.19 for boxwood and deutzia, respectively). The analysis suggested that irrigation should be scheduled following 6,227 W-m-2 for boxwood and 5,880 W.m-2 for deutzia plants.