The effects of genotype and environment on canopy development of sunflower (Helianthus annuus L.) have been the subject of much research. A framework was developed to analyse data from this research and predict leaf area of irrigated sunflower as affected by daily temperature, and the phenology of different cultivars and hybrids. Between emergence and anthesis, total leaf area per plant (TPLA) was determined as a logistic function of TPLA at anthesis and thermal time from emergence (TT). Total leaf area per plant at anthesis was linearly related to total leaf number for a range of cultivars and hybrids. Total leaf number was estimated as the product of the duration of the period emergence to head visible (in days) and the average leaf initiation rate in that period. Leaf initiation rate was related to the average temperature during this period. Senescent leaf area per plant (SPLA) was calculated as an exponential function of TT from head visible. Leaf area index (LAI) was determined as the difference at any time between TPLA and SPLA, multiplied by plant density and a density modifier, which allowed for the effect of density on leaf area per plant. Almost all variation in the TPLA and SPLA data was accounted for using inputs of TT and dates of phenological stages. When the model was tested on independent data it accounted for 93% of the observed variation in LAI.