This study aimed to estimate the accumulated temperature requirements for phenological changes in Lilium. Eight cultivars of three lily types were cultivated in open field conditions for phenological observations based on floral organ development. Growing degree days (GDD) requirements for phenological changes were calculated and verified using Lilium LA hybrid ‘Serrada’ under greenhouse conditions. Lilium Oriental hybrids exhibited higher GDD requirements compared to Lilium FA and LA hybrids for their phenological development. Estimations of phenological change dates in greenhouse cultivation were accurate within 1–3 days. These results provide a reliable description for predicting lily development stages across diverse cultivation environments by quantifying the accumulated temperature requirements for key phenological events.