SummaryPredicting plant development is a long-desired goal of crop physiology. Plant growth divides into two main, interwoven components: continuous growth, and the succession of growth stages (phenology). Growth is commonly modeled on species-level asper seresponse to temperature that is scaled to cultivar-specific intrinsic growth rates. Differences in phenology—exhibited by cultivars in different environments—are interpreted as interactions with remaining environmental covariates. We question this fundamental assumption and suspect that cultivar-specific temperature responses largely influence phenology.To gather evidence, temporally resolved plant organ tracking and field phenotyping data were collected over multiple years in winter wheat and soybean. The response of traits to temperature was examined using models of increasing complexity (ranging from linear models to hierarchical splines and neural networks). Additionally, the trait level (leaf growth, canopy development, stem elongation), the covariate level (soil temperature, air temperature) and the covariate measurement level (below/inside canopies, at a reference weather station) were varied.The cultivar-specific non-linear models explained, in contrast to thermal time, large proportions of phenology-related cultivar-by-environment interactions.We conclude that such unbiased predictions are essential in breeding and for the cultivation of crops in view of increasing heat, drought, and other adverse climatic conditions.