Many fruit and nut species require cold temperatures during the dormancy season to initiate flowering and bear fruit. Quantifying these chilling requirements is crucial for identifying appropriate cultivars for a given site, for timing applications of rest-breaking chemicals and for predicting consequences of climate change. We present a new method to test temperature models describing chilling and heat requirements of perennial plants, and use this method to compare the ability of four chilling models (Chilling Hours, Utah Model, Positive Utah Model and Dynamic Model) to explain walnut phenology in California.