Temperature (T) and water potential (y) are two primary environmental regulators of seed germination. Seeds exhibit a base or minimum T for germination (Tb), an optimum T at which germination is most rapid (To), and a maximum or ceiling T at which germination is prevented (Tc). Germination at suboptimal T can be characterized on the basis of thermal time, or the T in excess of Tb multiplied by the time to a given germination percentage (tg). Similarly, germination at reduced y can be characterized on a hydrotime basis, or tg multiplied by the y in excess of a base or threshold y that just prevents germination (yb). Within a seed population, the variation in thermal times to germination among different seed fractions (g) is based on a normal distribution of yb values among seeds (yb(g)). Germination responses across a range of suboptimal T and y can be described by a general hydrothermal time model that combines the T and y components, but this model does not account for the decrease in germination rates and percentages when T exceeds To. We report here that supra-optimal temperatures shift the ψb(g) distribution of a potato (Solanum tuberosum L.) seed population to more positive values, explaining why both germination rates and percentages are reduced as T increases above To. A modified hydrothermal time model incorporating changes in ψb(g) at T > To describes germination timing and percentage across all T and ψ at which germination can occur and provides physiologically relevant indices of seed behaviour.