Hydrothermal threshold models can describe the germination response of carrot (Daucus carota) and onion (Allium cepa) seed populations across both sub‐ and supra‐optimal temperatures

• The effect of temperature on the minimum (base) water potential for seed germination (Ψb) was investigated in Daucus carota and Allium cepa and then described in two hydrothermal threshold models. • Germination was recorded over a wide range of temperatures and water potentials. • At temperatures of 15°C and below the base water potential for germination of the 50th percentile (Ψb(50)) was constant, but in both species, above a temperature (Td) around 16–19 °C, Ψb(50) increased linearly with temperature. Hydrothermal time (HTT) and virtual osmotic potential (VOP) models were altered so that the effective base water potential (Ψb(G,T)) for any percentile of the seed population (G), above Td, was given by Ψb(G)d + m(T – Td), where Ψb(G)d is the uncorrected base water potential for that percentile. The coefficient m is the slope of the linear relationship between Ψb(50) and temperature above Td. • Germination response to all temperatures and water potentials can be adequately described in both the HTT and VOP models by incorporating changes in Ψb(G,T) with temperature.