Frost seeding alfalfa (Medicago sativa L.) can be convenient and economical for establishing or renovating forage stands; however, premature seedling emergence triggered by unusually warm temperatures followed by fatally cold temperatures can lead to seedling mortality and stand failure. Delaying germination could improve establishment success in frost-seeded stands. Our objective was to measure the effect of temperature and water potential (Ψ) on germination across a range of alfalfa varieties. Germination rate (1 divided by days to 50% germination) was estimated for 11 varieties at nine constant temperatures (−1.1 to 10°C) and three Ψ (0, −0.2, and −0.6 MPa). Linear regression between temperature and germination rate was tested for all variety–Ψ combinations. Base temperature (Tb; minimum temperature for 50% germination) and thermal constant (DD; time to 50% germination in growing degree-days) were determined by calculating the intercept on the temperature axis and the inverse of the slope parameter from each regression, respectively. The Tb ranged from −0.55 to 0.49°C across varieties and was greater in low Ψ conditions. The DD was negatively correlated with Tb, which supports the hypothesis that seeds with lower Tb require more degree-days to germinate. Selecting alfalfa varieties with higher Tb could delay germination and reduce the risk of frost mortality; however, these varieties are also likely to have higher DD, which would expedite seedling emergence and potentially offset delays from high Tb. Based on small variations in germination parameters compared with field temperatures, no variety was identified as superior for frost-seeding success.