Here we determined the critical temperatures (Tb, To, and Tc)for calendula seed germination by examining germination ratesover 9 d across temperatures ranging from 1 to 40◦C. Base temperature (Tb) is the temperature below which germination is 0%,optimum temperature (To) is the temperature where germination is maximal, and the ceiling temperature (Tc) is the temperature above which germination is 0%. Joly et al. (2013) reported the Tb for calendula to be 5.5◦C based on soil hydrothermal time to 50%field emergence. Hydrothermal time is a well documented measure where soil temperature and water potential are used to predict germination rates for different species. Using controlled temperature studies we sought to confirm the Tb for calendula and to deter-mine the To and Tc, which can be used to facilitate seed sowing and agronomic production of calendula. Additionally, we determined the time period over which Tc is exerted by a series of heat-shock experiments.