Base temperature (Tb) is the temperature below which germination is 0%,optimum temperature (To) is the temperature where germinationis maximal, and the ceiling temperature (Tc) is the temperature above which germination is 0%.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.