text: Daily crop heat units are calculated from these min and max temperatures. There are separate calculations for day and night. The daytime relationship uses 10 C (50 F) as the base temperature and 30 C (86 F) as the optimum because warm-season crops do not develop when daytime temperatures fall below 10 C and they develop fastest at about 30 C. The nighttime relationship uses 4.4 C (40 F) as the base temperature and does not specify an optimum temperature because nighttime minimum temperatures very seldom exceed 25 C in Ontario. The nighttime relationship is a straight line on the graph (Figure 1),while the daytime relationship is a curve that records greater crop heat units at 30 C than at higher or lower temperatures. Daily crop heat units are calculated by using the average of the two daily values from the equations given in Figure 1 or they can be read from the matrix in Appendix I.