Heat Units An objective measure of the time required for the development of the fruit to maturity after flowering can be made by measuring the degree days or heat units in a particular environment. It has been found that a characteristics number of heat unit or degree-days is required to mature a crop under usually warm conditions, maturity will be advanced and under cooler conditions, maturity is delayed. The number of degree days to maturity is determined over a period of several years by obtaining the algebraic sum from the differences, plus or minus, between the daily mean temperatures and a fixed base temperature (commonly the minimum temperature at which growth occurs). The average or characteristic number of degree-days is then used to forecast the probable date of maturity for the current year and as maturity approaches, it can be checked by other means.