The quantity of the essential oil accumulated in the aerial parts of peppermint attains a maximum level in the growing season and then diminishes rapidly. In order to identify the best possible harvest date, the optimum time for the greatest fresh weight yield, maximum oil yield and desired chemical composition must be identified. It is generally recognized that the period just preceding flowering is the preferred harvest time. In order to more precisely identify this time in the peppermint plant life cycle, the floral development and flowering process have been studied. During the period just before normal harvest time takes place, plants were examined for average height, number of leaf pairs, fresh herb weight and oil yield. Also shoot apices of the terminal buds were examined by electron microscropy to determine the first signs of flowering. From these observations, it was found that the optimal harvest time was when the calyces were only barely visible formations at the stem extremity.