Heat plays a role in determining the time of fruit maturity and the level of fruit quality. The heat requirement is determined by the number of hours that the temperature is above 55-F from the time of bloom to maturity. In general, grapefruit have the highest heat requirement, about 11,000 heat unites. In contrast Valencia oranges about 10,000 heat unites, and Washington navel about 8,000 heat unites. Citrus varieties that have lower heat requirements mature earlier than do those with high heat requirements. Also, in general, varieties that mature early in the season require less time between bloom and fruit maturity than do varieties that mature later in the winter or spring. - Millet