A term often associated with fruit trees is "chilling hour". When some fruit trees (apples, peaches, plums, pears to name a few) are dormant, a certain amount of hours below 45 degrees F are required to trigger the development of leaf and flower buds. This is referred to as chilling hour requirement. Each variety has its own specific requirement that has been quantified by researchers. Once the chilling hour requirement has been met, the plant will bloom and leaf whenever warm weather occurs. If the chilling hour requirements are not met, the plant will produce leaves sporadically over the tree with little or no fruit. For this reason, selecting varieties of fruit trees that match the chilling hours for the area is essential for successful fruit production.
Go here and look at numbers 47 + 48 for details from Texas A+M.
http://generalhorticulture.tamu.edu/lec ... tents.html