Deciduous fruit trees, bulbs and other plants that go dormant during the Winter need a minimum number of chilling hours (hours where the temperature is between 32 and 45 degrees Fahrenheit) in order to break out of dormancy and properly set their leaf buds and flowers for the Spring. For example, most apple and pear varieties need between 750 to 1,500 chilling hours during the Winter.
Central Florida receives between 100 and 300 chilling hours in a season, significantly reducing the number of plants that are adaptable to the region.
|Source: Sustainability Assessment of Fruit Crops for North and North Central Florida|
Some other interesting sites:
"Just Fruits and Exotics" discussion on Chilling Hours
Challenges of Gardening in Zone 9