What are chill hours?
In order to bloom in spring and then produce fruit, deciduous fruit trees such as peaches, plums, and nectarines, and some varieties of berry bushes, such as blueberries, require a dormancy period during winter with a certain number of chilling hours; when the temperature drops below 45 degrees Fahrenheit. Winter hours a region experiences above 60 degrees are subtracted from the total hours that drop below 45 degrees.

The number of chilling hours a specific plant will require will depend on the variety of plant. For example, not all blueberry plants require the same number of chilling hours. There are hundreds of varieties of blueberry plants and some will require more or less chilling hours than others. Some fruit bearing plants will require only a hundred chilling hours while others will require thousands.

The chill hour map seen below is a result of research done at the University of Maryland. The map is not exact, and Mother Nature can always change her mind from one year to the next, but it can be used to estimate the chill hours for your area. The chill hours for a region are below and between the line on the map and the next line down.

Chill Hours Map