If you grow peaches for a living, you know all about chill units. They measure how much cold a plant experiences during a winter. And chill units are essential to a good crop of peaches. When plants go dormant at the end of summer, many of them have to experience a certain amount of time in cold weather before they can grow on schedule in the spring. If a peach tree doesn’t get enough chill units, it can’t respond promptly to the warmth of spring. It is still in a zombie-like dormancy, and it can only rouse itself after the spring gets even warmer.

From a Wired article on greenups , or when plants begin growing in Spring.