Learn something new every month. This from Jeff Gillman out of the University of Minnesota:
"Trees have developed a tool to tell them that winter has passed and it's safe for them to open their delicate buds. Here's how it works: Although trees may look dormant in winter, certain important chemical reactions are taking place inside them. When the temperature is between 45 and 33 degrees, certain chemicals are produced in most trees. The longer the tree spends between these temperatures, the more of these chemicals are produced. Only when the chemicals reach the right level is the tree ready to respond to the warm air rushing through its branches.
People who grow fruit refer to the time the tree spends in this narrow temperature range as "chilling hours," and they select trees to plant based on their requirement for chilling hours. For example, a tree grown in Minnesota might require 1,200 chilling hours, while a tree in Florida might require 150....
To avoid late frosts, some trees also use what's called warm days. Even after they've met their chilling requirements, trees such as oaks wait until we've had plenty of warm days before opening their buds. This ensures that oaks almost always avoid late frosts. That's a good thing because oaks are not good at producing new buds. Other trees, such as maples, break bud soon after their chilling requirements are satisfied. If maples lose leaves to a late frost, they can produce more."