As spring approaches, trees feel the temperature difference just like we do. But unlike us, they aren't ready to start enjoying the warmth right away. They need a certain amount of cold weather before they can spread their leaves.
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.
So what happens when a tree that requires less chilling is placed in a northern environment? It will receive all the chilling hours it needs early in the winter. If we have a just a few warm days in the late winter, the tree will break bud and leaves and flowers will begin to emerge. If the temperature drops again, all the young growth on the tree could be killed. If the tree is strong, it may be able to produce new buds and continue for another season, but if the tree is young or weak, it's probably done for.
Trees that require lots of chilling hours but are planted in the South don't fare much better. If a tree doesn't receive the chilling hours it needs, it won't be ready to respond when spring comes. It will take much longer than it should for buds to open. And when they finally do open, they'll do so sporadically across the tree.
Here in Minnesota, this was a cold winter. Most of the trees in your yard -- and the forest -- have probably been able to meet their chilling-hour requirements. So there shouldn't be any delay in trees flowering and leafing out from a lack of chilling time. But trees use more than just chilling time to determine when their buds break.
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.
Forsythia, that early-blooming beauty, breaks bud even earlier than maples. But all it risks by doing so is its lovely golden flowers. And the leaves, which produce the sugars that the tree needs to grow, are more important, though less looky, than the flowers.
Jeff Gillman is an associate professor of horticulture at the University of Minnesota. He's also the author of "The Truth About Garden Remedies" and "The Truth About Organic Gardening" (Timber Press, $12.95).