How Do Buds Break Winter Dormancy?

By May 15th, many of our broad-leafed trees will have broken their winter dormancy and begin showing their fresh spring green leaves. Although you may not notice it, pines and other evergreens will add new shoots, cones and needles this year.

How do tree buds key in to the subtle environmental cues ensuring that their leaves won't get frostbite? Exposure to winter chilling or freezing temperatures, increasing daylight, warming temperatures, inactivation of growth inhibitor hormones, and increase in growth promoter hormones all play a part in the swelling and growth of dormant buds. The interplay of all these external and internal factors varies with each type of tree. Even more remarkably, the timing of breaking of bud dormancy can vary among the buds on the same tree. Phytochromes intercept photoperiod cues and hormones chemically transmit the message that spring has arrived. Among the growth promoter hormones are auxins, gibberellins, and cytokinins, while abscisic acid is a main bud growth inhibitor.