Understanding Blueberry Rest Periods

In most of the state, blueberry bushes take the winter off. Whether they are managed as deciduous or evergreen plants, they do not grow actively during this period. This temporary stop in visible growth is called dormancy. Dormancy is a natural response to internal and external stimuli. Plants have two kinds of dormancies: endodormancy and ecodormancy. Understanding how each one works is important, because together they play a major role in determining when plants resume growth—and ultimately, when we harvest fruit.

Endodormancy is a localized dormancy that affects buds. Each bud enters and exits dormancy at a different rate. Shortening days and cooling temperatures queue buds to enter endodormancy. Notably, these same stimuli lead the plant to transition vegetative to floral buds. So, flower bud initiation and endodormancy onset occur simultaneously.

Endodormancy protects the delicate tissues that will become leaves and flowers from freezing temperatures. During this phase, plants are essentially waiting out the winter. Plants track how much winter has passed by the accumulation of chill hours (hours between 32°F and 45°F). When a critical number of chill hours has been accumulated, the plant is ready to resume growth. 

Blueberry cultivars can have very different chill hour requirements. Vegetative buds have higher chill hour requirements than flower buds, which is why leafing out is often delayed compared with bloom. Some varieties in the north require up to 1,000 chill hours to come out of dormancy. These varieties would never have adequate chill accumulation in our state. 

Florida blueberry production is possible thanks to the pioneering breeding work of Paul Lyrene, Wayne Sherman, and Ralph Sharp, who developed cultivars with much lower chilling requirements. Today, most blueberry cultivars grown in Florida require between 0 and 400 chill hours, making them better adapted to our mild winters. However, weather records from Florida Automated Weather Network (FAWN) and National Oceanic and Atmospheric Administration (NOAA) show a clear trend toward fewer chill hours being accumulated each winter, which helps explain why spring growth has become more variable in recent years.

Most blueberry varieties meet their chill hour requirement in the middle of winter. So, what is keeping them dormant until the early spring? The answer is ecodormancy. Ecodormancy is a pause in growth caused by the environment rather than by the plant itself. In winter and early spring, that limiting factor is cold temperatures. Even though the plant is ready to grow, low temperatures keep buds from developing. As winter fades and warmer spring days become more consistent, blueberry plants gradually come out of ecodormancy and resume normal growth. 

There is no such thing as a perfect winter, but it is useful to imagine what an ideal one might look like for blueberries. In that scenario, winter temperatures would stay consistently cool—but mostly above 32 °F—allowing plants to meet their chilling requirement early come out of endodormancy. After that, a steady warm-up would move plants out of ecodormancy. Flower buds, and later leaves, would develop quickly and uniformly, setting the stage for a strong, concentrated harvest. Of course, that is not the kind of winter we usually get. Still, understanding the difference between the two types of dormancy in blueberry plants can help explain why bushes respond the way they do to today’s unpredictable winters.

CREDIT
GERARDO H. NUNEZ