Why do leaves on trees come out at different times?

A good question that I receive often, especially in times when the weather is off a bit, at least, compared to weather that we might be used to. Although we have a few tropical species, most of the trees in our landscapes are native to the temperate climatic zone, which means that their growth cycle is tied to our spring, summer, autumn, winter cycles.

In the temperate climatic zone, typically, our summers are very hot, compared to our winters. Deciduous trees lose their leaves in the fall, in response to the arrival of cooler temps and shorter days.

In the spring, when temps are warming up and days are getting longer, that’s a signal to deciduous plants that it’s to wake up from dormancy.  Their tender leaves, if they had them year-round, would be susceptible to frost damage if kept all winter, so the plants “go to sleep” and wait for better times.

Well, if spring weather arrives earlier than usual, trees and plants will be tricked and may think that it’s time to wake up. The warm days that we tend to have during winter now often throw a kink into our temperate zone plants natural cycles, causing them to wake up earlier than usual.

Also, many plants require a certain number of hours of cumulative cold during the winter before they will wake up in the spring. Our stone fruits, peaches and plums, as well as trees that form spring flowers before they form spring leaves, very typically have this growth habit, leading to different times of “waking up,” depending on both their internal time-keeping mechanism, and the weather.