From Sleeping Trees to Spring Leaves

I have always been captivated by how trees withstand the harshest of winter weather and know when to grow their new leaves in the spring. This isn’t magic. The process actually depends on a variety of factors, such as temperature, precipitation and species type, and the timing can even change from year to year! Bud burst, or in other words “leafing out,” in the spring, is a more remarkable process than many of us know!


In cold climates, such as ours, most physiological processes of trees occur when environmental factors, such as temperature and precipitation, are favourable for growth and reproduction. However, in the winter, when temperatures are freezing, trees go into a state of dormancy to survive these harsh conditions. During dormancy, everything within the tree slows down, including metabolism, photosynthesis, respiration, energy consumption and growth. Instead, trees conserve energy until the warm weather returns.


Contrary to popular be-LEAF, tree buds, which are underdeveloped leaves, actually start to form the previous year and lie dormant over the winter. In order to survive these cold temperatures, most buds are covered with scales which act as a protective layer. In the spring, these scales are prompted by climatic signals and drop off during a process called bud burst. Favourable climatic conditions, such as experiencing a certain number of days with cold temperatures followed by an adequate number of days with warm temperatures, allow trees to properly grow and avoid damage from late frosts. Until these exact favourable conditions are present, the tree’s buds will remain in a dormant state.


The timing of bud burst varies according to a number of factors including year, weather patterns, microclimates, species and individual trees. Interestingly, this timing can even vary within individual branches on one tree! Tree buds on maples and birches are more apt to burst earlier in the season whereas oak buds tend to burst later to protect against sudden drops in temperature. Furthermore, newly planted trees often take a little longer to leaf out in the spring because they have experienced some transplant stress and tend to focus a lot of their energy on their roots in the first few years after planting.


In some cases, however, a tree’s dormant season may be unexpectedly interrupted by warmer than usual temperatures, which can prompt trees to leaf out unusually early. In these instances, trees begin the spring growth processes as though long-lasting warm weather has actually arrived; they start leafing out and some even start producing flowers and fruits. Following this, if temperatures then plunge, it may cause the tree and its buds to become extremely stressed. As a result, new bud growth becomes shocked by the sudden decrease in temperature and the newly burst buds are exposed to freezing. That said, native species that have evolved to withstand local climate are less likely to experience increased stress from early bud burst!


There is no set date for bud burst on trees, but we can use environmental clues to predict when trees will leaf out year to year. Ultimately, trees are in tune with their physiological processes and can sense temperature changes, which help wake them up in the spring!


If you'd like to plant native trees and shrubs on your property, we provide subsidized planting programs for homes, multi-units and commercial properties. Our certified arborists consult with you to select the right tree for the right place and provide you with the tools to care for the tree as it grows and establishes itself! Learn more about our programs.


Natasha Keshavjee is an ISA certified arborist and the Residential Planting Programs Operations Supervisor at LEAF.


The Backyard Tree Planting Program is supported by the City of Toronto, the Regional Municipality of York, the City of Markham, the Town of Newmarket, the Town of Ajax and Ontario Power Generation.