Aspen Catkins in January! How Trees Know When to Leaf by Mary Small

Aspen Catkins Photo by Anna Wilson

How do trees know when to leaf out?  I’ve pondered this since I was recently sent a picture of aspen buds beginning to flower – in January!

Tree leafing and flowering is not completely understood and the process varies not only among species, but within species. Location’s important, too. A tree may leaf out earlier in the city than its relation in the mountains.  One growing on a warm southern exposure is more likely to leaf out earlier than the same species on a colder northern exposure. Trees originating in the southern part of their range often leaf out earlier than ones originating in the northern part.

Both cold and warm temperatures play a large role in leafing and in some species day length is involved, too. During early warm spells, day length is an additional layer of protection, keeping the tree from leafing too soon despite temperature signals. 


Trees must be exposed to a certain amount of cold, known as a “chilling requirement”. Generally this equals the number of hours that have passed at temperatures between 32 and 50 degrees Fahrenheit.  Temperatures less than 32 or above 50 degrees F generally don’t contribute to a plant’s requirement. Exactly how much chilling time trees must have before leafing varies with species.  Once the chilling requirement is met, another block of time must pass above some threshold temperature (for example 55 degrees F).  After that a signal is given that it is okay to leaf.

Some tree species leaf early, including silver maple, aspen and poplars. An advantage of this is the potential for increased food production through photosynthesis. The down side is potential freeze damage. Amazingly, early leafers tend to be species with small vessels (water transport tubes) which are less susceptible to freeze injury than later leafers (like ash) that have large spring-formed vessels.  For good health and survivability, it’s in the best interest of ash(and others) to delay leafing and avoid freeze damage.

So now, why is the aspen at the beginning of the article beginning to emerge? First, aspen have a low chilling requirement and it has apparently been met. Second, the aspen was located on the southern exposure of the home. This created a microclimate with warm temperatures, so time spent at or above the warmer threshold temperature has also been met. The signal has been given that it’s okay to forge ahead.

Photo by Anna Wilson

But we know there’s lots of winter left, so what will happen to this poor tree? It’s hard to say. Fortunately temperatures continue to play a role in leafing, so if weather is cold, the process will slow. Those small water transport vessels in aspen will aid in reducing freeze damage as well.  All we can do now is wait and see!