Why Do Trees Leaf Out?

This is a question that fascinates me. When I look around in spring, I notice that although the maples and the magnolias are out, the hickories and sweet gums are not. Why is that? Today I'd like to take a shot at answering that question.

I've figured out there seem to be three major factors that influence when trees emerge from dormancy:

The scene in my backyard right now - pure emerald - as the trees leaf out.
The scene in my backyard right now - pure emerald - as the trees leaf out.

The first is temperature. I know this may seem odd, but autumn plays an important role in when the trees break dormancy in spring.

Warmer autumns cause the trees to leaf out later, while cooler autumns induce the trees to start growing early. I found this hard to believe, because that would mean that our winters are almost the same lengths every year, but evidently they are. It’s just that they start and end at different times, but (give or take a week) generally last as long.

This is where chill hours (which are the number of hours under 45 degrees that a tree needs to break dormancy) come into play. I often hear that term associated with orchard trees, but it applies to every tree. Each tree has a set number of hours in its genetic code that prompts it to grow after the amount of chill hours have been reached.

Since some of the trees around you are imported from other countries, they have different requirements than those native to your area. Even the trees in your area vary for one reason or another. Some leaf out early (like the magnolias) and risk being caught by a late frost to get a head start on the competition. Others prefer to wait until there is absolutely no chance of frost before they attempt to grow.

Of course, there are always those crazy years when even the later group gets caught by a frost, but that is rare. Keep in mind that this is nature we’re dealing with. Although it may not make sense to us, the trees are nature, so they understand perfectly.

 

The maple in my front yard responds to heat - when the temperature reaches 60 degrees (even in February) it's ready to grow!
The maple in my front yard responds to heat - when the temperature reaches 60 degrees (even in February) it's ready to grow!

The second is light. Some trees (especially in tropical areas) use light as their cue to wake up. Since light almost never varies each year, it is an extremely reliable way of determining the seasons, as long as there’s not frost in the equation.

This is how growers induce Poinsettias, which go dormant in the summer, to flower at the right time. They simply restrict the light supply, and the plants respond by flowering.

The last factor is water. Since all ecosystems have some sort of weather cycle, the trees use it as a signal to break dormancy.

For instance, where I live, there is almost always a dry spell in the late summer and autumn. This prompts the trees to start changing colors and drop their leaves. The only tree in my yard that does not readily respond to this signal is the Tree Of Heaven (Ailanthus altissima), since it is nonnative and has different ways of determining when to go dormant. The same principle applies to the spring. When the rainfall decreases (combined with other factors such as temperature and light), the trees break dormancy.

I hope this has shed some light on such an interesting (and rarely talked about) topic!

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Travis

About Travis

Hi there! I'm Travis and have been gardening for over 10 years now at my home in Ruston, Louisiana. I currently run a garden consulting business and enjoy experimenting with new plants and techniques in my own garden. My goal is to help everyone by providing advice based on experience and enabling people to garden confidently. My greatest passion: connecting with the younger generations (including my own) and helping them see gardening for what it truly is.