Why do the leaves change colour?

Each fall we see the summery green trees turn to brilliant shades of yellow, orange, red, and even some deep purple. It can be stunning to witness, but why does it happen and what determines the colour the leaves will change to? How do the trees know when it’s time to shed their leaves?

The main reason for these changes is shorter days and cooler temperatures, but other factors affect the fall foliage change as well. Everything you wanted to know about the why the leaves change colour is below!

Shorter Days

As summer comes to a close and the fall season begins, the days become shorter and there is less sunlight. Trees are sensitive to this change and use it as a signal to prepare for winter, prompting a series of physiological processes in the tree.

Cooler Temperatures

As temperatures drop, chemical reactions within the leaves are affected. These lower temperatures, especially at night, act as a cue to start the process of senescence, which is the gradual aging and deterioration of the leaves.

Chlorophyll Breakdown

Chlorophyll is the green pigment in leaves that plays a main role in photosynthesis, the process by which trees convert sunlight into energy. As the days shorten and temperatures drop, trees receive less direct sunlight and the chlorophyll in the leaves begin to break down, revealing the other pigments in the leaves.

Leaf Pigment

In addition to chlorophyll, leaves contain other pigments, such as carotenoids and anthocyanins.

  • Carotenoids produce yellow and orange colours and are always present in leaves. They are usually masked by the green chlorophyll, but as it breaks down, the yellow and orange pigments become more visible.
  • Anthocyanins are not part of the leaf in the growing season and are sugars that get trapped in leaves. Through a chemical process, it creates new pigments of dark red and purple colours, and are only present in some tree species.

Tree Species

The exact colours and timing of the change can vary depending on the tree species. Each species contains their own pigment combinations, and can respond differently to environmental cues. For example, maples are known for their vibrant red and orange leaves in the fall, while certain oak species tend to turn yellow or brown. Oaks also keep their leaves much longer than other tree species and can still be green as other trees turn colour.

Other Environmental Factors

Decreased daylight and cooler temperatures are the main reasons for the changing leaves, but other factors have an impact as well. Soil moisture levels, rainfall, and the health of the tree also play a role in the changing leaves. Those under stress, whether due to drought or disease, may have less vibrant fall colours.


Now that you know all the basics of the brilliant colour changes, it’s time to get out and witness this once a year change for yourself! Head outside because it always seems to go quickly; one day they are lighting up the autumn forest, and the next they’ve all fallen to the ground.

Wondering where to go? Check out our blog on some of the best places at HCA to visit in the fall!



More Stories Like This...