What causes leaves to change color?

J. H. Osborne • Sep 24, 2019 at 2:58 PM

We all know that leaves change their colors in the fall, making that time of year a favorite for its spectacular colors. But what causes the leaves to turn?

For answers, we turned to the Farmers’ Almanac, which provided the following:

What Signals A Leaf To Change Color?

• The vivid, often simple colors on the outside are the products of the complex chemistry of growth inside a leaf. Take the yellows and oranges, for example — the dominant colors of aspen, ash, birch, beech, hickories, maples, some oaks, tulip poplar, and sassafras. Generally, these colors come from compounds called carotenoids (also responsible for the color of carrots), which are present in the leaf during the growing season.

• The green chlorophyll, the workhorse of photosynthesis, dominates and covers up those carotenoids in summer. As the days grow shorter and the temperatures cool, chlorophyll degrades and goes from green to colorless, allowing the oranges and the yellows to show up. These colors are present in the leaf during its growing season.

The Vibrant Reds

• Those gorgeous scarlet, crimson, and ruby hues of the red maples, black gums, dogwoods, sourwood and oaks are what make fall the most breathtaking.

• Red pigments are not present in the leaf during summer. Trees that turn red actually produce this pigment, called anthocyanin, in the autumn.

• However, though we might like to think so, plants don’t make this red pigment for our appreciation. These pigments play a key role in readying the tree for the next spring. Researchers discovered that anthocyanins act as a sunscreen, protecting leaves from bright seasonal light when it is cold outside. Other researchers have discovered that the sunscreening effect protects leaves from too much light, which can interfere with late-season transport of nutrients from the leaf back to the twigs, something trees do as a conservation mechanism.

• Still, other scientists believe the red color serves to ward off insect pests. A healthy, strong plant has lots of anthocyanins; certain insects laying eggs in the fall may seek other, weaker host plants for their offspring. While anthocyanins may ward off insects, there is no doubt that they are magnets for “leaf peepers” for fall color tourism. A little bit of red goes a long way — and more is even better.

When Do Leaves Change?

• Fall color starts earlier at higher altitudes and in northern regions than it does in lower elevations or farther south. Fall color is about location, location, location — and good timing!

Weather Conditions

• Weather affects both the leaves’ color intensity and its duration. Ideal conditions for spectacular coloring are a warm, dry summer followed by a rainy autumn. In autumn, warm, sunny days with cool nights trigger brilliant color formations. An early frost lessens the intensity of red. Rainy or overcast days intensify the brilliancy of color.

What About Trees Down South — Do They Change Color?

• Where palm trees, live oaks and cactus grow, there is little to “rake home about” in autumn. The Gulf Coast and the arid Southwest are mostly fall colorless.

See Farmers’ Almanac’s average peak foliage dates by state here. The Farmers’ Almanac projects peak foliage dates for our region to be Oct. 12-28.