As summer turns to autumn and the leaves start to fall, your kids might wonder why leaves change color. Luckily, we’re here to share the simple science behind this seasonal phenomenon, in terms even the itty bittiest botanists can understand.
Before we begin our explanation, we’d encourage you to head out for a walk at your local park to observe and collect various types and colors of leaves. Look at them together and talk about their size, shape, and color. Trace them on paper or try a leaf rubbing. Compare the supple leaves of a house plant to the brittle leaves you collected.
Now let’s talk color. First, we need to understand why leaves start out green. We know that plants need sunlight and water to survive. But how do sunlight and water keep a plant alive? Doesn’t it need food? The answer is yes. Plant cells contain tiny little food factories called chloroplasts that convert the energy from the sun into food for the plant (pretty cool, right?). And chlorophyll, a green pigment found inside each chloroplast food factory, is a key part of the process. It’s the chlorophyll that soaks up the sun’s energy so that the chloroplasts can turn it into food. And, guess what? Chlorophyll is green, which is why plants and their leaves are green.
But why aren’t leaves ALWAYS green? Well, it turns out that leaves have lots of colors inside of them all year long. For example, in addition to chlorophyll, plants also contain carotenoid, a pigment that causes a yellow, orange, or brown color. Foods like corn and carrots get their color from this pigment. Some plants also produce anthocyanin, another pigment found in plants like apple trees and strawberry bushes that causes a deep red color. But with the chlorophyll working so hard to soak up the spring and summer sun, its green color is the one we see in those months.
In the fall and winter months, the days are shorter and the sun is weaker, so there is less sunlight to absorb and less work for the chlorophyll to do. Consequently, plants slowly stop making chlorophyll. Less chlorophyll means less green pigment, which means we can finally see the other pigments in the leaves, resulting in beautiful oranges, reds, and yellows.
With less food to support its structure, leaves also become weaker in the fall and winter months. This includes the stem that holds the leaf to its tree. When the stem weakens and breaks, the leaf falls and voila! The fall ground is littered with colorful leaves.
Weather can affect the colors of the leaves as well. Things like drought, rain, frost, and freezes can kill leaves, turning them brown before they fall, or decrease the intensity of the colors we see. The best fall colors come with a warm, wet spring, followed by a mild summer, and a sunny fall.
So there you have it! The simple science behind the changing colors of fall leaves.