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The Science Behind the Colors of Fall

Audrey Hillman
Adams County Penna. Master Gardener

This fall promises to be a great one if you enjoy the changing colors of the trees around us. Weve had plenty of rain, so the trees are not stressed, and warm sunny days with cool nights also help out. But why are some years good for fall color, when others are not? Many factors such as soil conditions, weather, and genetics all contribute to the equation.

The whole process is a slow one and begins as the length of the nights increase. This change in the light causes the plant to produce phytochrome. Phytochrome is the chemical that starts the process of dormancy. A layer of cells is produced between the branch of the tree and the leaf stalk. This layer is called the abscission layer and it blocks the passage of water and nutrients (carbohydrates) to and from the leaf. The production of the green pigment, chlorophyll, which is the predominant pigment, begins to break down.

Without the chlorophyll to color the leaves green we begin to see the other pigments, carotenoids, give the leaf its yellow, orange and brown color. Now here is where the genetics fits in. Some trees also have the ability to form another pigment known as anthocyanin, which gives leaves a red or purple color. For anthocyanins to form there must be sugar present so any weather condition that enhances the production and accumulation of sugars in the leaf helps with the intensity of the red color.

Sunny days result in a high production of carbohydrates in the leaf and cool nights help to break those carbohydrates down into sugars. The cool nights also help to keep those sugars in the leaf instead of going to other parts of the plant. When the skies are cloudy and the nights warm, less sugars are produced and more are moved from the leaf, leaving us with less intense color.

As the abscission layer gets bigger it divides into two layers. One layer is protective and forms on the branch. The other is a separation layer and forms on the leaf stalk (petiole). Once both layers form there is not much left to hold the leaf in place and down it comes. A popular myth about fall color is that we need a frost to produce good fall color. Killing frosts and freezing temperatures stop the color change and kill the leaves. So lets hope the nights stay cool (40-45 degrees F.), but not cold.

Hope this helps demystify why a wonderful display of fall color is predicted for this year. Enjoy it while it lasts. Soon enough well have to be raking all those leaves up and its nice to think that at one point we really did enjoy them.

Read other gardening articles by Audrey Hillman