The whole process is a slow one, and it begins as the days start to get shorter in late June. This change in the amount of light causes the plant to produce phytochrome, the chemical that starts the process of dormancy.
When chlorophyll no longer colors the leaf green, we begin to see the other pigments that were previously masked. These carotenoids give the leaf its yellow, orange and brown colors.
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, fewer sugars are produced and more are moved from the leaf, leaving us with less intense color to enjoy.
A popular myth about fall color is that we need a frost to produce good fall color. Killing frosts and freezing temperatures actually stop the color change and kill the leaves. So the best color comes when the nights stay cool (40-45 degrees Fahrenheit), but not freezing cold.
So that’s the process that brings about a wonderful display of fall colors. Enjoy it when it happens. Soon enough you’ll be raking up those leaves, and it’s nice to remember that at one point we really did enjoy them.
Master Gardener Audrey Hillman is a landscape and garden designer in Emmitsburg, Va. This story originally appeared at www.emmitsburg.net.