It’s October and here in Atlanta and there is almost no autumn color. Very few trees have started changing. Temperatures are finally starting to cool—we had weather in the 90s until just two weeks ago—so we will begin to see the leaves changing soon. About this time every year I try to think through the explanations I’ve heard of what determines when the leaves turn to yellow, red, and orange, and purple; and what determines how vibrant the colors will be from year to year?
I’ve looked it up and written it down. Here is what brings about autumn color.
- Less Daylight: Autumn colors happen as a result of longer nights and less daylight — which affects the production of chlorophyll, the ingredient that keeps plants green during the growing season. When the nights become longer and there is less daylight, the production of chlorophyll slows down and finally stops all together. When there is no more chlorophyll, then all that is left are carotenoids — which are responsible for the yellow, orange and brown leaves, and the anthocyanins responsible for the red, blue and purple colors in leaves that are sometimes seen with such trees as dogwoods.
- Temperature: The amount and brilliance of the colors that develop in any particular autumn season are related to weather conditions that occur before and during the time the chlorophyll in the leaves is dwindling. A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays. During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out.
- Moisture: The amount of moisture in the soil also affects autumn colors. Like the weather, soil moisture varies greatly from year to year. The countless combinations of these two highly variable factors assure that no two autumns can be exactly alike. A late spring, or a severe summer drought, can delay the onset of fall color by a few weeks. A warm period during fall will also lower the intensity of autumn colors.