Dear Tom,
Why have so many trees failed to turn color this fall?
— Bob Marovitz, Glenview
Dear Bob,
Three factors influence autumn leaf color: length of night, weather and leaf pigments. The increasing length of night is the primary influence of the timing of color change. Consequently, autumn colors at a given location occur at about the same time every year. However, the brilliance of colors that develop in any particular year is related to weather conditions that occur before and during the time when chlorophyll in leaves is dwindling. During the growing season, chlorophyll is constantly being produced and the leaves appear green. With increasing length of night, its production ceases and other previously masked colors appear. Temperature and moisture become important. Warm, sunny days and cool, but not freezing, nights produce the best color displays.
