As days get shorter in fall, the diminishing hours of sunlight trigger processes that turn summer-green leaves into a kaleidoscope of color. These processes, which are also influenced by temperature, moisture, and genetic makeup of each plant, are the way deciduous plants prepare themselves for winter.
Several types of pigments are always present in plant leaves. The pigment chlorophyll gives leaves their green appearance throughout the growing season. Chlorophyll is the only pigment involved in the process of photosynthesis — the absorption of light energy and its conversion to sugars, which are then converted into usable energy by plants. It is relatively unstable and degrades easily in sunlight or cool temperatures. The progressive breakdown of chlorophyll at the end of a leaf's life cycle allows other pigments to be "unmasked" and become visible.
The only constant factor influencing fall color year-to-year is shortening day length. Reduction in day length signals changes in specialized cells called the abscission or separation layer in the petiole, the stalk at the base of a leaf. Changes in these tissues result in the accumulation of sugar in leaves as veins connecting the petiole to the plant's stem gradually close. Sugars trapped in the leaves behind the abscission layer may aid in the production of different pigments in shades of red and purple. The predominant fall colors — yellow, orange, red, purple — vary by species, individual plants within a species, and even leaf by leaf.
Each season's show may be different than in previous years. Predicting fall color is challenging because conditions vary annually. Range and intensity of color are greatly influenced by weather conditions. Early frost may kill leaves before they have the chance to accumulate sugar or before chlorophyll is sufficiently depleted to allow other pigments to be seen. Rainy or continually overcast days reduce the amount of available sunlight. Generally, dry, sunny days followed by cool, dry nights result in the most spectacular displays.
Autumn brings magnificent new colors to the Garden, from the vibrant golds and reds of its trees and shrubs to garnet chrysanthemums, lavender asters, buttery sunflowers, and the mixed hues of its rustling grasses — feather reed grass, flame grass, and Dallas Blues switch grass, to name just a few. Come on over and see for yourself!