Scott Aker, gardens unit leader at the U.S. National Arboretum, blogged about the myths of why tree foliage turns colors in the fall.
Scott writes:
Like a lot of people, I remember being taught when I was young that the brilliant autumn foliage of deciduous trees was caused by the cold temperatures of autumn frosts. I believed this until I became a horticulturist, studying the intricate system that plants use to prepare for winter’s harsh weather. Where I work, at the U.S. National Arboretum, we grow about 10,000 different kinds of trees and shrubs and have an overwhelming variety of fall color right now.
Daylength (or more correctly, the length of nights) is the real cause of trees’ transition to autumn colors. (Read more here about how longer nights translate into the vibrant color of autumn.)
So what role does frost play in the changing color of autumn leaves? Frost actually ruins vivid fall colors by causing cells in the leaf to rupture and die.
The appearance of fall colors shift each year due to weather. Heavy cloud cover limits ultraviolet light, so the chlorophyll lasts longer, and leaves stay green longer. High night temperatures cause cells to burn accumulated sugars, so less red and purple pigments are formed and yellow and orange are the predominant colors. Drought stress or a windy interlude cause leaves to drop prematurely.
The ideal conditions are warm, sunny days and cool nights with little wind or drought stress.
At the Arboretum, you can watch this process in action through the course of many months this time of year. Dogwoods begin to turn in late September, and Chinese fringetree ends its show of colors in early December. Even some of our herbaceous plants have great fall color—most notably the willow amsonia, which blooms in response to the longer nights of autumn. You’re your next visit to the Arboretum, the willow amsonia is planted at the columns with ‘Raydon’s Favorite’ asters.)
Scott is lecturing about fall color at the U.S. National Arboretum from 1-2 pm on Nov. 5, followed by a fall color tour led by Horticulturist Mariya Navazio.