Bartlett on Gardening: Colorama

My husband and I first visited this part of the world in the month of October. We were captivated by the brilliant fall foliage. We really had never seen anything like it. First impressions being the lasting ones, autumn remains my favorite season. In the arid West, where we had lived before, leaves just turn brown and fall to the ground. Why, oh why, do trees here display such vivid color?

Leaves contain chlorophyll, a pigment essential to photosynthesis. This is the process by which plants use sunlight to transform water and carbon dioxide into oxygen and glucose, making life on earth possible. Chlorophyll gives leaves their green color. Plants are constantly replacing it because it breaks down quickly in sunlight. As nights grow longer, sunlight becomes a scarce resource. Deciduous trees and shrubs cut off the supply chain to conserve energy as they prepare for winter dormancy.

​​Leaves contain other pigments, yellow xanthophyll and orange carotenoids. These are masked by the more abundant green chlorophyll most of the year. Once the plant stops replacing the chlorophyll, yellow and orange are rapidly revealed. At the same time, the plants transform the sugars trapped in the leaves into red and purple pigments called anthocyanins. As for those leaves that just turn brown, that is caused by tannins.

Because the color change is triggered by lengthening nights, it occurs on a predictable schedule every year. I once had a coworker from Vermont who planned his vacation around the peak of foliage color. He zipped home for the spectacle there and then ambled along the Blue Ridge Parkway on the return leg.

Prevailing weather conditions do impact the intensity of the colors. Ample moisture during the growing season followed by a dry, sunny, cool autumn is optimal. Cool nights and sunny days lead to more anthocyanin production and thus bright red maple leaves. Frost causes production of this pigment to cease. The show may be curtailed by wind and rain because the leaves are no longer well attached to the trees. Drought stress may also lead to an early leaf drop in which case there is no time for pigment production.

The forests of Eastern North America have the best displays of fall foliage in the world. The Appalachian Mountains are among the oldest on Earth. In the distant past they were as tall as the Himalayas are today. Over eons, the elements have whittled them into their present form. The northern forest regions are dominated by by a few tree species leading to a brief, spectacular display.

Here in the Southern Appalachians, we have the largest diversity of hardwood species in one area, giving us a prolonged, unparalleled kaleidoscope of fall foliage color.

Master gardener Ann Bartlett never lets lack of familiarity with a plant stop her from trying it in the ornamental beds around her home.

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