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Fall color - Why and how leaves change colors
The beautiful crisp days of fall, and all those beautiful colors, will soon be here. We can wax poetic that Jack Frost will get out his paintbrush and start dabbing his palette on the leaves. However, there is no Jack Frost, nor Jane Frost, nor any other little nipper frosts, though you can be nipped by frost, and David Frost might interview you sometime.
Fall color makes or breaks the autumn. When the vivid reds and oranges are coloring the trees, we oooh and aaah, just like we do when we watch fireworks. People will get jealous when their trees don't turn, blaming it on the tree and some insidious government plot.
Color is dependent on basically two factors - weather and plant type.
No matter what you do, a birch just won't turn red. It's not in its genetic make-up. Getting orange from the tree is just as hard. In most cases, you're stuck with a yellow and then the ubiquitous brown all trees share.
On the other hand, sugar maples will turn the oranges, yellows and reds. Red maples, which are different from Norway maples, such as 'Crimson King,' have a strong red color in the fall, which is their major asset and the reason so many are planted.
Okay, now bear with me. We're getting into the whys of the fall colors.
The fall colors come from five major pigment groups.
First, there's green. That color is a product of chlorophyll, the primary pigment in photosynthesis. Without it, plants don't grow. It's usually present in all leaves, even those that don't look green during the summer.
Yellows are attributed to xanthophyll pigments, better known as one of the few "x" words you can use in Scrabble or crossword puzzles.
The oranges, as well as the yellow and red colors, are a result of the presence of carotenoid pigments. Think carrots and winter squashes. The showy shades of dark red scarlet and crimson are due to anthocyanins.
The least noticeable pigments seen in fall are the tannins, which are responsible for the deep browns of oaks and most other plants once all other color is drained out.
Light, water, temperature and nutrition influence the amount of color that develops each fall and so does the soluble sugar produced by the plant.
The yellow colors are there all season. It's just that chlorophyll is so much more vivid and abundant that it masks the xanthophylls and carotenoids. However, as the temperatures get cooler and it gets darker earlier, the green starts breaking down, and exposes the yellows.
That's why yellow is one of the predominant fall colors, and also the first one to show up. A light frost will destroy the chloroplasts and essentially remove the chlorophyll. But the yellow pigments can keep producing some sugars. Sugar accumulation in the leaves is the most important factor in production of anthocyanins and the intensity of the red and orange colors. Trees and shrubs kept healthy during the summer with full sun and adequate nutrients and water have the greatest potential for intense fall color from anthocyanins. Whether the color develops depends on fall weather conditions. Sunny fall days followed by cool (40-45 degrees F.) nights favor accumulation of sugars in the leaves.
This means a great show. Indian summers, with the quick October frost, followed by warm days, are ideal.
Cloudy autumn days and warm nights result in decreased sugar production and a movement of sugar out of the leaves, resulting in less fall color. The yellows may remain, but the reds have left for another year.
Finally, when all the colorful pigments have broken down, the tannins remain, turning the leaf brown.
Why and how leaves fall off the tree doesn't really have an impact on plant color. That's next year's topic.
David Robson is an Extension Educator, Horticulture, at the Springfield Extension Center, University of Illinois Extension. You can write to Robson in care of Illinois Country Living, P.O. Box 3787, Springfield, IL 62708.
16 ILLINOIS COUNTRY LIVING • OCTOBER 2000 |
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David Robson