In the Garden:
Western Mountains and High Plains
October, 2005
Regional Report

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The golden hues of aspens glitter among the evergreens in the Vail valley of Colorado.

The Magic of Autumn Colors

The golden leaves of autumn that blanket the Rocky Mountains and the brilliant foliage of our landscape plants are wonders of nature. We often take it for granted that this colorful foliage -- its spring and summer work finished -- has reached old age and will soon fall to the ground. But although the foliage is dying, it does not signify finality. The death of the leaves in autumn sets the stage for next spring's rebirth of plant life.

Coloring Up
The processes that lead to colorful fall foliage started back in summer, and they are determined by genetics and the environmental conditions of a region. We're fortunate to have sunny days and cool nights, which results in bright yellow and orange hues. Additionally, drier soils with higher iron content tend to bring on more intense colors. The abeyance of hard freezes also can produce a more colorful and long-lasting autumn foliage show.

Have you ever noticed how much more showy the aspen foliage is in the mountains? Higher elevations have greater sunlight intensity and much cooler nights. Mountain soils generally are drier and drain water better than the clay soils in home landscapes, which hold moisture longer. Plus most gardeners provide additional irrigation to lawns and surrounding areas. Whereas home landscape soils are alkaline, high country soils are usually more acidic, which makes iron more available to the trees and shrubs.

Leaves are green because of the substance known as chlorophyll, a complex pigment needed for photosynthesis. As days shorten in late summer and early fall (and night temperatures cool down), synthesis of chlorophyll stops and the remaining chlorophyll in the foliage is broken down by enzymes and sunlight. With the disappearance of chlorophyll, other colorful pigments become apparent.

The scarlet and red colors are made in the leaves late in summer from sugars trapped in the leaf tissues and converted into a pigment called anthocyanin. This pigment is water soluble and can be washed out. Therefore, if there is a rainy period during this time of transformation, the red coloration will be diminished.

Yellow is a leaf's basic color, present from the beginning. All summer long it has been masked by the green chlorophyll. With shortening days and the cessation of chlorophyll, the yellow pigments, including xanthophyll and carotenoids, are revealed. These pigments and the tannins (russet and brown colors) are least affected by rainfall.

The miracle of fall colors also signifies new life. Before the leaves drop in autumn, they send many important nutrients back through the tree's vascular system. This transfer takes place before the abscission layer forms between the leaf stem and twig; then all circulation ceases and the leaf dies. Fallen leaves truly leave a legacy to the plant that produced them.


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