Three factors influence autumn leaf color-leaf pigments, length of night, and weather, but not quite in the way we think. The timing of color change and leaf fall are primarily regulated by the calendar, that is, the increasing length of night. As days grow shorter, and nights grow longer and cooler, biochemical processes in the leaf begin to paint the landscape with Nature's autumn palette.

 

There are several factors that affect the brilliance of fall foliage. Color development in trees and shrubs are determined by both genetics and environmental conditions. We're fortunate to have sunny days and cool nights, which prompt bright yellow and orange hues. Additionally, drier soils, with high iron content, help bring on more intense colors. The absence of hard freezes also means a more colorful and long-lasting foliage show.

Have you ever noticed how much more showy the aspen foliage is in the mountains compared to aspen leaves in lower-elevation, metropolitan landscapes? Higher elevations have greater sunlight intensity and much cooler nights. Mountain soils generally are drier and drain water better than the clay soils common in home landscapes. 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 comes to a halt, and enzymes and sunlight break down the remaining chlorophyll in the foliage. A transformation occurs as this chlorophyll breaks down and other colorful pigments become more apparent.

The Reds
The scarlet colors are made in the leaves late in summer from sugars that are 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.

The Yellows
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, that give the russet and brown colors, are least affected by rainfall.

 

Certain colors are characteristic of particular species. Oaks turn red, brown, or russet; aspen and poplar, golden yellow; and beech, light tan. Maples differ by species. Sugar maples display the most brilliant fall color, but will not grow in our alkaline soils.  Leaves of some species such as the elms simply shrivel up and fall, exhibiting little color other than drab brown.
 

The timing of the color change also varies by species. The differences in timing among species seem to be genetically inherited.

The amount and brilliance of the colors that develop in any particular autumn season are related to weather conditions that occur before and during the time the chlorophyll in the leaves is dwindling. Temperature and moisture are the main influences.

 

A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays. During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions-lots of sugar and lots of light-spur production of the brilliant anthocyanin pigments, which tint reds, purples, and crimson. Because carotenoids are always present in leaves, the yellow and gold colors remain fairly constant from year to year.

 

The amount of moisture in the soil also affects autumn colors. Like the weather, soil moisture varies greatly from year to year. The countless combinations of these two highly variable factors assure that no two autumns can be exactly alike. A late spring, or a severe summer drought, can delay the onset of fall color by a few weeks. A warm period during fall will also lower the intensity of autumn colors. A warm wet spring, favorable summer weather, and warm sunny fall days with cool nights should produce the most brilliant autumn colors.