Rocky Mountain Region

USDA Forest Service
Rocky Mountain Region
740 Simms St
Golden, CO 80401
303-275-5350

Telephone for the
Hearing Impaired
303-275-5367

Fall Colors

In the 1800’s the allure of gold brought people to the Rocky Mountains.  Today, that same allure still brings people to the Rockies.  Instead of shiny flakes of metal, the golden attraction today is the sight of hillsides painted with the golden leaves of aspen.

Around Labor Day each year our thoughts turn to fall:  harvests, hunting, football, and, in the Rocky Mountains…fall colors.  As the day length begins to shorten, the deep green color of summer begins to pale, and we see the first hints of yellow in the trees.  In most years, we expect to see the height of the fall color display in the Rockies around the third and fourth weeks of September. 

Fall colors follow a typical progression, beginning in the north and spreading to the south in response to shorter day lengths.  The color display also begins earliest in the higher elevations and works its way down the slopes.

While the annual transition from summer to fall is determined primarily by the shortening day length, many factors contribute to both the precise timing of the change and the intensity of the display.  The ideal conditions for the formation of brilliant fall colors are warm, sunny days, and cool, clear evenings.  Drought conditions can place the trees under stress.  One reaction to that stress is a slowing or cessation of active growth.  Stress can even cause trees to begin the process of dropping their leaves for the year.  Consequently, fall colors often develop a little earlier than normal in drought years.  Extended periods of cool, wet weather will decrease the intensity of the color display.  Strong winds or heavy snows could put an early end to the fall color season.

The early season color change allows fall color viewers to observe a unique characteristic of aspen growth.  Aspen propagate primarily by sprouting from an expanding root system, creating groups of trees, or clones, ranging in size from several trees to many acres. These clones are genetically identical.  One factor in how and when a tree changes color is the balance of various chemicals in the plant.  This balance is, to some degree, genetically determined and varies between clones.  Because of these differences, it is possible to see hillsides with one small group of trees that have already changed to vibrant gold colors standing among otherwise green aspen.  This color change allows viewers to readily see individual aspen clones.