In Colorado, the most definitive way to know that summer is over and winter is not long off comes when the high-country aspen groves put on their brilliant yellow coats.
Aspens trigger the change from green to gold -- plus mutant reds and oranges -- as the days get shorter and photosynthesis loses strength. Generally, that means fall foliage in Colorado begins in the north and spreads southward.
Other variables -- elevation, unseasonably warm temps, summer precipitation, disease -- can alter timing and brilliance. So leaves in drought-stressed Western Slope will drop quicker than those in the north and on the Front Range.
High-elevation locales put aspens on display before lower locations, so the Colorado ski and snowboard resorts are prime places for "leaf peepers" to congregate during September and October. And if you start in the north and leisurely wander downstate, you'll get an eyeful of colors -- over and over again.
Around mid-September, head to Steamboat. Nearby Buffalo Pass signals the start of the aspen turn. Large groves quilt-patch the mountain. Check out the massive base reconstruction while you're there.
No fall foliage trip would be complete without a stop in Aspen itself. Drenched with aspens, Snowmass runs gondola on weekends through September -- with scenics of Maroon Bells plentiful aspen groves. A deep system of MTB trails beckons. If your timing is right, you can get the Colorado trifecta: Blue skies, patches of early white snow, and bright yellow aspens.
Next stop should be Powderhorn. On the skirt of flat-topped 10,000-foot-high Grand Mesa, groves tend to turn at the same time. Golden aspens mix with rusty-red scrub oaks for a brilliant autumn portrait. Bring an MTB as the resort has amped up its trail system. The chairlift runs through September, which is the leaf-peeping peak there.
Then straight south to Telluride. A free gondola runs from town up and over to the resort village -- a perfect platform for checking out groves on the slopes of the Wilson Range. A Jeep ride gets you up into the highest aspen groves on the way to Tomboy, Ophir Pass and Imogene Pass.