When Do the Aspens Change in Colorado?
The peak color of changing aspens only lasts for about a week in Colorado. However, the shoulder weeks bring to the state about a month of golds, greens, oranges, and reds from mid-September to mid-October. So, the best answer to the question, “When is the best time to see the fall colors of Colorado?” is best answered as, between September 15th and October 15th, but there will be an optimal 7-10 day period sometime within that date range. That optimal time when the aspen colors are most intense and brilliant, depends on four factors: temperature, moisture, sunlight, and altitude. For instance, a dry season will dull the fall colors. An early frost can cut short the season, too. The best kind of mountain weather requires a bit of moisture, gradually decreasing temperatures, and a good amount of sunlight.
If you are interested in why certain aspens turn red, orange, or yellow, or wonder why they change color in the first place, then read on. I also give a set of recommendations for places to hike where you can take in the Fall colors of Colorado near Denver. If you are looking for hikes to take this fall, then check out our Top 10 Best Fall Hikes Near Denver.
Why Do the Aspens Change Color in Colorado?
Let’s start off with the familiar. We know that the compound, chlorophyll, is what provides food to trees through the process of photosynthesis–and that chlorophyll is what makes leaves green. It helps to think of the power of Chlorophyll as similar to the intensity of the sun that blocks out the stars during the day. During the Spring and Summer, the other chemicals in a leaf that produce the yellows, oranges, and reds get overpowered by the green of chlorophyll. When the chlorophyll flow gets shut off in the Autumn, these other colors get to shine. This is triggered when trees respond to longer and colder nights by essentially blocking off the flow of chlorophyll into and out of the leaves. At this point, three other compounds get trapped in the leaf and begin to show their hues. Let’s take a look into each of the colors.
Why Do Aspens Turn Red?
Red is the most rare, and it is created by anthocyanins. These are generated by sugars that get locked into the leaves in the fall. You may notice that certain groves of aspens will turn red year after year, and that others will never turn red. Researchers as Colorado State University found that this is due in part to genetics: some apsens will produce anthocyanins and others just don’t. They also found that weather plays an important role. The reds will become more intense if the fall season starts out warm and sunny then gradually cools, without getting down to freezing at night.
Why Do Aspens Turn Yellow?
Another compound, Xanthophyll, creates the yellow tones in fall leaves. Going back to our sun and stars metaphor, Xanthophylls might be similar to the moon. Its yellow pigment can stand up to the intensity of the green in chlorophyll and can be seen year-round–similar to how you can often see the moon during the intensity of the day. However, once the chlorophyll is blocked, its golden hue radiates!
Why Do Aspens Turn Orange?
You probably guessed it, the orange is caused by another trapped compound with a funny name! Carotenoids (think carrots) produce and orange pigment–Go Broncos!
Carotenoids are not as strong as xanthophylls, but they are more prevalent than their red cousins, the anthocyanins. That’s why you’ll see more oranges in the mountains than reds when the fall colors carpet the landscape.
Once all the yellows, oranges, and reds are spent, the only thing left are the tannins, which turn the leaves brown. Then comes the white of winter.
The Best Places to View Changing Aspens Near Denver
I’ve written an entire post with the Top 10 Best Fall Hikes Near Denver–check it out. But if you are looking for a quick guide, here are a few of my recommendations.
Drive: Guanella Pass Road to 285 to Kenosha Pass
Trail Ridge Road in Rocky Mountain National Park
Maroon Bells – Worth the Drive
Easy access from Denver; pick up Guanella Pass Road by taking interstate 70 to Georgetown, Colorado. The road winds up to the top of Guanella Pass, where hikers can get out and go for a light stroll, or make the demanding climb up the 14er, Mt. Bierstadt (prepare accordingly). The road continues over the pass and terminates at highway 285 in Grant, Colorado. Taking a right (going West) onto 285 leads to Kenosha Pass about 7.5 miles down the road. Kenosha Pass is arguably the best place to view the changing aspens within 90 minutes of Denver. Check out our Kenosha Pass and Guanella Pass Trail profiles for more details.
Trail Ridge Road is further from Denver, but worth the drive. It will also require drivers to purchase a Rocky Mountain National Park pass. I find it most affordable to buy a yearly pass because I make multiple trips into the park each year. September is a great time to visit the park because the Elk are in rut and bugling. However, that does mean that the traffic in RMNP is high, so I recommend going on a weekday. We have profiled over 50 hikes in Rocky Mountain National Park, so for more info, explore our Rocky Mountain National Park Hiking Trails page.
Maroon Bells is about 4 hours from Denver, and if you have the time, it’s worth taking a long day-trip to witness the breathtaking landscape. There are several hikes around the Maroon Bells area and we’ve profiled them, as well as several campsite options, in our Ultimate Guide to Hiking Maroon Bells. Be sure to take time to read it because there is limited parking, an access fee, and you may need to take the shuttle to access the trailhead.
Thanks goes out to Pacheco for the photos of the orange and red aspens; to Matt Santomarco for his photo of the changing aspens at Mt. Sneffels; Gwendolyn Stansbury for her photo of Trail Ridge Road;and to Alistair Nicol’s photo of the golden stand of aspens.