Labor Day is nearing, pumpkin spice is about to be pumped into countless hot beverages (and, for the first time, into Spam?!), and some grocery store shelves are already being stocked with Halloween candy. The autumn season brings many changes, but none are more spectacular than this: Billions of leaves will turn from green to auburn and gold.
When days begin to grow shorter, deciduous (green leafy) trees start signaling their leaves to stop producing chlorophyll, the green pigment responsible for the leaves’ color and photosynthesis.
Because the color change is more dependent on light than temperature, it takes place at basically the same time year after year, according to the US National Arboretum.
Temperature and weather conditions, though, can impact the intensity of fall colors and how long they linger. They can also subtly affect the timing of when the leaves start to change. And drought can change the rate at which the leaves turn.
Because of all the variables at play, it can be tough to predict precisely when fall colors will peak, and how long they’ll last, in a particular area. But here’s an admirable effort:
The website SmokyMountains.com (a site promoting Smoky Mountains tourism) created this interactive map to determine peak fall colors across the United States by county. You can slide the bar at the bottom of the map to see how the peak foliage spreads out across the country over the coming weeks. Use it to figure out the right time to do some leaf-peeping in your area or on a weekend trip.
The map pulls historical data and seasonal forecast predictions from the National Oceanic and Atmospheric Administration and makes a best guess about peak timing. “Although the scientific concept of how leaves change colors is fairly simple, predicting the precise moment the event will occur is extremely challenging,” Wes Melton, the chief technology officer of SmokyMountains.com, says in a press statement. So the predictions may not be 100 percent accurate. (Here’s another easy way to figure out whether the map is accurate for your home: Go outside!)
Why do the leaves turn red, orange, or yellow?
When the chlorophyll disappears from the leaves, the National Arboretum explains, other chemicals persist and show their colors:
Chlorophyll normally masks the yellow pigments known as xanthophylls and the orange pigments called carotenoids — both then become visible when the green chlorophyll is gone. These colors are present in the leaf throughout the growing season. Red and purple pigments come from anthocyanins. In the fall anthocyanins are manufactured from the sugars that are trapped in the leaf. In most plants anthocyanins are typically not present during the growing season.
Different trees will reveal different colors, as the US Forest Service describes on its website. The leaves of oak trees, for example, turn reddish brown or russet. Here are a few others:
- Hickories: golden bronze
- Aspen and yellow-poplar: golden yellow
- Dogwood: purplish red
- Beech: light tan
- Sourwood and black tupelo: crimson
The color of maples leaves differ species by species:
- Red maple: brilliant scarlet
- Sugar maple: orange-red
- Black maple: glowing yellow
- Striped maple: almost colorless