Labor Day has come and gone, pumpkin spice is wafting through the air, and Halloween candy is everywhere. The autumn season brings many changes, but none are more spectacular than this: billions of leaves are turning 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 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.
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. The map pulls historical data from the National Oceanic and Atmospheric Administration and makes a best guess. Right now, for many areas, the leaves are peaking or are just past their peak color intensity.
(Here’s another easy way to figure out whether the map is accurate for your home: Go outside!)
The colors might be particularly glorious in New England
The intensity and duration of peak fall foliage can vary depending on weather conditions, but the science is a bit tricky. A moderate amount of drought can actually intensify colors, producing more red pigments, as National Geographic explains. But too much drought can hasten browning and cause leaves to fall off earlier than usual.
According to an Associated Press report, the conditions in 2017 hit the sweet spot for colors, particularly in New England. "The biggest thing that can go wrong with foliage is a really wet couple of weeks leading up," Jim Salge, a New England leaf forecaster, told the AP. "We'll really need that typical fall weather in New England — warm, sunny days and cool, crisp nights — to make it pop. But we've had a great setup."
Why do the leaves turn red, orange, or yellow?
When the chlorophyll disappears, the National Arboretum explains, other chemicals in the leaves 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