The how and why of fall color

Dec 21, 2015
Why do leaves change color in the fall? Why do fall colors some years seem brighter and bolder than others? Get some insights from our weather expert on-staff, Zak Boone.

Ever wonder exactly why leaves change color in the fall? And why are some years better for fall color than others? Our amateur meteorologist on staff, Zak Boone, has the answers:

Q: Why do leaves change color in the fall?

A: As trees grow through the spring and summer months, chlorophyll is constantly replaced in the leaves and gives them their green color. As the nights get longer in the late summer on the high desert, the cells near the stem form a layer that blocks the the transportation of materials from the leaf to the branch and from the roots to the leaves. As the chlorophyll is slowly blocked from the leaves, it begins to disappear. The lack of chlorophyll allows the yellow and orange pigments to be visible, while the red and purple pigments come from sugars which get trapped in the leaf once the transport system closes down. These pigments become the vivid color in the fall and vary by species of tree. 

Q: Why are some years better for fall color than others?

A: Weather plays a huge role in the vibrancy of fall color. Many factors affect how the leaves will look in the late summer and early fall, especially oncoming freezing temperatures in Central Oregon. Here are a few things that directly affect the splendor of fall color in central Oregon:

  • Cool temperatures (especially at night) with lots of daytime sunshine promote the formation of more red and purple pigments, especially in ash and maple trees.
  • Freezing conditions destroy the leaf's ability to manufacture the red and purple pigments. Early frost will end the colorful foliage for most species, though aspen and larch are hardy enough to continue to hold color beyond the first freeze.
  • Drought during the growing season can cause the leaves to drop before they change color.
  • Heavy wind (or an early snow) can cause the leaves to fall before they fully develop color.

The best weather for brilliant fall foliage in central Oregon is a growing season with ample moisture followed by a dry, sunny late summer/early fall with warm days and cool, but not frosty, nights. In other words, this year is shaping up be a good one!