SALT LAKE CITY — The first hints of fall color are starting to show up across Utah’s mountains and canyons, signaling that autumn is just around the corner.
But what actually tells a leaf it’s time to change?
According to Katie Wagner, a horticulture faculty member with Utah State University Extension, the biggest trigger is something that doesn’t change much from year to year: daylight.
“The initial trigger is shorter days and longer nights,” Wagner said.
As daylight decreases, trees begin slowing their production of chlorophyll — the pigment responsible for giving leaves their green color.
Once that green pigment begins to fade, other pigments become more visible, producing the yellow, orange, red and purple colors we associate with fall.
“So, it's carotenoids, which are your yellows. And then anthocyanins are kind of our reds and our purples,” Wagner said.
But while the timing of the color change is largely controlled by daylight, the weather can influence just how vibrant the display becomes. The ideal conditions for brilliant fall color include warm, sunny autumn days and cool nights — but not freezing temperatures.
Moisture during the growing season also plays an important role. Utah received some monsoon moisture later this summer, but that came after a prolonged stretch of heat and drought. Wagner says that could mean somewhat less vibrant fall color across parts of the state this year.
“Although we have had some monsoonal storms come through, we did have kind of a prolonged period of heat and drought earlier in the year," she explained. "And so it's gonna cause us to have a little bit less vibrant color this year."
The summer stress can also show up as brown or scorched edges on leaves. During drought, it becomes more difficult for trees to move enough moisture all the way through the leaf.
That doesn't mean Utah won't have beautiful fall colors. Local conditions can vary considerably from one canyon or mountain range to another.
For people enjoying the outdoors, the changing leaves are also part of what makes fall a favorite time of year. Jackson Burton, an avid mountain biker, says the changing aspen leaves make the season especially enjoyable.
“We're like big mountain bikers, so the aspen leaves, mountain biking is the best time of year,” Burton said.
So as the leaves begin changing across Utah, take a closer look. That colorful display is more than just a sign of the seasons — it’s the result of changing daylight, plant chemistry, and the weather conditions the trees experienced throughout the year.
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