Falling into Fall - Why Leaves Change Color and Your Foliage Forecast

Pumpkin spice lattes are back at coffee shops, and the Halloween décor has popped into retail locations, which means fall is here. In fact, meteorological fall began on September 1st, and earlier on Sunday the Autumnal Equinox kicked off the astronomical season at 8:44 AM ET. Mother Nature heralds in the tell-tale signs that Autumn has arrived with the changing colors of the leaves.  

But forecasting the changing leaves is a difficult task with variables that can sometimes be difficult to track.  Weather plays an important role in the timing of when leaves will change, and while some locations have seasonal averages for peak foliage, nature has the final say on when the brilliant colors will arrive. 

WHY LEAVES CHANGE COLOR: 

The change in season sparks the transformation within deciduous trees. Deciduous trees typically have large, broad leaves and use flowers to reproduce; think your oak and maple types. These trees get the deciduous classification from the Latin word “dēciduus” which means “to fall” because of their tendency to drop their leaves once they reach maturity.  

Coniferous trees are your evergreen trees. These trees tend to have needles or scales and reproduce using cones. Coniferous trees typically hold their needles year-round as they help with water conservation for the tree to continue producing food for itself during the long winter and summer months. 

Because of the deciduous tree's leaf shape, they are the ones that transform into brilliant plumes of color as the days get shorter.  

So, what’s sparking the change? 

According to the Smithsonian, in fall, the days start getting shorter, meaning less sunlight, and in turn, cooler temperatures. This signals the trees to end their food making process. The chlorophyll within the leaves that helps conduct photosynthesis begins to fade allowing other chemicals within the leaves to shine.  

WHAT DO FORECASTS LOOK FOR: 

“The ideal weather we’re looking for is sunny, warm afternoons, and clear, cold nights,” said Evan Fisher, the founder of ExploreFall.com. Too much drought or rain can vastly impact the colors produced by the trees.   

Heat can induce stress on the trees and allow them to show color early. Fisher said maples are susceptible to stress, such as a warm summer, like many along the Eastern Seaboard have seen in the 2024 summer season.  

“The hardest part of predicting fall color is accounting for the extremes.” Fisher elaborated. “[Drought] that can do crazy things to the fall color. If you have a little bit of drought it may delay the color. If you have a lot of drought, it may accelerate it. But we don’t really know exactly where that line is exactly in between. And sometimes the same goes for really moist conditions.”  

Another important factor for forecasting foliage is the amount of daylight too. “I like to say that daylight opens the door for fall colors. Once the days get short enough and the temperatures are cool enough, we start to see the colors progress,” Fisher said. 

You can learn more about forecasting fall colors below: 

WHY ARE THEY DIFFERENT COLORS:  

The color each tree presents depends on what’s present in the leaves. According to the State University of New York’s College of Environmental Science and Forestry, xanthophyll and carotenes are already present in leaves as the green chlorophyll fades away. These pigments give leaves yellow and orange coloring, similar to how they color other plants such as carrots. 

Other chemical reactions can happen at the same time as the fading green color which produces anthocyanin pigments, which give leaves a reddish and purplish color. The Forestry Services of Scotland says if temperatures stay above freezing, more of these anthocyanin pigments are produced. If autumn remains sunny and dry, even more trees will begin to produce pigments to give even more red color. The process is slow though, which is why you sometimes see trees with a mix of greens, yellows, and reds all at once. 

WHY DO LEAVES DROP: 

In the spring, trees create little tubules between the leaves and the branches to transport nutrients back and forth between the leaves. As the fall creeps in, the tree begins producing a hormone that builds up a waxy layer between the leaf and the branch that reduces nutrient transportation. Eventually the leaf will be weak enough to be carried away by the wind, rain, or snow, said Michael Davidson on his Molecular Expressions website via Florida State University.  

By dropping their leaves, deciduous trees will spend less energy through the winter and converse moisture throughout their systems. The dropping of their leaves also prevents external stress that can occur when heavy snow weighs down a tree’s branches.  

THE FORECAST AHEAD:

For the Week of September 19 - September 26, 2024, the foliage map highlights some areas across the country that will be reaching peak color.

This includes the Rocky Mountains of Colorado, where some of the best spots for leaf-peeping those aspens will wrap up this upcoming week. So get out, be safe, and enjoy!

For the latest foliage forecast, be sure to join us live on WeatherNation!