Fall is one of my favorite seasons. We finally get some relief from the oppressive summer heat, and the cooler weather brings a new palate of fall colors. This is also the time of year that drivers struggle to keep their eyes on the road and off the brilliant autumn hues that drape the roadsides.

Japanese maple

Many Japanese maples are a favorite for red fall color.

Photo Credit: Bryce H. Lane

Red maple

‘Red Sunset’ red maple shows how anthocyanin pigments can produce both red and purple leaves.

Photo Credit: Bryce H. Lane

Sassafrass

Check out the bright oranges and yellows produced by pigments in this sassafras.

Photo Credit: Bryce H. Lane

Sugar maple

Sugar maples are a winner when it comes to displaying soft, consistent yellow color in fall.

Photo Credit: Bryce H. Lane

One neat thing about fall is that every year it looks different: Sometimes the seasonal transition is earlier; sometimes it’s later. Sometimes it’s spectacular; other years it’s slightly less impressive. There are lots of factors that weigh into how colorful fall will be, but the main one is weather – that’s what determines when leaves will change, as well as if a given year’s autumn show will be something worth talking about.

Leaves contain pigments that visually tell us that a tree is undergoing a transition. In the fall, we observe this transition by watching the leaves change color. A given season’s weather determines how much pigment we see. Here’s the neat little science lesson behind it all:

Leaves are green thanks to the presence of chlorophyll, a green pigment that’s necessary for plants to produce food. As temperatures decrease in fall, chlorophyll production slows. As this happens, the green fades, allowing other pigments – usually hidden by the chlorophyll – to show through. Just like “chlorophyll” is the name for “green pigment,” the other pigments have “fancy” names, too. “Carotenoids” are the yellow and orange pigments present in leaves (easy to remember because “carrots” are orange). “Anthocyanins” are the purples and reds. Weather conditions actually determine the presence of these pigments and the intensity of the color each year.

“What types of weather conditions,” you ask? Well, fall’s lower temperatures and shorter days stimulate the formation of the abscission layer between the leaf and the branch. This is what causes leaves to fall off trees. The formation of this layer slows the flow of sugars in and out of the leaves. Warm days and cool, crisp nights promote good sugar production. Since the abscission layer slows the exit of sugars, they build up in the leaves – which is a good thing if you want a colorful fall.

Nights below 45 degrees F combined with warm, bright days stimulate the formation of red and purple pigments, but only when there’s extra sugar in the leaf. If you’ve got nights below 45 degrees, but your days aren’t bright and warm, fall’s gorgeous reds will be muted – or not even there. The reason the mountains are notorious for brilliant fall color is that they have the ideal climate for producing the anthocyanin pigments that make autumn so “warm.” So just remember: warm, bright days + many nights below 45 degrees F = amazing fall color!

Unfortunately, not all trees and shrubs are able to produce red and purple pigments. But the Eastern seaboard is blessed with many species that produce some of the most brilliant colors, making it one of the fall tourist destinations. Some common Eastern thrillers are red maple, sugar maple, sassafras, sumac, blackgum, sweetgum, scarlet oak, sourwood, Japanese maple and dogwood.

So, what will your autumn color be like this year? Turn to Mother Nature and take a look outside. If you’ve got cool nights and warm, sunny days, you’re likely to fall into something amazing!