Cool Nights and Sunny Days Needed For Colorful Foliage

Weather factors such as temperature, sunlight, precipitation and soil moisture influence fall color arrival, duration and vibrancy. According to United States National Arboretum, a wet growing season followed by a dry autumn filled with sunny days and cool, frostless nights results in the brightest palette of fall colors. Changes in weather can speed up, slow down or change the arrival time of fall’s colorful foliage. For example:

• Drought conditions during late summer and early fall can trigger an early “shutdown” of trees as they prepare for winter. This causes leaves to fall early from trees without reaching their full color potential.
• Freezing temperatures and hard frosts can kill the processes within a leaf and lead to poor fall color and an early separation from a tree.

Trees actually begin to show their true colors in autumn, and here’s why. 
The four primary pigments that produce color within a leaf are: chlorophyll (green); xanthophylls (yellow); carotenoids (orange); and anthocyanins (reds and purples). During the warmer growing seasons, leaves produce chlorophyll to help plants create energy from light. The green pigment becomes dominant and masks the other pigments. 

Trees must replenish the chlorophyll because sunlight causes it to fade over time. As days get shorter and nights become longer, trees prepare for winter and the next growing season by blocking off flow to and from a leaf’s stem. This process stops green chlorophyll from being replenished and causes the leaf’s green color to fade.The fading green allows a leaf’s true colors to emerge, producing the dazzling array of orange, yellow, red and purple pigments we refer to as fall foliage.