I thought the foliage this year was pretty weak and I wondered why. In conversations with friends I heard many different theories but no one seemed sure as to what was really going on. So, I did a little research.
There are three factors that influence leaf color: leaf pigment, length of night, and weather. Leaves are green because they have the pigment Chlorophyll which enables plants to make sugars for food. It gets used up but is continually replaced by the tree during the growing season.
Also present in the leaves is a pigment called Carotenoid. This is a yellow / orange pigment which is masked by the Chlorophyll. As night length increases in the fall, Chlorophyll production slows down, stops, and eventually all Chlorophyll is destroyed.
There is another pigment called Anthocyanin which is not present throughout the growing season but is produced in late summer. This pigment gives the reds and purples of foliage and its formation depends on the breakdown of sugars in the presence of bright light.
The brilliance of fall foliage is also directly related to weather conditions occurring before and during the Chlorophyll breakdown. Warm, sunny days and cool but not freezing nights seem to cause the best foliage. The sun allows lots of sugar to be produced in the leaf but the cool nights and the gradual closing of veins leading into the leaf prevent the sugars from moving out.
This is when the reds (Anthocyanin) really shine. The yellow (Carotenoid) is fairly constant because it is always present in the leaf. A warm period during the fall (and cloudy) will lower the intensity of color, which is what I felt happened this year. If anything seemed bright it was the poplars which are yellow and so the yellow being a constant it would stand out in a dull foliage. Also a late spring or a bad summer drought can delay the colors by a few weeks. However, what usually is best for a fantastic foliage is a warm wet spring, nice summer weather, and warm sunny fall days with cool nights.