It’s pretty much de rigueur for New England news outlets to run some sort of “ever wonder why leaves change color each fall?” story each fall.
Since this is a blog, I don’t have to write it – I can steal it! There are plenty of places to swipe from; I like the explanation from the National Arboretum, which you can read here.
Like most plants, deciduous trees and shrubs are rather sensitive to length of the dark period each day. When nights reach a threshold value and are long enough, the cells near the juncture of the leaf and the stem divide rapidly, but they do not expand. This abscission layer is a corky layer of cells that slowly begins to block transport of materials such as carbohydrates from the leaf to the branch. It also blocks the flow of minerals from the roots into the leaves. Because the starting time of the whole process is dependent on night length, fall colors appear at about the same time each year in a given location, whether temperatures are cooler or warmer than normal.
Yes, so the annual freaking out about “when will it start, when will it be peak?!?!” is mostly pointless. Timing is largely dependent on daylight, which doesn’t change. What does change is weather which can alter the brightness or amount of change – heavy rain knocks leaves off trees; dreary days make the leaves look dreary no matter their chemistry.
Want more numbers? Check out my piece from last year calculating the number of leaves that change color in New Hampshire each fall. (607 trillion, if you’re curious.)