It's that time of year, and trees are gorgeous! Why do green leaves turn yellow, red and orange.
Actually, the colors are always there. They're just masked by the green chlorophyll in leaves, which is busy making food by photosynthesis while the sun shines.
Come autumn, shorter days and cooler temperatures cause trees to switch into energy-storage mode, at which point their leaves stop producing chlorophyll. For the few weeks before the leaves fall to the ground, they are colored only by their natural pigments. It's these colors — red and purple anthocyanins, yellow and orange carotenoids — that make fall foliage in four-season climates so glorious.
Of course, some years the show is more dramatic than others. The best conditions for intense leaf color to develop are dry, sunny days followed by cool (but not freezing) nights.
A warm, wet autumn will almost surely result in less-than-spectacular foliage because the process of chlorophyll loss will be less consistent. Freezing temperatures, meanwhile, can cause leaves to drop suddenly, denying plants the opportunity to enter their slow, colorful dormancy.
Finally, trees that are under stress — because of pests, disease, injury or drought — may drop their leaves with no color change at all.
