In the lower deserts of Arizona, it’s open to reasonable debate as to whether we have a legitimate, four-season climate. Winters are mild, summers are brutal, and spring is wedged somewhere in between, able to swing one way or the other, depending on the year. Autumn, however, is more difficult to grasp, and therefore the one we miss the most. Come mid-October, we know something has changed: the sky is a deeper blue, the nights are longer, the days cooler, and it seems that we’re pulling down the brims of our hats and lowering our cars’ sun visors a little more each day.
But these restrained autumn subtleties aren’t enough to satisfy our collective lust for a meaner and cleaner, seasonal transition. We need the equivalent of a stronger drink. We want to see russets and golds waving from the trees, and feel the crunch of fallen leaves under the soles of our shoes. We want rain and the smell of leaf mold forming from thick layers of decomposing leaves, all moist and sour. And then, we want a stiff, cool breeze to blow away all memories of summer.
We can book a flight in October to see the oaks, basswoods, and maples near Montpelier, Vermont, or drive a few hours to the much closer aspens on Escudilla Mountain above Nutrioso, Arizona. Even better, we can do both and still have plenty of time to take in the late November color of Chinese pistachios, Arizona ash, and willows here at Boyce Thompson Arboretum. But wherever we travel to steep ourselves in the color and transitional power of autumn, the science of why all of these leaves morph from a pastoral green to those of a day-glow Thomas Kinkade painting remains the same.
There are three basic pigments in leaves. Carotenoids produce the yellows, oranges, and browns and are found in all leaves. Anthocyanins produce the reds and the purples but require bright sunlight to be produced. Chlorophyll is the green pigment that is necessary for photosynthesis and effectively masks the other pigments during the growing season.
As days grow shorter in the fall, chlorophyll production begins to slow and eventually stops. It gradually breaks down and allows the other, more colorful pigments to prevail. Though the calendar remains constant, a recipe of other factors, including cool nights, sunny days, adequate soil moisture, and high leaf sugar content, must also be whipped together for a fall color frappé—even then, the timing of when the best color will happen can be difficult to predict.
Generally, the peak color at the Arboretum is sometime within a three-week period between mid-November and early December. By celebrating our Fall Foliage Finale Festival on Thanksgiving weekend, we usually get the timing just about right. And though most of our honey locusts have dropped their leaves by then, pomegranates, canyon hackberries, willows, pecans, and our famous grove of scarlet-orange Chinese pistachios will (most likely) be in their full regalia.