Q. When can we expect the peak of fall foliage color this year?
A. The quick answer is perhaps a tad earlier this season.
Fall color is controlled by the plant's genetic factors and the environment. Carotenes and xanthophylls are yellow pigments produced in foliage all year, along with chlorophyll, the green pigment. In autumn when short days and cool temperatures slow chlorophyll production, the remaining chlorophyll breaks down and disappears. Then, yellow pigments previously masked by chlorophyll will show, giving ginkgo its clear yellow color. Redbud, larch, hickory, birch, and witch hazel turn hues of yellow and gold.
Some plants produce anthocyanins (red and purple pigments) which can overpower yellow pigments. Some maples, dogwood, black tupelo, oaks and burning bush seem to be on fire with reds and purples.
Anthocyanin production increases with increased sugar in the leaves. A fall season with sunny days and cool nights increases sugar content of the leaves and intensifies fall reds. This also explains the two-tone effect on green ash which exhibits yellow on leaves inside the tree and purple on the outside leaves where they are exposed to sunlight. It also explains the serviceberry tree, which may be red on top and yellow on the bottom. Clear days, cool nights, and dry conditions promote high quality fall color.
These colorful displays are often cut short by heavy winds, rainfall, and freezing temperatures, which can actually kill leaf tissue and cause the leaves to fall early. The tans and browns of oaks are caused by tannins, which accumulate as the chlorophyll disappears.
The fall color forecast depends on the weather. Part of the reason the coloring of the leaves could vary is because much of the country is in a drought. Colors may come in earlier because the leaves are drying out more quickly than in a typical year.
Drought-stressed trees can turn color more quickly compared with years when precipitation is more “normal” So, instead of mid-October, leaves may have begun to turn toward the end of September. If the weather remains cool, fall color may come early and bright but will be short-lived. On the other hand, if temperatures stay warmer, leaves will dry out in a slow display where the colors never quite get as bright, they're a little longer-lived, and then they fall to the ground in stages, so it doesn't create quite as spectacular a display.
A fall color prediction map was developed by SmokyMountains.com co-founders David Angotti and Wes Melton using publicly available data to predict when peak fall would happen, county by county, for the entire United States.
Each year, Angotti says, their model gets more and more accurate as they work out kinks in the system, which is based which is based on meteorology and other data. This year they predict peak fall color for northern Indiana began Saturday and will last until Oct. 20.
Frost and freezing temperatures will ultimately stop the coloration process and blacken the leaves.
The Plant Medic, written by Ricky Kemery, appears every other Tuesday. Kemery retired as the extension educator for horticulture at the Allen County Purdue Extension Service. To send him a question, email firstname.lastname@example.org.