UNIVERSITY PARK, Pa. -- Hot autumn days and mild nights -- many wish for Indian summer this time of year. But if you want a bright, colorful foliage display in October, you shouldn't, according to an expert in Penn State's College of Agricultural Sciences.

"Weather in the next month or so is very important," says Marc Abrams, professor of forest ecology and physiology. "We need to have cool temperatures falling into the high 30s or 40s at night, as well as bright sunny days with little rain."

If temperatures cool over the next four weeks with little rain falling, fall foliage watchers in Pennsylvania should be treated to a superior display of color this year, Abrams predicts. "If we cross our fingers and Mother Nature cooperates, we should have good color by the middle of October," he says.

For 15 years, Abrams has studied how seasonal precipitation and temperature influence timing and intensity of fall colors in central Pennsylvania. "We believe that clear, bright days, low -- but not freezing -- temperatures and dry conditions promote the best fall colors," he says.

Cooler temperatures signal deciduous trees to stop producing chlorophyll, the green pigment responsible for photosynthesis, he explains. The chlorophyll breaks down and disappears, unmasking other leaf pigments. It's these other pigments -- called xanthophylls and carotenes -- that create the yellows and oranges seen in the leaves of yellow poplar, hickory, sycamore, honey locust, birch, beech and certain maples.

After chlorophyll production stops, trees also produce another pigment in their leaves called anthocyanin, Abrams explains. The anthocyanins create the brilliant reds and purples seen in maple, sassafras, sumac, black gum and purple oak.

The amount of anthocyanin produced each year is related to starch levels in the tree. Trees often produce less starch during droughts. Much of Pennsylvania experienced a significant drought this summer, but Abrams doesn't believe most trees were stressed enough to significantly impact fall colors.

This year's drought, however, may cause some highly stressed trees to change color earlier than normal or to display rather dull coloration. Overall, there is still a high potential for most trees to show their true, vibrant colors this year, Abrams predicts.

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EDITORS: Marc Abrams can be contacted at 814-865-4901 or e-mail agl@psu.edu.

Contact:

Jeff Mulhollem jjm29@psu.edu 814-863-2719 814-865-1068 fax