In the Pittsburgh corner of Pennsylvania, the first few days of autumn have been perfect. Dry sunny skies carried the region from summer into fall.
But the great recent weather is not great news for fans of colorful fall foliage. Despite predictions of a rainy late summer and early fall, the absence of substantial rain may cause a premature dormancy in some trees, resulting in brown leaves that could drop early.
Weather and its effect on temperature, precipitation and soil moisture have some impact on leaf coloration. But the main factor that dictates autumn leaf color is light. The Earth’s wobbly orbit results in seasons brought on by the changing duration of daylight in a day.
After the summer solstice in June, each day grows a few minutes shorter in the Northern Hemisphere. Waning periods of daylight signal many deciduous tree species to begin preparing for winter. Step 1 is the development of corky cellular walls that grow between twigs from their leaf stalks. The walls block the supply of water and nutrients halting the production of chlorophyll, a green pigment that helps plants to absorb and process sunlight. The green colors are dominant in leaves throughout spring and summer. When the chlorophyll dies, other pigments — anthocyanin (reds) and carotene (yellows) become exposed.
Autumn leaves don’t actually “turn” colorful in the fall. The colors were there all along, but were obscured by the green pigment.
“Weather simply influences the vividness of the color and how quickly the leaves fall off the trees,” stated George and Becky Lohmiller in a 2018 feature article in The Old Farmer’s Almanac, a steady source for weather information since the early 1800s.
The current edition predicts, “[Autumn] precipitation will be above normal from the Deep South and Southeast northeastward to New England.” In the Ohio Valley including Pittsburgh, it says, “September and October will be warmer and rainier than normal.”
So far, all we’ve seen are a few scattered morning showers, not even enough to threaten an overflow in the Parkway’s Bathtub depression.
“It has been dry recently,” said Glen Bupp, master gardener coordinator for the Penn State Extension’s Pittsburgh office. “If it continues it could affect leaf color. Whether we’ll have future rain, who knows?”
As those corky walls block the leaf’s intake of nutrients, they also block the exit of fluids, trapping simple sugars within the leaves.
“Sugar trapped in autumn leaves by the corky wall [the abscission layer] is largely responsible for the vivid color,” said the Lohmillers. “Some additional anthocyanins are also manufactured by sunlight acting on the trapped sugar. This is why the foliage is so sparkling after several bright fall days and more pastel during rainy spells.”
Wet growing seasons followed by dry sunny days and cold frostless nights generally result in the most vibrant autumn leaf colors, especially in trees with red leaves. In southwestern Pennsylvania that includes second-growth forests of sugar maples and red maples.
“The browns in autumn leaves are the result of tannin, a chemical that exists in many leaves, especially oaks,” said the Lohmillers. “... Drought conditions during late summer and early fall can trigger an early ‘shutdown’ of trees as they prepare for winter, causing leaves to fall early from trees without reaching their full color potential.”
Mr. Bupp said wet weather helps the leaves to hang on a little bit longer.
“But if it gets wetter,” he said, “we could see some fungal issues that hasten the leaves’ dropping, too.”
Peak autumn foliage in southwestern Pennsylvania was expected Oct. 5-21, but it may be already too late. Without some good dousing rains in the next couple of weeks, fall colors could flop.
“It’s funny, we just had this conversation the other day among office staff,” said Mr. Bupp. “The truth is, we’re just not sure. But if I had to guess I’d say the second week of October going into the third week will still be the best time to see fall foliage, regardless of whether it will be a spectacular year.”
If leaves drop prematurely in our corner of the state, however, drive a couple of hours for your fall leaf-peeping fix. Recent weeks of warm dry days and cool nights in New England could result in bright bold colors as far south as New York. The Appalachian Mountains to the southeast and east of Pittsburgh were spared a late-summer drought. Expect bright and long-lasting colors from Chestnut Ridge to the Blue Mountains.
By John Hayes: 412-263-1991, jhayes@post-gazette.com.
First Published: September 26, 2019, 2:37 p.m.
Updated: September 26, 2019, 3:16 p.m.
