As summer gives way to fall, the green leaves blanketing Penn’s Woods change into vibrant reds, oranges and yellows before they drop to the ground for the winter.
The process is scientific, not spontaneous. The colors are specific to species. And the diversity of Pennsylvania’s native trees makes the state one of the best places in the world to experience the splendor of autumn.
Like bears finding burrows to hunker down, the trees are preparing for colder days.
"The driving force behind all of this is the plants are getting ready for winter, days are getting shorter … they don't want their leaves to freeze. So they're going to shut down. In that shutting down process, we're getting I'd say, an explosion of colors, right?” said Vince Cotrone, Penn State Extension Educator and Regional Urban Forester.
Pennsylvania is blessed, said Julian Maza, service forester for the Pinchot Forest District with the state’s Department of Conservation and Natural Resources (DCNR) Bureau of Forestry.
"We have such a diverse forest," he said. "There's over 130 different species across the state, and again, each one of those, they have their own slightly different kind of color variations, different hues. They're going to change colors at different times of the year,” he said.
From green to red, orange and yellow
All plants have chlorophyll, which makes leaves green. It allows plants to absorb energy from the sun and creates food.
"The day length gets shorter, the plants slow down, they produce less chlorophyll," Cotrone said.
When chlorophyll production stops or diminishes, different colored pigments, which build up because of sugars in the plants, start to take over. That process guarantees the leaves will change color.
Carotenoids bring out yellows. Anthocyanins produce reds and purples.
Maza said the peak week for fall foliage typically stays the same from year to year — again because the length of the days is the major factor.
"What's going to affect the fall colors and how vibrant they are, or how long the peak lasts, is really weather-dependent," Maza said.
Wet springs and summers can cause fungal diseases on the leaves which then affect how they look. A drought in the summer causes the leaves to turn then drop off trees quickly.
The petiole is the small stem that connects the leaves to the trees. Cotrone said when a tree is stressed one of its defense mechanisms is to plug up the petiole and shed the leaves.
"But the prediction is tough in a sense ... We're always going to get fall color. It just depends on, you know, have the insects eaten all the leaves off? Has drought taken the leaves in August?” he said. “That wasn't the case this year.”
Perfect timing
Finding the most vibrant fall foliage comes down to timing.
"Go up to Susquehanna County, go to the Northern Tier, drive Route 6 in early October, and you will see some wonderful colors,” Cotrone said.
The timing also has to do with the types of forests in Pennsylvania.
Different parts of the state are home to different species of trees.
For example, the state’s Northern Tier has northern hardwood forests — sugar maples, red maple, birch, beech, black cherry, tulip poplar.
"And naturally they kind of turn color a little bit quicker than the other ones do,” Maza said.
Maza said there’s a dividing line in the middle of Lackawanna County, where hardwood forests become more oak and hickory trees.
"In the northern part of the county, it's going to be early [with] those northern hardwood forests, and the southern part of the Lackawanna County area could be completely green still from all the oaks,” he said.
How those species spread goes back to the glacial period. The glaciers were full of seeds left behind as they melted.
"Really the driving factor is just the parent material that's there, so it's the soils that are present that in those areas," Maza said, "that those trees favor those types of soils."
Some trees like the limestone soils in the north, others grow better in rocky acidic soils in the south.
Weather woes
The trees are healthy this year, Cotrone said. But the days need to stay dry and sunny and the nights cool for prime fall foliage.
Cotrone said there is something that could derail the fall foliage season altogether — a windy, powerful storm that knocks all the leaves off the trees.
"If you're starting to get fall colors and then all of a sudden you get wind and rain — man, what could have been a nice show for a week-and-a-half turns into a two-day show,” he said.
But, Cotrone said, nature is resilient.
'A huge economic driver'
Fall drives a huge portion of Pennsylvania’s tourism industry. The season generated $83.9 billion in economic impact, supported more than 514,000 jobs, and welcomed 201.6 million visitors in 2024, according to the Pennsylvania Tourism Office.
Beginning on Oct. 1, DCNR will resume its weekly fall foliage reports each Thursday, which include regional updates, photos and expert tips.
The Pocono Mountains Visitors Bureau Poconos also has an online Fall Foliage Forecast.
Beginning on Oct. 1, DCNR will resume its weekly fall foliage reports each Thursday, which include regional updates, photos and expert tips.
Maza is the coordinator of the DCNR report for the 54,000 acre-Pinchot State Forest district in Lackawanna, Luzerne, Wyoming Susquehanna and Wayne counties.
The Pocono Mountains Visitors Bureau also has an online Fall Foliage Forecast, which is also updated every Thursday through the end of October.
Maza said one major use of state forest land is just people driving through to see the fall foliage.
"It's a huge economic driver, really for recreation in the area. So, it is really important,” he said.
Cotrone said more communities are tapping into the splendor of fall by hosting festivals connected to the fall foliage.
He said people come in busloads to Wellsboro and the Grand Canyon of Pennsylvania in Tioga County to see the leaves. Cotrone even suggested that the best fall foliage is sometimes in a backyard.
"You don't need to spend gobs of money on gas this year going to Boston or Vermont," he said. "Stay in Pennsylvania, spend your money in Pennsylvania."
