The rainy summer could make for glorious New England foliage, but leaf-lovers may have to wait a little longer for the show to begin, experts say.
Wet and humid conditions in July and August have primed trees to burst with red, yellow, and orange hues, said Richard Primack, a professor of biology and plant ecology at Boston University. But warm temperatures and rain in September are pushing back the day the leaves will turn.
“I can give you a strong assurance that it’s going to be a good year for fall foliage, but a late year,” Primack said.
By experts’ estimates, peak foliage season will be mid-October.
The Massachusetts Office of Travel & Tourism website predicts that the Greater Boston area will experience peak season between Oct. 1 and 14. The majority of the state can expect the brightest hues between Oct. 5 and 18.
In 2020, peak foliage was evident in Berkshire County, the Pioneer Valley, and Worcester County in early October, the Globe reported. Further north, Maine began seeing color changes as early as Sept. 16.
Foliage appears brightest when trees have had sufficient precipitation during the spring and summer, followed by cool, crisp temperatures in the fall, according to AccuWeather senior meteorologist Paul Walker.
“A warm, wet summer produces more growth in the leaf,” Walker said. “They need that moisture to make the bright colors.”
Primack said the trees are healthy and full now from that wet season. They will not shed their green tint, fueled by chlorophyll, until they lose moisture. That’s why the drought in 2020 caused trees to shed early and quickly.
If it gets sunnier and cooler now, the leaves will be well-positioned to turn, added Andrew Gapinski, the director of horticulture at the Arnold Arboretum of Harvard University.
Foliage is “driven by the days shortening,” he said. “That’s the switch that turns it on and off. … But temperatures in the 60s or 70s and cloudless days are also helpful contributing factors.”
The brightness of the colors, Gapinski said, relies on the coming weather. “If we start drying out today, I’d say we can expect a really vibrant fall,” he said. “If it’s cloudy and warm, the colors will be more muted.”
To date, the average temperature in September is 70.6 degrees, according to the National Weather Service.
Climate change plays a part in the delay, too, Primack said.
Frost encourages trees to undergo autumnal senescence. Then the chlorophyll deteriorates, making way for pigments like anthocyanin to create red and orange colors. Those same pigments are responsible for a pumpkin’s orange skin and a sunflower’s bright petals.
Year after year, the warming of the world nudges the first frost later. In recent years, Primack said it occurred in the beginning of October.
“There’s still potential for the trees to put on a great show in the fall,” Gapinski said.
The foliage season typically ends near the first hard frost, when both the ground and air reach freezing temperatures, Primack said. Strong winds could also strip leaves off trees earlier than expected.
Nicole Keleher, the forest health director at the Massachusetts Department of Conservation and Recreation, said she is expecting a shorter season of colors this year — around two weeks.
A longer season requires healthy leaf tissue. And the intense summer rains stressed trees statewide, leading to insect damage and fungal pathogens, Keleher said. Trees like oak could clutch onto their leaves through November, though, after they have browned.
The season is “not always as long. It’s not always as vibrant as other years,” Keleher said. “But there’s still so many different types of trees out there. It’s always great to see.”