Philadelphia's trees are sending out a distress signal this fall as drought and heat are forcing them to shed their leaves weeks ahead of schedule – and potentially with less vibrant colors.

According to data from the National Oceanic and Atmospheric Administration's Drought.gov website, a large portion of Philadelphia County experienced moderate drought in July and drier-than-normal conditions in June and August.

As a result, experts at Morris Arboretum & Gardens report that the city's trees are turning color two to three weeks earlier than normal – an early transition that not only shortens the fall foliage season but may also result in duller leaf colors.

“The short answer is, it's just the drought, you know, it's just been so dry,” said Vince Marrocco, horticulture director at Morris Arboretum & Gardens. “If you have sunny days, cool nights, but no moisture, the plants have to make a decision and it's basically sort of like a risk, reward analysis. If I keep holding on to my leaves, are they going to produce enough energy to offset the amount of effort and water I have to put into making the process go? And  because we're in such a drought deficit, a lot of plants are deciding, nope, 2024 is done.” 

Marrocco explains the process: "When this process starts, an abscission layer forms between the branch and the leaves. Once the abscission layer forms, additional rainfall like this past weekend won't extend how long the leaves stay green."

The early transition is likely to affect the vibrancy of fall colors, particularly the reds. "When you get drought stress on plants, they typically turn yellow and drop. You don't get bright reds," Marrocco said. Some leaves, he said, may dry up quickly and turn a "dirty green and brown" without ever changing to more vibrant hues.

However, not all trees will be equally affected. Native species like oaks and maples are more likely to maintain their typical fall colors, Morocco said, and may still turn a more reddish hue. 

"After millions of years, they've seen droughts come and go in the fall, and they kind of know how to behave," Marrocco explained. "It's the non-native plants that tend to cover up a little earlier and that's kind of what we're seeing."

Older trees are particularly vulnerable to drought stress, which can shorten their lifespan. Marrocco advises residents to water older trees during drought months to help protect them. At the arboretum, staff go a step further by irrigating older trees in their care.

"Trees are generally super efficient at everything they do. They don't waste energy doing anything," Marrocco said. "At the end of the season, as that abscission layer is forming in the leaves, they break down all the chemicals in the leaf and extract them into the plant. They're saving all those chemicals and all the sugars and everything to store it up for next year, when you have a drought like we're having this fall there's a certain point for the plant to realize, if we don't do something soon, we're going to start losing a lot of energy."

The effects of drought stress on older trees often become visible the following year. "You think, what happened to my tree this year? Really, the answer is; it was the drought the previous year that caused the issue," Marrocco explained.

Marrocco notes that while fall has been coming later in recent years, this year is different, and notably drier. So while residents can still expect some decent color this fall, some trees will be noticeably duller.