Poison ivy is expected to be one of the big winners in a warming world

More than a decade ago, when Peter Barron began removing poison ivy for a living, he decided to document his work.

“Every year I always take pictures of poison ivy in bloom,” said Barron, known as Peskey Pete, of poison ivy removal company Peskey Pete.

He still remembers the photos he took of the first small, bright-red poison ivy leaves that appeared in Massachusetts and southern New Hampshire, where he worked.

“When I first started, it was May 10 or 11,” he recalls. “I was so excited. I thought, ‘Wow, the season’s here.’ ”

Now, if he cataloged all his photos from 14 years ago, the first sighting would have come about a month ago. In 2023, the first sighting was on April 18.

Barron may have inadvertently documented the impact of climate change.

Poison ivy is expected to be one of the big winners in this global human-caused phenomenon. Scientists expect the creepy three-leafed vine to take full advantage of warmer temperatures and rising levels of carbon dioxide in the atmosphere to grow faster and larger — and become more toxic.

Experts who have studied this plant for decades warn that it is likely to have effects on human health. They say hikers, gardeners, landscapers and others may want to take extra precautions — and get better at identifying this plant — to avoid developing an itchy rash. (Learn how to recognize it and test your knowledge with this quiz from WBUR.)

Barron believes the early start to the season is due to changing weather patterns.

“The weather has gotten warmer, and the plants have gotten warm enough to bloom earlier and earlier every year in Massachusetts,” he said. “It’s very noticeable.”

Testing the theory

There is science to back up Barron’s hunch.

In the late 1990s, a team of researchers designed an ambitious study to find out how plants — and even the entire forest ecosystem — would respond to rising levels of carbon dioxide in the atmosphere.

They built large towers around six huge circular forest plots to pump gas into the air. The experiment was carefully computerized: If the wind was blowing from the west, the towers to the west would release the gas, so that it could float above the rest of the patch of forest and out the other side. The idea was to simulate what scientists believe conditions will be like in 2050.

“The cylinder of the future is how I like to call it,” explained William Schlesinger, now a professor emeritus at Duke University, who worked on the study with scientists from the federal government.

Over the course of a few years, the researchers watched plants grow faster with more carbon dioxide. This was expected since plants mainly use the gas as food. Trees grew about 18% faster in forest plots with a high concentration of carbon dioxide.

However, the vines grew faster, and poison ivy was the fastest of all, growing 70% faster than it would have without the additional carbon dioxide.

“It was the ceiling. It topped the growth of everything else,” Schlesinger said.

And that’s not all: researchers have discovered that poison ivy has become more toxic. The higher carbon dioxide levels stimulated the plant to produce a more potent form of urushiol, the oily substance that causes those nasty rashes we all try to avoid.

But we don’t know why, said Jacqueline Mohan, a professor at the University of Georgia’s Odum College of Ecology, who participated in the study.

In another experiment, Mohan found that vine leaves became larger as carbon dioxide increased.

More recently, Mohan has been working on an ongoing study at Harvard Forest in central Massachusetts, where researchers are artificially warming the top layer of soil by about 9 degrees Fahrenheit. The idea is to simulate the impact of climate change and measure how plants respond. Poison ivy seems to like warmer conditions.

“Congratulations Betsy, she’s taking off,” she said. “Poison ivy uproots more than any tree, and more than any shrub.”

One possible reason for this growth is that, unlike shrubs and trees, vines can invest all their energy growing taller, Mohan said. They do not need to build trunks or thick branches. In addition, artificially warm soil appears to promote fungi that thrive in warm soil and help poison ivy grow.

More itching?

With climate change already starting to affect global weather and weather conditions and atmospheric carbon dioxide levels rising, both Schlesinger and Mohan believe it is plausible that poison ivy will change.

To date there are no observational studies on this topic. “It’s a bad plant to work on,” Schlesinger noted. “It’s an understudied species,” Mohan agrees.

Some Massachusetts conservationists report they are seeing more vines growing around driveways and yards. Doctors say they’re seeing more rashes caused by poison ivy, including the kind that takes people to the emergency room.

“Every one of us sees this every week,” said Louis Kushner, a dermatologist with 10 doctors in the suburbs west of Boston. “And I mean the kind of situation where people can’t sleep and are covered in blisters.”

Nearly 80% of the population is allergic to poison ivy, but Kushner said only a small percentage of cases make it to the doctor. The severity of the reaction depends on how the individual’s immune system responds to the oil in poison ivy.

“Some people will have a massive allergic reaction to poison ivy, and others don’t seem to have any allergic reaction at all,” he said.

Kushner suspects there may be another reason to consider the spike in poison ivy reactions in recent years: an epidemic that has shut down indoor activities and pushed people into their gardens and onto trails.

Just as more people are taking to the trails, conservationists are noticing more poison ivy on the trails and climbing trees. In Lincoln, Gwen Loud has been monitoring Pom Ivy’s expanding estate.

“There’s a lot. (It’s) everywhere,” said Loud, a member of the board of directors of the Lincoln Land Conservation Trust who has lived in the area for 55 years.

I’ve noticed another change, too: the leaves are getting larger.

She pointed to a patch of poison ivy growing at the edge of the forest, and noted that its leaves were the size of a book. “I don’t think I’ve ever seen leaves this big before,” she said.

Loud would like to see some hard data, but if her observations are correct, that’s not good news for the vast majority of people who are allergic to poison ivy.

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