California's poison oak problem is bad in the wintertime. In 20 years, it could be much, much worse. - SF Gate

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On a cold, wet winter day in early 2021, Mitch Crispe donned full rain gear and went for a hike at College Cove, 300 miles north of San Francisco, with his partner. The adventurous pair trudged through forestland, across coastal bluffs and down into creek beds.

It didn’t occur to Crispe that this might be an area where poison oak would grow; because the leaves drop off in the wintertime, poison oak can easily be mistaken for twigs or other plants. A day later, Crispe paid the price.

“I actually had blisters that were weeping,” he says. “My face was oozing liquid. Then it was crusty, and I didn't want to go out in public.”

Three weeks later, the toxin was still circulating in his blood, causing new blisters to form. Crispe began to worry he would have poison oak forever. “I kept thinking there was oil in the house,” he says. “I was like, 'We’re going to have to burn the house down.' I was being dramatic, but I couldn't help it.”

When hiking, keep an eye out for signs about poison oak.
Paul A. Souders/Getty Images

With winter again upon us, Crispe and the rest of California must face poison oak in its less obvious but still poison-spewing form. Worse still, as temperatures rise in California due to climate change, scientists expect the horrible plant to become even more abundant.

For the blissfully uninitiated, poison oak is a woody vine or shrub that’s common in western regions of California at elevations below 5,000 feet, and is most often encountered on grassy hillsides, in forests and by the coast. It’s a member of the anacardiaceae family, which also includes mangos, pistachios and cashews. The plants all carry an oily irritant called urushiol, and in poison oak, the oil leaks out as a protective sap.

“You have to have your chemical defenses when you have a stationary lifestyle,” explains botanist Jonathan Lee, who works as a revegetation specialist and environmental planner with Caltrans.

Because urushiol is exceedingly stable and hardy, when it gets on the skin or under the nails, it tends to stay there a while. And in addition to coming in contact with the plant itself, people can contract poison oak by touching oils that have gotten on clothing, shoes, gloves, pets and tools. Even smoke from burning plants can lead to an exposure.

“It’s a really sneaky compound,” says Rais Vohra, Medical Director for the Fresno/Madera Division of the California Poison Control System.

In the fall, poison oak leaves turn a fiery orange-red.
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For those who are allergic, which includes most people, exposure results in a rash about one to six days after coming in contact. Interestingly, the rash is a result of a person’s own immune system destroying the skin cells in the exposed area, as the urushiol has tricked it into thinking our own cells are foreign bodies. The rash itches and then forms water blisters (which, contrary to popular belief, do not transmit poison oak). There are creams and lotions that treat mild cases, and for more severe ones, victims must see a doctor to get a steroid cream or pill prescription.

The best way to deal with poison oak is to avoid getting it in the first place. For most of the year, identifying it is straight-forward. The dense, leafy shrub grows from one to six feet high and has three leaflets. Hence the saying, “leaves of three, let it be.”

In the fall, the leaves turn a fiery red color. But in the winter, the leaves all fall off and the plant looks just like upright little twigs. “Most people don't think anything of it, but even then, the buds are covered by oil that will give you an allergic reaction,” Lee says.

There are four Poison Control divisions across California, and they all work together to answer calls on a 24-hour hotline. The division could not respond by press time to a request for recent statistics on calls related to poison oak, but Vohra says there are a lot of them.

“These plants are not uncommon, they are one of the most frequent calls that poison centers handle because the dermatitis can last for days to weeks and prompt a visit to the doctor,” he says. The calls continue through the winter months, he adds, as the branches contain the toxic fluid even when the leaves fall off.

In the wintertime, poison oak looks like harmless, upright twigs.
Ashley Harrell

Unless people get much better at recognizing poison oak — leaves or no — the number of calls is likely to rise as the climate warms.

“The research is showing that the northwest is going to get more arid and hotter, and poison oak tends to thrive in hot and dry locations,” Lee says. The plant can also grow in the shade, he adds, but in sunny, coastal environments like chaparral, it’s so abundant that it turns into thickets. And with climate change, those environments will expand."

“I would expect if California’s climate continues on its current trend of drier rainy seasons and hotter summers, that we’ll eventually see massive shifts in the spectrum of what native vegetation grows where,” Lee adds. “In 20 years, it’s very likely vegetation that thrives in hot and dry conditions, like poison oak, will see its growing range expand at the expense of plants that have an affinity for wetter, more sheltered site conditions.”

If the climate tips faster than current models suggest, Lee continues, we could be looking at a future where vegetation goes from forest to grassland without any steps in between. “With mega-forest fires becoming the new norm, we’re watching the process of shifting plant communities unfold in real-time instead of over millennia,” he says.

A close up of a relatively minor rash caused by poison oak.
Darren415/Getty Images/iStockphoto

When SFGATE gave Crispe the bad news about California’s poison oak-filled future, he groaned and suggested that people stock up on Tecnu, a product that can strip the oil from the skin for a few hours after exposure.

These days, Crispe uses it after hiking just to be safe. “I never want to go through that again,” he says. “It was awful.”