Monumental Questions - Can I still get Poison Oak in the Winter?

Kristie Ehrhardt • February 6, 2025



YES!


Leaves of three - let them be. Right but it’s winter and what if there are no leaves? 

 

Poison Oak (Toxicodendron diversilobum), Pacific Poison Oak or Western Poison Oak is a widely distributed woody shrub or vine that belongs to the Sumac (Anacardiaceae) family. The Sumac family is also known as the Cashew family which contains the tree that gives us that delightful buttery nut (it’s actually a seed) as well as the tree that provides mouthwatering mangos and the tree that produces savory pink peppercorns. How can such a delectable family also give us such an annoying plant? 

 

Poison oak (PO) is not even remotely related to oaks but the leaves sort of resemble oak leaves. They are divided into three leaflets that are about one and a half to four inches long. The species name, diversilobum, refers to the diversity of leaf size and shape; the edges can be toothed, scalloped or lobed and vary greatly. PO is deciduous and the new leaves are usually dark golden or bronze when they are just beginning to unfold in early to mid-spring. They turn a bright then darker glossy green and ultimately transitioning a stunning scarlet red then fading to pink in late summer and fall. The small flowers are greenish-white and appear in late spring to mid-summer and if they are fertilized they become small creamy white berries. Growth forms vary depending on where it is occurring, but it thrives in full sun as well as shady environments. In open sunlight it can grow as a tall, dense shrub or tree with a thick trunk, or a woody vine winding around and up into the canopy of tall trees, fences and other vegetation. In shady areas it can occur as a dense thicket, vine or anything in between. It is prolific and spreads by seed dispersal and through underground rhizomes.

 

It is widely distributed in Western North America from British Columbia, Washington, Oregon, California, Nevada and Baja Mexico and occurs extensively throughout the Berryessa Snow Mountain National Monument. It occurs in chaparral and woodland habitats as well as coastal sage scrub, grasslands, oak woodlands and mixed evergreen forests below 5,000 feet. It’s a common understory plant in riparian corridors and grows alongside Giant and Coastal Redwoods. PO is even a constant quagmire in dry-farmed mountain orchards, urban parks and recreation areas. In fact, it’s kind of hard to find a biome in the Central Valley of California that doesn’t support Poison Oak unless you go into the Great Basin or the southwestern edge of the Mojave desert.

 

The genus Toxicodendron contains many plants that cause allergic reactions. The oil of the plants contains a compound called Urushiol that causes the reaction and it is found in all parts of the plant except the pollen (ironic!). Direct contact with the plant or anything that has touched the plant such as clothing, pets, tools or vehicles can result in painful, itchy blisters. Not everyone is allergic to it, however, repeated exposure can lead to heightened sensitivity. If an area dense with PO is burning, inhaling the smoke can cause a severe internal reaction.

 

Which leads me to why you can still have a reaction to PO in the winter after all the leaves have fallen. It’s a lot harder to spot when it’s just a twig, branch or bare vine along the edge of the trail. Twigs and stems still contain the oil that produces reactions and you may not even realize that you’ve brushed up against it until it’s too late. Keep an eye out for exposed, reddish-brown whip-like sticks coming out of the ground, especially if you know you’re in an area where PO is present. Sometimes the berries remain on the plant even though the leaves have fallen off and occasionally the stems show black marks where the sap may have oozed out and dried. If you know you’ve been in contact with PO, washing your exposed skin with dish soap is a great way to remove the oil from your skin and washing your clothes as soon as you can so you don’t transfer the oil to yourself, others or additional laundry is also a good idea. If your pets have been scampering about where PO could be present, give them a good bath before you snuggle up with them and consider wearing long rubber gloves like those made for doing dishes to avoid the oils transferring to you.

 

PO isn’t all bad; Black-tailed Deer, California Ground Squirrels and Western Gray Squirrels among other species consume the leaves of the plant which is rich in phosphorus, calcium and sulfur. Many birds eat the berries and utilize the plant for shelter. Research has shown that livestock, domestic dogs and other native wildlife species do not have adverse reactions to Urushiol. Native Americans cooked with it, used the stems to make baskets and used the juice from the fruit to dye materials black.

 

Don’t stay off the trail, just take precautions and enjoy winter hiking in your favorite monument! 


-Kristie Ehrhardt (kehrhardt@tuleyome.org)

Tuleyome Land Conservation Program Manager

RECENT ARTICLES

Wildfires and California Oaks – Nature and You Lectures

By Geoff Benn • October 7, 2026
Blue oaks on the Berryessa Peak Trail. Photo by Geoff Benn. Our September Nature and You speaker was Tuleyome Education Associate Geoff Benn, who gave a lecture on one of his favorite subjects, California’s amazing oaks. In the lecture, Geoff discussed how California’s oaks evolved and how they deal with a range of challenges, including drought, fire, and diseases. If you missed it, be sure to check out his talk here. 

Welcome Interns!

By Geoff Benn • October 7, 2026
Interns Deana Martinez, Yaquelin Gonzalez, Gabriella Limon, Ben Hubbard, and Keira Lazaro at Woodland Regional Park Preserve. Tuleyome is excited to welcome our largest ever cohort of interns, including four Horticultural Interns and one Communications Intern! Our Horticultural Interns are paired with volunteer mentors at Woodland Regional Park Preserve where they learn about managing invasive plants and monitoring populations of rare native plants. This year’s mentors are Jennifer Hogan and Teri Barry, both longtime volunteers and leadership team members at the Preserve. Our Communications Intern works closely with Program Director Nate Lillge to shape and execute our communications, including social media, e-newsletters, and more. Keep reading to meet each of our interns: Horticultural Intern Yaquelin Gonzalez has lived within the bounds of Sacramento County her whole life, where her mother’s garden and love for rock collecting sparked her appreciation for nature. Her desire to enter the environmental field was further fueled via nature-based field trips during elementary school. After high school, she began studying at American River College and eventually transferred to the University of California, Davis in 2025 to study Environmental Science and Management, focusing on Ecology, Biodiversity, and Conservation. Here, she has gained an interest in soil science as well as plant ecology and hopes to pursue a research career in restoration ecology. In her spare time, she likes to go on nature walks, play the guitar, read, bake, and listen to podcasts. Horticultural Intern Ben Hubbard grew up in Roseville, California where he spent a lot of time outdoors with friends, competing in sports, hiking, as well as instructing elementary school field trips about the environment, specifically wetlands. He just recently transferred from Sierra College to the University of California, Davis and is working towards completing his Bachelor’s degree in Environmental Science and Management with a specialization in Ecology. When he completes his education, he hopes he can make a difference in the world and help to preserve wildlife and natural land for the next generation to enjoy as much as he has. When he’s taking a break from studying you can find him practicing with his recreational sports team, grabbing a coffee, hiking, enjoying a good story, or just hanging out with friends. Horticultural Intern Keira Lazaro is a third year undergraduate at University of California, Davis where she majors in Environmental Toxicology. She’s specializing in pharmacology and biotechnology with hopes to work in a research lab on specific pathogens that attack native California crops. She is also a part of the University Honors Program in Davis and is taking the first couple of steps towards her fourth year signature work on the oxidation chemistry behind electronic cigarette devices, specifically with exposure in indoor environments. Keira grew up in Los Angeles County and hopes to find a career in Southern California once she graduates. When she is able to go back home she spends most of her free time at the beach or hiking. Communications Intern Gabriella Limon grew up in Bishop, California, within the Sierra Nevada and Owens Valley where she became connected with wildlife and stewardship. She spent her later teenage years in Southern California and committed to the University of California, Davis, where she is currently pursuing a Bachelor's Degree in Environmental Policy Analysis and Planning with a focus on conservation management. In addition, she is also a scholar staff member of the UC Davis Women’s Resources and Research Center where she provides STEM-based programming and academic support to students of marginalized identities. Gabby is especially passionate about wildlife justice, land protection, and environmental education. She spends her free time writing, playing video games, tide pooling, hiking, and logging critters. Horticultural Intern Deana Martinez grew up in Arbuckle, California, surrounded by rice fields and almond orchards, which grew her interests in agriculture. She is currently in her 4th year at the University of California, Davis, where she is pursuing a Bachelor’s Degree in Entomology with a Soil Science minor. Along with classes, she is involved in a Nematology research lab, working with plant parasitic nematodes and their interactions with common crop systems in California. She has also held a student position with the Plant Biology research greenhouses as a technician for the past 3 years. Outside of school and work, Deana enjoys hiking, reading, and spending time with her family.

Policy Corner: California Fights Back on Endangered Species Protections

By Shai Demus • October 7, 2026
California’s condors and sea otters are two of conservation’s great comeback stories – species that were once on the brink of extinction and have rebounded thanks in large part to the Endangered Species Act (ESA). That 50-year-old law is now facing one of the most significant rollbacks in its history, and California is fighting back in court. Last week, Attorney General Rob Bonta, joined by a coalition of 21 state attorneys general, filed two lawsuits challenging the Trump administration’s recent changes to the ESA. At the center of the fight is a single but enormously consequential change: the administration rescinded the longstanding definition of “harm” under the Act – a definition that, since the early 1980s, has made it illegal to destroy or damage the places endangered species live, breed, and feed. Without it, habitat destruction itself may no longer count as illegal “harm” to a species, even though most biologists agree habitat loss is the single biggest threat facing endangered species today. The changes don’t stop there. The administration is also making it easier for federal agencies to avoid designating “critical habitat” for at-risk species, and it has eliminated automatic protections for newly-listed threatened species – meaning wildlife officials must now write custom protection rules for each species one at a time. That’s a heavy lift for an agency that has reportedly lost more than a third of its staff, and which currently has 400 species waiting for a protection decision. Why this matters here at home California is home to more threatened and endangered species than any other state. It’s natural to assume the state’s own Endangered Species Act would fill any gap left by federal rollbacks – but legal experts point out that California’s law, as written, doesn’t clearly protect habitat at all. It primarily prohibits killing or injuring individual members of a listed species, not damaging the places they live. That means the very thing under threat federally – habitat protection – may not have a reliable backstop at the state level either. What this means for our work At Tuleyome, so much of what we do, protecting oak woodlands, wildlife corridors, and watershed habitat across our region – depends on the basic principle that a species' home matters as much as the species itself. A condor or a Clear Lake hitch can’t survive court-ordered protection from hunting alone if the habitat it depends on is quietly bulldozed, drilled, or logged out from under it. This lawsuit is about defending that basic principle at the federal level, and it’s a reminder of why local land protection and legislative advocacy matters now more than ever – because the safety net above us may have real holes in it.