About 80% of the population is allergic to the oily resin urushiol, the compound found in poison ivy that causes dermatitis — a burning, itching rash. Reactions to poison ivy range from mild to life-threatening, and I am among those who have ended up in the ER due to poison ivy exposure. As this three-leaved menace is in its peak season, I am revisiting my conversation with Dr. Susan Pell, a botanist with a deep expertise on the family of plants that poison ivy belongs to.
Susan is now the executive director of the U.S. Botanic Garden in Washington D.C., where she has worked since 2014. She relocated to D.C. from New York in 2013 for an AAAS Science and Technology Policy Fellowship at the National Science Foundation, and has stayed there ever since. Before joining the U.S. Botanic Garden, she was the director of science at Brooklyn Botanic Garden and the botany program manager at the New York Botanical Garden.
Susan has been traveling the world – including this trip to Vietnam – to research the diverse plant species which are members of the cashew family. (photo: Dr. Susan Pell)
Susan holds a bachelor’s in biology from St. Andrews University in North Carolina and a doctorate in plant biology from Louisiana State University. Her doctoral thesis is on her research focused on the molecular systematics of the cashew family of plants — and believe it or not, poison ivy is one of 800 plant species worldwide that belong to the cashew family.
For an in-depth recounting of my conversation with Susan, see the show notes from the original airing in 2019. Or you can read on for a more tightly summarized recap.
Anacardiaceae
Mangos, pistachios, sumac, smoke bush, pink peppercorns and poison ivy are all are members of the diverse cashew family, Anacardiaceae, and all of them contain urushiol in at least some of their plant parts.
You will never see unshelled cashews at the supermarket because the fruit and shell of cashew nuts contain urushiol and can cause dermatitis. Small, hard black bits stuck to cashew nuts is actually urushiol that has oxidized. Susan suggests paying a little extra for higher-quality cashews, free of oxidized urushiol.
She also suggests avoiding peppercorn mixes because pink peppercorns, specifically, can cause a painful reaction on your lips and in your mouth if you, like most people, are allergic to urushiol.
The flesh of these cashew fruits, along with the shells of the nuts they contain, are just as likely to cause a rash as poison ivy. (photo: Dr. Susan Pell)
A Nearly Ubiquitous Plant
Poison ivy is found throughout the contiguous United States, except in California. Poison ivy doesn’t thrive in the Mediterranean-like climate of the Golden State, though California is home to poison oak, which also has three leaves and contains urushiol in its foliage, stems and berries.
Identify and Avoid Poison Ivy
“Leaves of three, let it be” is age-old advice for avoiding poison ivy. The leaf form of poison ivy is one terminal leaflet above two lateral and opposite leaflets on a single stem. The group of leaflets is considered a single leaf, and and the trios of leaflets grow up the vine, alternating sides. In other words, a three-leaflet leaf on the right side of the vine, and the next one further up the vine on the left side, repeating.
The leaves are wavy along the margins and pointed, with a slight rounding at the tip. The leaves vary in size from around an inch up to several inches in length, and they turn a brilliant red in fall. Small black spots are common on poison ivy foliage. The spots are urushiol that has oozed out of the leaf through a cut and oxidized.
The vine is a climber with a woody trunk and red, adventitious roots — roots that sprout from parts of a plant other than the root zone — growing from the trunk.
Poison icy prefers to grow in a disturbed area, such as the edge of a cleared forest or a hiking trail. Once an area has been disturbed, it doesn’t take long for poison ivy to move in, and it can quickly outcompete other plants for the space.
How Poison Ivy Spreads
Poison ivy seeds spread for miles in the bellies of birds, which, like most animals, are not allergic to urushiol. Various animals love the white, cream-colored and red berries of poison ivy. The seeds in the berries pass through the digestive system of the animal and then germinate easily when in contact with soil.
Mature poison ivy develops a rhizome system, which makes poison ivy difficult to eradicate. Even a tiny piece of rhizome left behind in soil can produce a new poison ivy vine.
Urushiol Dangers and Protection
Susan has found that urushiol can remain active for up to a century, if not longer. Urushiol in pieces of a poison ivy vine that has been dead for 100 years still has the potency to cause dermatitis.
Rumors persist that exposing yourself to poison ivy will help you build up immunity to the allergy, but the opposite is true. The more you expose yourself, the more virulently your immune system will react. So if the discomfort isn’t enough to keep you away from poison ivy, maybe the thought of increasingly severe reactions will be.
When you must confront poison ivy in your landscape, protect your skin with gloves and long sleeves. Ivyx is a product that can be applied like sunscreen to protect from poison ivy before exposure. The urushiol compound binds to the Ivyx rather than your skin.
If you come into contact with poison ivy unexpectedly, remember there is a delay between contact and when the toxic compound binds itself to your skin cells. Wash the area of contact and any part of your body where you might have transferred urushiol after making contact. Sweat can transfer urushiol too, so your safest bet is to take a shower.
Because urushiol is an oil, wash with cold water to minimize the spread. Use plenty of soap to break down the oil, and wash all traces down the drain.
If you can’t wash, be sure to have a post-exposure product, like Technu, on hand. Tecnu circumvents a reaction by breaking the bond that urushiol makes with the proteins in our skin cells. When the compound doesn’t bond to your skin, your immune system doesn’t recognize a need to react.
Wash all the clothes you were wearing and any tools you were using or other items you were carrying when exposed to poison ivy. Pets that came into contact with poison ivy won’t have a reaction themselves but can transfer urushiol to you, so wash pets too.
Poison ivy is an aggressive climber. The vines will wrap themselves around and clamber up trees and fencing to reach for the light.
Poison Ivy Treatment
When you do experience a rash, treatment depends on the intensity of your body’s reaction. There are over-the-counter corticosteroid creams available, but the rash can be so severe that you might need to see a doctor to treat the inflammation with a prescription steroid that will promote quicker healing. You might also be given an antibiotic to prevent the blistering on your skin from becoming infected.
Calamine lotion will help to dry out the blisters. That will make you a bit less uncomfortable, but it won’t speed the healing. Any over-the-counter product designed for itch relief might help you make it through the day or get some sleep.
Each poison ivy leaf comprises three leaflets.
If you haven’t listened to my conversation with Susan Pell on poison ivy, you can do so now by clicking the Play button on the green bar near the top of this post.
What’s been your experience with poison ivy? Let us know in the comments below.
Links & Resources
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Episode 052: Why Organic Matters – with Maria Rodale
Episode 109: Garden Safety: When Shortcuts Have Consequences
joegardenerTV YouTube: How to Protect Trees from Deer Rut Damage
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Earthbound Expeditions: Discover South Africa with Joe Lamp’l
Duke Magazine, More Pernicious Poison Ivy
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