Botanist's Lens: Poison Ivy

Powerful and Potent, Poison Ivy's Defense Mechanism: Urishol


Young leaves of Poison Ivy, Toxicodendron radicans. Leaves have smooth edges.

"Leaves of three, let them be."


Any camping trip, wilderness trek or working around the woods shouldn't begin without a check of the potent leaves of Poison Ivy, especially if you live in the East Coast. Poison Ivy, Toxicodendron radicans, is a rather familiar plant for some painful reasons, its oily sap: Urishol Oil. Toxicodendron itself means a "poison tree."


One doesn't have to necessarily come in contact with the oil. Any contaminated clothing, tools, or pretty much anywhere Urishol had made a contact can cause a rash. Urishol can stay on a surface for as long as 5 years!


It is key to remember that one can develop an allergy to Urishol even years after reaction free exposures. Once sensitive to Urishol, every time you come in contact with it, your reaction can become worse.


But how would you know where the sap Urishol has spread? You can be careful while working with it, wash your clothes, disinfect your tools, but is there any other way to take off the Urishol when you can't see it? Well, there just might be: "Rebecca Braslau and her colleagues at the University of California, Santa Cruz, describe a nontoxic spray that will fluoresce in the presence of urushiol. Braslau was able to develop a nitroxide-based solution that would react only with urushiol. By adding a fluorescent dye that would show up only when the reaction occurred, she could spray her nitroxide solution on a surface, and if the urushiol was there, the surface would light up." Source


But why? Why is Urishol so potent? "Once they get into the body, the chemicals in urushiol become oxidised and stick to proteins in skin cells. This changes their shape, making them seem strange and foreign to other cells in the immune system. The immune system then mounts an attack on these healthy skin cells as if they were foreign invaders, creating swelling, inflammation, pain and blistering." Source


Urishol is Poison Ivy's defense mechanism to prevent getting infections. When urishol reacts with our protein cells, our bodies think they are foreign and start attacking, causing a rash.


ID Poison Ivy


Before we look into this, lets remember that Urishol can remain on a surface for 1 or 5 years and "Only 1 nanogram (billionth of a gram) is needed to cause a rash." Source


Mature and yound leaves of Posion Ivy. Notice how the leaf petiole has a top leader leaf and two side leaves starting way below it.

The pictures with this post might be helpful to ID leaves, but plants adapt. The leaf shape changes slightly considering the canopy covers it is growing under and the time of the year. But the main difference is in the margin/edges of the leaf, they are smooth, meaning there are no serrations/teeth or spike or hairs. Also, the center leaf is wider than the side two leaves.


Poison Ivy on a tree

Tiger Lilies, Lilium lancifolium (NOT Poison Ivy)

Many plants have interesting common names. As much as 'Tiger Lilies' are pretty and might excite kids with 'tiger' dots to spot on their leaves, its the unattractive mundane looking vines like Poison Ivy, that kids need to know about so as to freely venture into the woods. As long as there is not contact with the oily sap at anytime on your skin, you are good to go.


Young vine of Virginia Creeper, Parthenocissus quinquefolia. Think the the Latin name is long? Wait till you have the vines run in your garden for miles. But Virginia creeper as a ground cover is any day better than Poison Ivy, which it often gets confused with.

Even with my taxonomy skills (I could ID Poison Ivy from 5 ft away!) I succumbed to a rash. Of course I cross contaminated after working with it! Remember Urishol can stay on an object for 5 years, so rinse off equipment, stay off the leaves, and definitely don't ever burn your brush piles. If you have Poison Ivy, they can be lethal - literally!


Beware and informed for Urishol has been around for millions of years and it isn't going anywhere. And don't forget to use wash off with Tecnu!

839 Londontown Road, Edgewater, MD 21037          410-222-1919         londontown@historiclondontown.org

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