Poison Ivy – Leaves of three let it be?


I have issues with the “Leaves of three let it be” guidance with regards to poison ivy avoidance. It is too non-specific and also anatomically incorrect. What is being referred to as “leaves” is actually the three leaflets of a single compound leaf.

Over the years I have guided numerous tree walks for garden clubs, museums, high school classes, and city recreation programs and one goal that I always had for those outings was to make sure that at the end, all the participants would be able to identify poison ivy. While is it not a tree, ignorance of it can ruin many outdoor experiences.

As a family physician, I would frequently see patients with poison ivy contact dermatitis who had no idea how they got exposed. My favorite was the teen whose mom was skeptical of my diagnosis because “she’s never outside and only on her phone or Playstation”. Upon further questioning we found out that the students in her biology class were assigned to bring in leaf specimens to pass around the classroom. Apparently someone had brought in poison ivy.

One of the challenges with identifying poison ivy is that it can take three forms: an upright ground cover as seen in the photo above, a small shrub that reaches up to 8 foot tall, and a vine climbing up trees, fences or buildings. But the way you identify it is always the same. You look at the leaf structure and how it is patterned on the stem.

Poison Ivy has a compound leaf. Compound means that each leaf is made up of three or more leaflets. With poison ivy, each leaf has three leaflets and the end leaflet always has an elongated petiolule or leaflet stem. In the photo below the red line encircles the compound leaf made up of three leaflets, and the arrow points to the longer petiolule of the end leaflet.

The other distinguishing feature with poison ivy is that the leaves and branches always arise in an alternating pattern from the stem or vine. See below.

Compare that to one of its “look-alikes”, a young box-elder tree, whose leaves arise opposite on the stem.

Also notice in the photo above that the leaf petioles (stems) of young box-elders are frequently red like poison ivy.

The other look-alike for poison ivy is Virginia Creeper. It is a native plant with intense red fall color. The leaves are again compound, usually with 5 leaflets, but young leaves can have 3. The leaflets arise in a palmate pattern, from a central point and there is no extended petiolule. See arrow below.

Virginia creeper is always a vine, either climbing up a tree or structure, or over the ground. In the winter you will see clusters of black berries on older plants. It is a very important food source for wildlife.

The Rash

The rash of poison ivy is caused by an individual’s immune system reacting to the urushiol in the plant and about 85% of the population is susceptible. Every part of the plant, including the vines and roots, contain the oil and therefore can cause the rash. Minor trauma to plant structures release the oil. I have seen patients get the rash by carrying firewood with leafless vines on them into their house. The oil is very susceptible to soap and therefore if you shower shortly after exposure you will frequently avoid the rash. You can however get the rash from a pet that has been in contact with the plant. Contrary to popular belief, the rash does not spread once you have showered. The areas with the greatest urushiol contact will show up first, usually within a day or two, and those with less contact, or with thicker skin, will show up later, causing the individual to think that it is spreading. The fluid that can ooze out of the rash is an inflammatory exudate and will not spread the rash or give it to another individual.

Fall is a great time to enjoy poison ivy … from a distance. It has great fall color with red, orange and yellow leaves, and after leaf fall, outstanding waxy white berries, that like virginia creeper, are important for wildlife. In the vine form I have seen the plant with horizontal branches extending 4-5 feet from the vine itself.

So get outdoors and try to build your confidence with poison ivy identification using the points mentioned above. Once you do, you will be the resident expert and hopefully save yourself and those around you from a bad experience. Look along fence rows, on trails, beside out buildings, or the edges of woodlands.

So let’s settle on, “If leaflets three, with alternating leaves or branching, let it be”.

If you would like additional information or are an audio learner I would recommend this podcast from The Nature Guys that I heard while I was planning this article.

Poison Ivy

Photo credits to Peggy Juengling Burns.


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