Featured article: Jelesko, J.G., E.B. Benhase, and J.N. Barney. 2017. Differential Responses to Light and Nutrient Availability by Geographically Isolated Poison Ivy Accessions. Northeastern Naturalist 24(2): 191-200, https://doi.org/10.1656/045.024.0210
“Leaves of three, let it be,” the saying goes. Anyone in the eastern United States who spends a significant amount of time outside is likely familiar with this plant and its three leaflets. In fact, about 80% of us are allergic to the infamous vine known as eastern poison ivy (Toxicodendron radicans). If we come in contact with the plant, it causes allergic contact dermatitis, another term for an allergic reaction on the skin. This can range from an annoying itchy rash to crater-like blisters resembling small volcanoes erupting on one’s skin. In extreme cases it can lead to hospitalization.
Poison ivy is native to North America and consists of two species. Eastern poison ivy is generally restricted to the eastern half of the United States including the Dakotas and Texas. Another species known as western poison ivy (Toxicodendron rydbergii) prevails in the western U.S. causing the same rash effects on our skin. Instead of a vine, the latter species is a low-growing shrub.
The fame of poison ivy can be traced back to the English-explorer, Captain John Smith, who compared it to English ivy.
“The poisonous weed, being in shape but little different from our English ivie; but being touched causeth redness, itchings, and lastly blysters, the which howsoever, after a while passe away of themselves without further harme; yet because for the time they are somewhat painefull, it hath gotten itself an ill name, although questionlesse of no ill nature.” – Captain John Smith 1624.
Though Smith provided the first written account of the species, Native Americans were well aware of poison ivy and its effects; some even used it for dyes and medicines.
Poison Ivy in the environment
The compound responsible for poison ivy’s effect on humans is called urushiol and it is the same chemical found in poison oak (Toxicodendron diversilobum) and poison sumac (Toxicodendron vernix); both species cause similar reactions upon contact.
The urushiol rash affects 10-20 million Americans per year (Pariser et al. 2003). Only humans and several higher primates are sensitive to urushiol. The compound exists in every part of the poison ivy plant including the roots, stems, leaves, and berries. Interestingly, other animals, such as deer, squirrels, goats, and birds all consume various parts of the plant without any ill effects.
Much research has explored the effects of poison ivy, including urushiol chemistry and pathophysiology, symptoms, and treatment. But relatively fewer studies have examined the ecology of this species. Poison ivy is unique in that it exhibits a wide range of appearances, depending on its location. It can grow low to the ground and appear as an ankle-high weed; it can also develop into thick, jungle-like vines causing entire trees to sag under its weight. A group of researchers recently explored the difference in growth forms of poison ivy from different regions of the U.S. Researchers gathered seeds from poison ivy plants located in Virginia, Texas, Iowa, and Michigan. The research team grew plants from these seeds and tested them under a variety of different environmental conditions: A portion of each group was grown in either full sunlight or shade, and subset of these groups was grown in nutrient-rich soil, soil with a moderate level of nutrients, or soil with a minimal level of nutrients. They found the Texas and Michigan groups grew better in sunlight compared to shade, but both Iowa and Virginia grew equally well in both conditions. They also found that poison ivy has greater growth in soils with higher nutrient levels. This was true for all groups. Finally, the plants from Texas outgrew all the other groups, producing the tallest plants with the most biomass. These differences in growth patterns within a single species helps explain how one species can perform well in a variety of locations. Although it was unclear exactly why seeds from Texas outperformed others, the significant differences in their growth highlighted how diverse a single species can be. Uncovering differences in growth patterns and genetic diversity within poison ivy could partially explain its ability to exploit a variety of different habitats.
Many questions about poison ivy are still unanswered. The genetic diversity and variation in growth form are only the start. Ecologists are still uncertain about the function of urushiol – the chemical’s purpose in the plant remains a mystery. How this species responds to changes in its environment is another unknown which scientists are only beginning to explore.
Climate change effects
Poison ivy is expected to get “bigger and badder” in the coming decades. This prediction comes from another study by a group of researchers who examined the effects of higher atmospheric CO2 levels on poison ivy. The plant significantly increases in both growth and biomass in response to elevated CO2 levels; in fact, its increased growth rate outpaces most other woody species, such as oak trees, birch trees, and pine trees for example (Mohan et al. 2006, Curtis and Wang 1998). The group further found poison ivy produces a more toxic form of urushiol under these higher CO2 concentrations. In short, we can expect poison ivy to become more widespread, aggressive, and harmful in the coming years (Mohan et al. 2006).
The group further found poison ivy produces a more toxic form of urushiol under these higher CO2 concentrations. In short, we can expect poison ivy to become more widespread, aggressive, and harmful in the coming years
If you touch poison ivy this summer…
Quickly and thoroughly wash your skin with soap and water. Urushiol is an oil and can be removed before it is absorbed. Some research suggests that all, if not most of the oil can be removed within 10 minutes of contact but this declines with time (Gladman 2006). So, the sooner you can wash your skin, the less likely you will develop a rash. Long sleeves and gardening gloves are effective if working in areas where the plant is prevalent. However, even if you are wearing pants and you kneel on poison ivy, the oil can penetrate the fabric and reach your skin (author’s experience). Avoiding poison ivy is the best strategy, but for those of us who love the outdoors, this is often hard to do. Keep wet wipes on hand and be aware of the plant’s appearance – these are simple steps one can take to reduce the risk of developing poison ivy’s itch. For more basic information, click here; if you experience a severe reaction to poison ivy, consider consulting your primary physician.
Curtis, P.S. and X. Wang. 1998. A meta-analysis of elevated CO2 effects on woody plant mass, form, and physiology. Oecologia 113(3): 299-313.
Gladman, A.C. Toxicodendron dermatitis: Poison ivy, oak and sumac. Wilderness and Environmental Medicine 17:120-128.
Mohan, J.E., L.H Ziska, W.H. Schlesinger, R.B. Thomas, R.C. Sicher, K. George, and J.S. Clark. 2006. Biomass and toxicity responses of poison ivy (Toxicodendron radicans) to elevated atmospheric CO2. Proceedings of the National Academny of Sciences of the United States of America. 103(24):9086-9089.
Pariser, D.M., R.I. Ceilley, A.M. Lefkovits, B.E. Katz, and A.S. Paller. 2003. Poison ivy, oak and sumac. Derm. Insights. 4: 26–28