Figure 1. A poison ivy trifoliate leaf.
In the past few weeks I've had at least five inquiries regarding a possible poison ivy pandemic. Poison ivy is a woody vine most commonly found along edges of wooded areas. The plant has trifoliate leaves (leaves of three, leave it be), and the leaflets are irregularly lobed. Virginia creeper is a woody vine found in the same habitats as poison ivy, but has five leaflets. A few people develop a rash when they come in contact with Virginia creeper. Poison oak is not found in Iowa.
The toxin in poison ivy is an oil known as urushiol. People vary widely in their sensitivity to the plant; I was blessed with a high tolerance and can handle the plant with no problem (Figure 1). It is a persistent chemical and remains active within plant parts even after the plant has been killed. While the majority of exposures are caused by people coming into direct contact with the plant, the oil can be moved from the plant to people's skin via other means. Pets running through poison ivy can get the oil on their fur, and unsuspecting people can then absorb the oil while petting the animal. Urushiol also can be transferred via contaminated tools or clothing. Finally, some people have developed serious problems after breathing smoke released while burning wood piles containing poison ivy plants.
Back to the reason for this article, is there more poison ivy this year, or are people simply getting out into nature more frequently? There are several studies that indicate increases in atmospheric carbon dioxide levels may benefit poison ivy. Poison ivy grown at current CO2 concentrations produced more biomass, and therefore more urushiol, than poison ivy grown under CO2 concentrations found in the 1950's. Other studies have shown that while most plants benefit from increasing atmospheric CO2, vines take advantage of the CO2 more than non-viney plants. Whereas other plants would need to use much of the increase in photosynthates to support an increase in foliage, vines can focus on foliage production since they don't need to use as much of these materials to support their growth - they use other plants for support.
Thus, research indicates that poison ivy may be becoming a bigger problem than it was in the past. However, increases in poison ivy growth from year to year would be relatively small and difficult to detect. If there are indeed more problems with people encountering poison ivy this year than in recent years, it is likely due to the local growing conditions this growing season rather than the long-term increase in atmospheric CO2.