Protective Mechanism with an Itchy Outcome


Feeling itchy after a hike at the Nature Center? You may have just encountered poison ivy.
If it has three leaves, let it be. Visitors to the Nature Center who ignore this maxim risk an uncomfortable few days if they touch poison ivy. Photo by FWNC&R Staff.

Have you ever considered the practical ways plants protect themselves from being eaten or harmed? Some have developed various self-defense mechanisms that are easily noticeable by hikers, including the sharp spines on a prickly pear cactus or the long thorns on a honey locust tree. However, some defenses, such as chemical defenses, aren’t as noticeable but can be felt after contact. One plant at the Nature Center by which visitors may become collateral damage to its defense mechanism is poison ivy (Toxicodendron radicans).

Poison ivy is part of the Anacardiaceae family, also known as the Sumac family. This plant species is native to North America and can be found in various growth forms, including vines or ground cover at the park. The plant produces pale green flowers with five petals that give rise to white fruit with a single seed known as drupes. Leaves are compound, with three leaflets, which has resulted in the common saying, “Leaves of three, let it be.” Notably, the middle leaflet stalk is longer than the two side leaflets, and the leaf edges can be either smooth or coarsely toothed, with the surface either glossy or dull.

Poison ivy is a plant that leaves a lasting impression on those who accidentally encounter it. The plant’s sap contains a natural chemical, urushiol, a defense mechanism against insects and fungi. Urushiol can be found throughout the plant, from its leaves to its roots. The name “urushiol” is derived from the Japanese word for lacquer, “urushi,” as many plants that contain urushiol belong to the same family as the Japanese lacquer tree. However, for humans, contact with this oily resin can cause an allergic reaction and rash. The reaction may not be visible immediately, but it can appear a few days later when our immune system is triggered. Interestingly, not everyone is allergic to urushiol, but prolonged exposure can increase the chances of developing an allergy.

Some plant species belonging to the poison ivy family can be found in stores and our kitchens. Mangoes, pistachios, and cashews are the edible relatives on the poison ivy family tree. In the wild, these plants contain varying amounts of urushiol. The vines of mangoes contain this protective resin, and the skin of the fruit contains some traces of the chemical. Urushiol is also present in the shells of cashews and pistachios. However, when sold in stores, cashews and pistachios are processed before being sold to consumers.

It may be hard to believe, but poison ivy is not intentionally trying to harm us. Wildlife benefits from poison ivy, as migratory birds depend on its white berries as a food source during harsh winter months when food is limited. These birds help scatter the plant’s seeds, which grow in hedges and under trees. Meanwhile, small bees pollinate the flowers in the spring. Even white-tail deer and cottontail rabbits nibble on the plant’s greenery. Poison ivy’s stunning fall foliage and ability to thrive in early successional environments make it a noteworthy species. The best defense against the plant’s defense is a good offense: learn what it looks like, avoid contact, and wash your hands with soap and water when in doubt.

By Laura Veloz, Naturalist, Fort Worth Nature Center & Refuge

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