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A Poison Ivy Primer: Everything You Need to Know

April 8, 2018

Spend enough time outdoors, and you’ll eventually have a run-in with poison ivy — the three-leaved scourge of the natural world. Famous for the incredibly itchy rash it causes, poison ivy is a common and benign-looking plant, which can wreck a camping trip and send you home with a long-lasting reminder of the encounter.

But, with a bit of knowledge, preparation and practice, you can learn to identify poison ivy at a glance and avoid it like a skilled pro. Below, we’ll cover some of the basic facts about the plant (and its close relatives), explain how the rash occurs and provide tips for avoiding the plant during your next outdoor adventure.

Poison Ivy Basics

Poison ivy is a broad-leafed, deciduous plant that is native to most of the United States east of the Rockies, as well as parts of Canada and Mexico. It usually grows along forest margins, where sunlight is abundant and pollinators (primarily bees and wasps) are common.

The plant can exhibit any of several different growth habits; it can grow as a ground-spreading creeper, a climbing vine or an erect, bushy shrub. Its leaves are green throughout the late spring to early fall, but they become bright yellow, orange or red as the days grow short. Shortly before winter arrives, the plant jettisons its leaves and remains bare until the early spring.

Poison ivy plants contain a yellowish oil called urushiol, which permeates most of the plant’s tissues. When bruised (which can happen with very gentle contact), the oil is released from the plant. This oil triggers an allergic reaction in many people, which leads to the itchy rash associated with the plant.

However, while humans may despise the plant, poison ivy is actually quite important for many ecosystems. Deer and other herbivores frequently consume the leaves without suffering any ill effects, and several birds and small mammals dine on the berries. Many animals make their homes amid the plant, and its roots and vines help to slow erosion. Poison ivy also competes with many invasive weeds and vines and helps to maintain the native species and biodiversity of the locations in which it grows.

So, while you may not want poison ivy growing in your backyard, and you certainly want to avoid it while camping, it should not be considered a “bad” plant. Instead, it simply deserves respect and deference.

Poison Ivy Vs. Poison Oak

There are a number of closely related plants that go by the name poison ivy, and scientists have long argued about the details of the plant’s family tree. Some scientists suspect that the numerous varieties of the plant are all members of a single species, while others recognize half a dozen distinct species or more.

Some, for example, contend that eastern poison ivy and eastern poison oak are distinct species, while others believe that they are the same species, and simply exhibit morphological differences based on the place in which they grow.

These types of debates may excite biologists, but they mean relatively little to outdoor enthusiasts. Most of the varieties/species look very similar, exhibit broadly similar biology and are capable of triggering a rash in sensitive humans.

Nevertheless, most authorities recognize the following species.

  • Eastern poison ivy (Toxicodendron radicans)
  • Western poison ivy (Toxicodendron rydbergii)
  • Eastern poison oak (Toxicodendron pubescens)
  • Western poison oak (Toxicodendron diversilobum)
  • Poison sumac (Toxicodendron vernix)
  • Japanese lacquer tree (Toxicodendron vernicifluum)

Aside from the Japanese lacquer tree (which is not found in the US) and poison sumac (which is not encountered nearly as frequently as the other listed species), each of the species above look relatively similar and feature leaves with three leaflets.

Most of the species labeled poison oak have lobed leaves, but this isn’t an infallible rule. They all contain the same active ingredient that causes humans problems, and they’re all close relatives of each other. Many of the species are very difficult to distinguish, even for experts.

As far as hikers and campers are concerned, these differences matter very little, and you can treat them all similarly.

Poison Ivy Myths and Misunderstandings

Many hazardous components of the natural world become the focus of myths and misunderstandings, and poison ivy is no exception. Below, we’ll clear up three of the most pervasive examples:

Poison Ivy Myth #1: If you itch after a hike, you probably touched poison ivy.

Poison ivy elicits an allergic reaction, which often takes some time to manifest. Although some people may develop the rash within 24 to 48 hours, others won’t see the rash development for a week. This means that any itching that occurs immediately following a trip through the woods is very unlikely to be the result of poison ivy. It’s probably the result of sweat, bugs or some other itch-inducing plant.

Poison Ivy Myth #2: Scratching poison ivy rashes can cause them to spread.

Spreading urushiol around will cause your body to develop rashes in several different places. However, by the time the rash appears, you’ve (hopefully) washed off any of the oil that was present. This means that scratching a poison ivy rash will not cause it to spread.

Poison Ivy Myth #3: Poison ivy is poisonous.

Technically, poison ivy isn’t poisonous at all. Urushiol isn’t a toxic substance used by the plant in the name of self-defense; it is actually an oil that enables the plant to better conserve water. This is important, as the plant often grows in areas with full sun exposure, where water stress is common.

However, urushiol enrages the human immune system (at least in many cases – individual reactions vary). This triggers an overzealous allergic response, that results in the rash and itchiness.

Identifying Poison Ivy: Defining Characteristics

The best way to avoid problems with poison ivy is by learning to identify it so you can steer clear. And while there are a number of plants that resemble poison ivy, you can usually identify it by noting a few important characteristics.

Leaves of Three; Leave It Be

The easiest way to identify poison ivy is by noting the leaves, which are grouped in threes. Additionally, the central leaf is borne on a longer stalk than the lateral leaves, which helps distinguish poison ivy from many similar-looking plants. There are plenty of other three-leaved plants in the forest, so just avoid them all unless you are skilled at identifying the plant.

In the interest of being technically accurate, we must point out that what you’ll think of as poison ivy “leaves” are actually leaflets – the entire group of three leaflets comprise the leaf. But this is only an academic point that doesn’t matter for outdoor enthusiasts.

Berries White; Dangerous Sight

Most berries growing in and around forests are blue, black, red or purple, but poison ivy berries are yellow-white to off-white. Most poison ivy berries are located relatively high in the trees, so be sure to peek skyward if you are trying to identify a climbing vine.

Hairy Vines Lead to Bad Times

Poison ivy vines have numerous tendrils that give them a hairy or furry appearance. And although there are other vines that also look hairy, you can just avoid all of them to keep yourself safe. Note that all parts of poison ivy plants – including the vines – contain urushiol and can trigger reactions.

No Thorns Required

A few poison ivy lookalikes have leaflets clumped in groups of three and can be confused with poison ivy. However, several (but not all) possess thorns to help deter predators. Poison ivy is not equipped with thorns, which helps to distinguish it from blackberry, raspberry and a few other plants.

Helpful Tips for Hikers and Campers

Now that you understand the basics of poison ivy, it is time to put those lessons into practice. After all, the goal is to avoid contracting the maddening rash in the first place.

Try to follow these tips during your next hike or camping trip:

  • Be careful what you burn. Inadvertently toss a few poison ivy leaves on the campfire and you may find a trip to the hospital is in order. The smoke from burned poison ivy can cause severe lung and throat irritation, potentially causing the throat to swell enough to necessitate emergency action.
  • Don’t trust prior immunity. Scientists estimate that somewhere between 25% and 75% of the public is allergic to poison ivy. However, this can (and often does) change without any warning. Many people who’ve long thought they were immune to the plant’s irritating oil have suffered severe rashes after carelessly contacting the plant.
  • Bring poison ivy wash if you are especially sensitive. You can wash urushiol off your skin with soap and water, but there are a few commercial products that may work better for some people. These types of products can also be especially convenient when hiking far from clean water.
  • Speak with your doctor. If you know or suspect that you are strongly allergic to poison ivy, ask your doctor what to expect and how to best treat a reaction. He or she may, for example, recommend taking an antihistamine following exposure.
  • Don’t forget about your dog. Dogs rarely suffer from poison ivy rashes, but they can become coated in urushiol if they run through a patch of poison ivy. This can represent a big problem, as they’ll not only get the oil all over you but your tent and sleeping bag too.
  • When in doubt, err on the side of caution. Don’t spend 15 minutes trying to determine if the plant in front of you is poison ivy or kudzu. Just avoid it and keep moving on. The upside of correctly identifying a poison-ivy look-a-like is rarely worth the risk.

Poison ivy can certainly cause problems, but it is easy to avoid these by simply using a bit of care when traveling through the great outdoors. Learn to identify the plant and familiarize yourself with the places it tends to grow, and you’ll find that it’s usually pretty easy to stay rash-free.

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