Readers from Europe and other continents can relax: poison ivy (Toxicodendron radicans) is not found in your country. The plant is strictly North American, although there is a sister species, T. orientale, in Taiwan, Japan, Korea and Sakhalin.
However, within North America, poison ivy is distributed from northern Mexico to southern Canada, so North Americans—and visitors to North America—must always be on guard. It’s by far the main cause of allergic skin irritations in the areas where it grows.
The Right Name
Poison ivy is in the cashew family (Anacardiaceae), a plant family known for its poisonous plants (cashews themselves are toxic unless properly prepared). It’s very closely related to sumacs (Rhus spp.) and indeed, is often called poison sumac (along with other Toxicodendron species). The genus name Toxicodendron is a new one and so you may well see poison ivy listed under its old name, Rhus radicans.
In spite of the common name, poison ivy is in no way related to true ivies (Hedera, in the Araliaceae family).
Contact Dermatitis
Poison ivy truly deserves the botanical name Toxicodendron: it’s highly toxic in all its parts. If ingested (although this rarely happens), it can cause death, as it affects the airways and digestive tract. Mostly, though, it causes contact dermatitis.: severe and sometimes debilitating skin rashes caused by touching the plant.
The toxicity comes from an oil called urushiol (sometimes called toxicodendrol). It can cause rashes through direct or indirect contact or by inhalation of smoke. The oil is present on all parts of the plant: leaves, stems, roots and even fruits. Dead branches and roots may still be toxic five years after they are cut.
“Indirect contact” is a common occurrence, usually caused by petting your cat or your dog after it has rubbed up against the plant in its wanderings. You can also “catch poison ivy” from contaminated tools. Lawn mowers and weed trimmers, in particular, are often a source of serious reactions.
Not everyone is affected by poison ivy. About 15% of the population is apparently immune to it. But even seemingly immune people can develop sensitivity through repeated contact, since this kind of contact dermatitis is an allergic reaction. Even sensitive individuals won’t necessarily react to it the first time they touch it, but will after a second contact, and then each additional contact will often make the suffering worse.
The reaction (itching, redness, blisters, etc.) can occur 24 hours to 7 days after contact and normally lasts about a week, but up to a month for very sensitive people. Most people recover fully and suffer no after-effects. However, some people end up in the hospital and, very occasionally, death can result.
Know Your Enemy
Poison ivy is very widely distributed and can be found in almost all non-aquatic environments, from forest to fields, shores, roadsides and swamps, although not in true deserts or in extremely cold regions. It adapts to most soil conditions, from alkaline to acid and humid to dry, and will grow in full sun or deep shade. It’s common in gardens as well, especially along fences. Why fences? Because birds eat the berries (only humans seem to be sensitive to urushiol), then settle on fences where seeds from their droppings germinate and result in new plants.
The most common form of poison ivy is the shrubby form: T. radicans rydbergii (sometimes treated as a separate species, T. rydbergii), most often seen as a creeping shrub under 1 foot (30 cm) tall, although in some regions it can grow to form dense thickets up to 7 feet (2 m) tall. More often, the plant will grow as a groundcover, extending by suckering to create an extensive carpet.
There is also a climbing form (T. radicans radicans) that can reach up to 65 feet (20 m), reaching the top of trees thanks to clinging aerial roots. The climbing form is most common in the eastern United States and as far north as southern Ontario and southwestern Quebec.
Leaves of Three
Poison ivy is best known for its trifoliate leaves. There is an old rhyme taught to school children in areas where the plant is common: leaves of three, let it be. However, many other plants also bear three leaflets, including strawberries, clovers and beans, so a plant with three leaflets is not necessarily a threat!
What makes poison ivy difficult to recognize is that its foliage is so variable.
It is usually shiny, but can be dull and often has a smooth margin, but sometimes has notches or even lobes. It may be helpful to know that the leaves are alternate, never opposite (a good way to telling it from a young box elder or Manitoba maple [Acer negundo], which has very similar trifoliate leaves at early stages of its life). Note too that the nerves are prominent and that each leaflet ends in a thin tip. Generally, the leaf is reddish green in spring, dark green in summer and bright red, yellow or orange in fall.
The greenish flowers are insignificant and the grayish-white berries—produced only by female plants—are especially visible after the leaves fall. They can still be on the plant in spring, when the snow melts.
Western poison oak or Pacific poison oak (T. diversilobum), from the West Coast of North America, and Atlantic poison oak (T. pubescens), from the southeastern United States, are close relatives and differ mainly from T. radicans by their trifoliate leaves that are lobed like oak leaves. Of course, they are not related to real oaks (Quercus, of the Fagaceae family).
Together, the three species cover essentially all of North America except the province of Newfoundland and Labrador and the far north.
A Summer Problem
Although the bare stems of poison ivy (and poison oak) remain poisonous all winter, it’s usually with the return of summer that doctors offices start to fill with poison ivy victims, often children. In summer, we’re outdoors more and dressed lightly to boot: plenty of bare skin for urushiol oil to sink into. Under those circumstances, itchy arms and legs are bound to happen.
Campers, forestry workers and hikers regularly run into poison ivy and gardeners find it a perennial problem. The worst suffering undoubtedly occurs when hikers inadvertently use poison ivy leaves as toilet paper.
One common myth about poison ivy is that once you’ve had it, the rash will come back every year on the same date. It’s not true, of course—it takes an actual physical contact with urushiol to stimulate a reaction—but if you carry out the same activity on about the same date every year, like opening the cottage at the end of May, your symptoms will likely reappear shortly thereafter.
Keeping Poison Ivy Under Control
Most weeds get into trouble with gardeners and farmers due to their invasiveness, but at least they don’t try to poison us, but poison ivy sure seems to want to. Of course, poison ivy has a place in nature and feeds birds and insects, so in many cases, you can just let it be. However, such a dangerous plant has no place near human beings. If you have some on your land, at least in a place where people or pets will likely be running into it—near paths, in lawns, in gardens, etc. —, it’s really your civil duty to control it. But how?
Here’s one technique. Put on long-sleeved clothes and waterproof gloves and tear out or dig out the entire plant. Next, clean all tools used with rubbing alcohol while still wearing gloves. Put the plant and gloves in a garbage bag, seal it and put out with the trash or bury the residue under 1 foot (30 cm) of soil. Wash contaminated clothes at least twice in hot, soapy water before reuse. And if you are very sensitive to poison ivy, it’s better to throw the clothes away!
If you are unable to dig it out (the poison ivy often settles in among tree roots or rocks where it simply can’t be extracted), try covering it with a thick black plastic tarp for at least 12 months. Without light, it won’t be able to survive. Or apply a non-selective weedkiller (total herbicide) with a brush, directly onto the foliage. It will usually be necessary to repeat this several times.
Even dead, stems (and roots) still remain toxic for many years, so you’ll still have to cut them back and bag or bury them.
Never burn poison ivy! Its smoke can enter the airways, causing a serious or even fatal reaction!
And don’t put poison ivy in the compost bin either. True enough, urushiol will decompose during composting, but it does so very slowly and the chance that a slight trace could cause a reaction in the user is just too much of a risk to take.
Oops! You Just Touched Poison Ivy?
Very quickly, within five minutes of contact if possible, wash the affected area with cold water (not hot water, which will open the pores and worsen the situation). You can use a mild soap, but nothing that will irritate the skin. To relieve any itching and redness that follows, cold compresses can be helpful, as can calamine lotion. Your pharmacist may have other suggestions. And don’t hesitate to consult a doctor if the reaction goes beyond an annoying but minor skin irritation.
Above all, study the photos of poison ivy included with this article and memorize the details: it’s when you don’t recognize poison ivy that you most readily become its victim.
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YUCK! California is one of only three states, along with Hawaii and Alaska, that lack poison ivy. However, poison oak is very common. I need to cut down a huge specimen that grew up with a douglas fir, and is not about seventy or eight feet up. I cut it at the base, and deal with what emerges from the roots later. It is fortunately relatively easy to kill. I leave the dea parts up in the trees. They deteriorate rather efficiently and do not stay toxic for long once they start to deteriorate.
This article on poison ivy is the most comprehensive I have ever read. Learned quite a bit. So thank you. Highly allergic.
I have a thick vine growing up a 45+ foot hardwood tree in my wooded yard. How does one kill it?
You could follow the instructions in the text, cutting back what you could, but you’d probably not be able to reach the upper parts. Still, with the stem cut, at least they’d die.
Two more identifiers I find useful: when the plants first emerge, the leaves are yellow-green (much brighter than the surrounding growth) and the new stems are dark red.. it’s a good way to find it early.
I’m allergic to it, have had some nasty bouts, and the vast variety makes it challenging to identify unless I leave all three leaves alone which I try to do. 🙂 I always wore shorts to garden but no more – long pants regardless of how hot it is, and I leave tanning to those at the beach. 🙂
I’m lucky: I seem to be immune.