Leaves of three

Poison Ivy

I made a second visit to the site I’m surveying for the City a few days ago. The place was quite active with birds, including a few promising migrants that I hope stick around. The area I’m surveying is primarily open meadow that’s being encouraged to succeed into forest (obviously some decades from now, since the trees being planted are mostly just little seedlings). It’s surrounded by some relatively narrow bands of mature deciduous forest that separate the meadow from the nearby rail lines and road. However, there’s enough remnant forest there to support forest species, including the migrant Wood Thrushes and Ovenbird I heard singing from it.

Since I don’t really venture into the forest during the survey, once I’m done, before I leave, I made a quick foray to poke around. I spotted a few wildflowers I think of as “deep forest” species, stuff that you don’t see outside of the forest interior, including a couple favourites of mine like Bloodroot. And, I spotted this stuff. Poison Ivy. A favourite of hikers everywhere, who often take home more than just fond memories.

Poison Ivy

I’ve seen some nasty reactions to Poison Ivy, so I was careful to not step off the trail. The culprit causing the ugly rashes is a chemical called urushiol. It’s an oil, found in plants of the family Anacardiaceae. The most problematic genus is Toxicodendron, which includes the dreaded three Poison plants: Ivy (T. radicans), Oak (T. pubescens), and Sumac (T. vernix), the latter being considered by some to be the most toxic plant in the US. Fortunately for me, neither of the latter two occur in my area of Ontario (Poison Sumac does occur in Ontario, but it’s either restricted to the more southern Carolinian forest regions, or it’s local in occurrence and I’ve never encountered it).

Urushiol is an organic chemical with two forms, one saturated and one unsaturated, much like the fatty acids we pay attention to in food. The term “saturated” means that every carbon atom in the molecule has the maximum number of additional atoms attached to it that it can carry. A single carbon atom can make four bonds – some of these may be to other carbon, oxygen, or other atoms, but the rest will be filled with hydrogen. Unsaturated means that the bond between some of the carbon atoms is doubled (they’ve ditched – or never had – the extra hydrogen atoms and instead bonded with each other. Awww…).

Unfortunately, these double bonds can be unstable, and the unsaturated urushiol molecules are the ones that cause the more severe reactions because of the way they’ll interact with our own body molecules. The more unsaturated the molecule, the greater the reaction. A plant’s particular urushiol composition varies from species to species, with the species that cause the worst reactions, such as Poison Sumac, containing the greatest proportion of unsaturated molecules. Less than 50% of people react to saturated urushiol by itself, but over 90% will react to molecules with at least two double bonds.

Poison Ivy

Poison Ivy has a milder composition of urushiol, such that there are some people who don’t react to the oils. Variably between 15% and 30% of people may not react when exposed to Poison Ivy. I happen to be one of the lucky folks in this segment of the population, as are Blackburnian and my mother. People with hyperactive immune systems can have very severe reactions. An earlier boyfriend of mine fell into this category, and I usually found myself taking care of his clothing for him after he’d had an unfortunate run-in. He could probably have broken out in a rash just by looking at the plant. More sensitive people may find that they react also to other species in the Anacardiaceae family, including mangos (which have urushiol in their skin) or cashews (which have it in the shell).

Unfortunately, just because you’ve not reacted in the past doesn’t mean you’re scot-free. People who are repeatedly exposed to the compound may begin to develop a sensitivity to it, or may find that other plants with stronger urushiol compositions may trigger a reaction. Therefore I don’t intentionally go prancing through the stuff, just to be safe, although I’m usually the person to venture in if it’s unavoidable. Interestingly, the same thing works the other way, too – some people who have reacted in the past may lose sensitivity to it.

Poison Ivy

Not all animals are bothered by this oil. Birds, for instance, show no adverse effect from exposure, and will happily gobble down the white berries the plant produces in the fall. Pets who run through patches of the stuff will carry the oils back on their fur, which is unfortunate for the owner who unsuspectingly then pets the animal, but does the animal itself no harm. In fact, grazers could eat the plant and not become sick from it. Since the oils are not just present in the leaves, but also in the roots such that pulling it up by hand even in the winter will still give you a reaction, perhaps getting a goat as an organic means of control might be a solution…

So what do you do once you’ve had a run-in with the stuff? Well, for one, avoid hot water which will open your skin’s pores and allow the chemical to sink deeper. It’s an oil, but does dissolve in water (barely), so washing with lots of cold water may help, but will take a while. Using soaps with the water is preferable, to help break up the oil the same way you’d clean your greasy dishes. It’s partially dissolvable in alcohol, which may be another solution. There are some home remedies, too, including the sap from the plant Jewelweed – useful if you find yourself in the bush with no easy access to other options. If you’re not fortunate enough to notice it or be able to do something in time, without treatment the rash will last 3-4 weeks, but will eventually go away on its own. Various creams and salves are available that will soothe the itching and speed up the healing.

Poison Ivy

Obviously the best solution is just to avoid it in the first place. There’s a rhyme to help you remember – “Leaves of three, leave it be”. I’ve heard tagged on to this, “Leaves of four, have some more”, though this may have been made up by kids as I don’t know if it even references anything. Wikipedia also suggests second-halves of “Berries white, danger in sight”, referring to the white berries of fall, or “hairy vine, no friend of mine”, in reference to the little root hairs that secure the climbing version of the species to a tree trunk or other substrate.

In fact, the three-leaves thing by itself is not useful for identifying the plant (although you’ll be sure to stay away from anything with three leaves, which happens to include Poison Ivy), as there are other species that also have three leaves. I was taught that the three leaves of Poison Ivy droop, rather than being held erect, and that the plant has red stems, but in recent years I’ve begun to wonder about the broad applicability of those statements, and I can find no mention of them on websites.

The main features are the three leaves, the branches for which come off the main stem in an alternating pattern (rather than opposite each other), and that the stem has no thorns (I’m not sure what species does have thorns that it might be confused with). Other look-alikes will either have opposite leaves, or will not be uniformly three-leaved. Poison Ivy is usually bright green in the summer, but can be reddish in the spring and fall. There are two forms, a ground plant and a vine, which are variously lumped into one species or split into two, depending on your source. The ground form can grow as high as waist-height, if conditions are good, and also depending on the particular subspecies.

Poison Ivy

There are two species of Poison Ivy, an eastern and a western, and between the two they’re found across the continent. The only places to be free of it are Newfoundland, Alaska, California and surrounding area, and northern Canada. The species is more common now than it was historically. It likes forest edges and clearings where it can get a bit more light, and development has helped to create more of this habitat.

Also, a recent study apparently indicates that climate change is having an effect on the plant. The increased levels of CO2 in the atmosphere encourage greater plant growth, with larger leaves, as well as higher urushiol production.

Poison Ivy

You really have to wonder why a plant would go to the trouble of producing a compound that hardly anything reacts to. And also why most animals don’t react, but humans can react so violently. The plants weren’t talking, though, so it remains a mystery.


Author: Seabrooke

Author of Peterson Field Guide to Moths. #WriteOnCon Mastermind. Writer of action/thriller SF/F YA. Story junkie. Nature nut. Tea addict. Mother. Finding happiness in the little things. Twitter: @SeabrookeN / @SeabrookeLeckie

17 thoughts on “Leaves of three”

  1. that was really interesting. I get it every year, usually not too severely and I rarely remember to look for it. Maybe after learning all this information, I will mentally treat it like birds or mushrooms or other things that I am intentionally seeking and learn to avoid it–goal: avoid poison ivy summer 2008.

  2. Good luck with it, Beth! That should be a challenge. It’s unfortunate that the places it chooses to grow are also the areas we tend to find ourselves in the summer, and it’s so easy to inadvertently brush up against it while distracted by something else.

  3. Ahh! Your pictures are too vivid. Unfocus! Even the sight of that plant causes me pain. I’m one of the lucky ones whose sensitivity has increased over time. I would get it as a child and it would last a few days — Calamine lotion sticking to the bedsheets on hot summer nights. By the time I was a grad-student, an encounter meant weeks of sleepless nights and cortisone. Then I moved to California, went herping with colleagues (in January) and discovered poison oak, which has deciduous leaves. Lesson: When clambering up steep Santa Cruz Mtn hillsides, don’t grab saplings for support.

    I miss the Carolinian flora, but I don’t miss that genus. Poison Ivy should be shot through Vaseline!



  4. There is a moth whose larvae feed on Poison Ivy – is the strange-looking Light Marathyssa, and by some remarkable coincidence you can view the adult on my blog, right now…

  5. Great ID photos — the glossiness and the amount of red in the leaves are something to watch for.
    A number of years ago I wore work gloves and pulled up a good deal of the stuff on my parents’ property, and wore weeping bracelets for about two weeks afterwards. My brother’s reactions are much worse — but he has various allergies, and I don’t, so I was interested in your mention of hyperactive immune systems. Very helpful!

  6. Hope you guys don’t run into it too much anymore, Lavenderbay. Unfortunately, the leaves lose their red during the summer months, being entirely green at this time of year, and the glossiness is more a function of the leaves being young than because of their species. Of course, in the spring they’re pretty distinctive because of these features.

  7. well this is the 1st time i have actually seen a poison ivy plant and i get it every year including right now its not as bad as normal but i itch really bad. do you know what poison oak and poison sumac looks like cause i have never seen these 2 either but with me being adventurous i get them too

  8. Sorry to hear about your plight, Mike! I’m afraid I can’t help you with the poison oak or sumac, since they don’t grown in my area and so I’m unfamiliar with them. If you do a google image search for photos I’m sure you’ll get many hits, however!

  9. If its leaves of three WITH thorns its probably wild raspberry. I know because I found some in my backyard and have been researching this for the last few hours.

    1. Mmm- Delicious wild black raspberries! Also called blackcaps. For years our family has been gathering and eating their wonderful food gift. How wonderful tthat you found them.

  10. We have encountered poison sumac in the northeastern corner of Frontenac Park– sorry we did not take a picture! There is also a 5 leafed climbing plant in the area that is often seen on stone buildings, it turns red in the fall and it will give a nasty rash too (Virginia creeper maybe?). I react very badly to poison ivy and can spot it at 20 paces, there is apparently a great deal of variation in size and appearance though because sometimes it has that red to it and sometimes it does not. It tends to do best in dappled shade near the edges of forest openings. Now if I could just train my dogs to stay out of it!

  11. hello all,

    i just had a quick question. I’ve been searching for this weed/plant my whole life….because it releases a specific smell….I was wondering if any of you had an idea of what it may be…..I actually found it the other day but am experiencing troubles identifying it…….when the leaves die they smell of fruit loops……the leaves are in threes…….if anyone knows….i would luv your help!!


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