Avoiding Rash Decisions: A Guide to Plants You Shouldn’t Touch

Avoiding Rash Decisions: A Guide to Plants You Shouldn’t Touch

Poison ivy can be quite beautiful, really. Foliage can appear shiny, dull, or red depending on the time of year and the age of the leaves. Photo by M.S. Henszey

"Plants are in the business of making themselves inedible,” says Tom Vogelmann, a plant biologist at the University of Vermont. Being unable to run and hide from predators, they’ve spent millions of years figuring out other ways to fight back. Some have developed armor – tree bark, for instance, or thorns. Others employ more advanced chemical defense systems. These chemical mechanisms range from shock and awe (an herbivore who eats an amanita-type mushroom could drop dead) to on-the-sly (some wild yam species contain a contraceptive molecule that works to render a predator sterile – a brilliant bit of lose-the-battle, win-the-war strategy).

Walk through a poison ivy patch, or brush up against some stinging nettle, and you’ll learn firsthand about plant defense mechanisms. The resulting rash you’ll get, the annoying pain you’ll feel, isn’t personal – the toxins in these plants weren’t meant to target humans specifically. Still, the fact that we’re merely collateral damage in a chemical war that has been raging for eons is of little consolation to someone soaking in a bathtub full of calamine lotion.

There are dozens of plants in the northern forest that contain rash-inducing chemicals, including members of the celery, rue, mulberry, pea, and aster families. The human reaction to these chemicals (called furanocoumarins) is generally mild – think of the “do you like butter?” game you played as a child, where a buttercup held beneath a classmate’s chin would sometimes cause – instead of the expected yellow reflection – a minor red spot. Common plants, such as sunflowers, wild grapes, and clematis can be irritating to touch.

Other plant’s toxins are a bigger deal, though, and can cause itchy rashes and painful boils. The following represents a rogue’s gallery of common plants that can “bite” you.

Gonna Need An Ocean Of Calamine Lotion…

The king of Northeastern rash plants is, of course, poison ivy (Toxicodendron radicans). The plant can be identified by its almond-shaped leaves that have subtle teeth along the edge. The leaflets appear in clusters of three, and the middle leaflet has a longer petiole. Poison ivy grows both as a ground cover and as a woody vine that climbs trees by means of aerial roots. Western poison ivy (Toxicodendron rydbergii) is also found in the region, and appears as a straggling shrub. Both types of poison ivy prefer warm, rich sites over cooler, thinner, higher elevation sites.

If you brush up against any part of a poison ivy plant (or come in contact with a pet or a garden tool that has brushed up against a poison ivy plant), you might contract a rash. Rashes typically appear a day or two after you’ve touched the plant and are characterized by red swelling and small blisters. The intensity of the rash varies among individuals.

To understand why some people seem immune to the plant while others seem to get a rash just by looking at it, one must first understand the science behind the body’s reaction. Poison ivy contains a chemical compound called urushiol. When you touch the plant, the urushiol is transferred to your body and absorbed through your skin, where it binds harmlessly to your skin cells. Left to their own devices, your skin cells would simply metabolize the compound, and you’d never even know you’d touched the plant. But if the body’s immune system gets wind of things, all hell breaks loose.

Inside your body, your T-lymphocyte cells are constantly roaming around looking for invaders (high-school science teachers often analogize these to “cop” cells). If there are no T-cells in the vicinity, the urushiol compound will not be discovered and no rash will ever develop. But if the chemical is spotted, your T-cells call for military backup (killer T-cells) that swarm in, latch on, and release toxic enzymes. This is why it sometimes takes several days before the rash appears – it takes time for the cells to multiply and for reinforcements to appear. The enzymes destroy both the invading compound and your healthy skin cells. The effect is red blistering and an itchy rash.

Now some people’s T-cells, for whatever reason, don’t recognize urushiol. These lucky folks can roll in a patch of poison ivy with immunity…and impunity. If you’ve brushed up against the plant, though, with no ill effect, don’t be so sure you’re one of the lucky ones. Touching poison ivy is like playing Russian roulette – the more contact you have, the greater the chance of its “discovery” in your body. And once you’ve been sensitized, every time you touch the plant, the reaction will happen more quickly. This is why gardeners are often hypersensitive to poison ivy: they’ve been over-exposed, and their T-cells are just waiting to pick a fight.

All parts of the plant can give you a rash. If you burn poison ivy, urushiol can attach to smoke particles, which can land on your skin and infect you; worse, you can inhale the toxin, and it can severely damage your lungs. Washing does remove the oil, but it can also spread it. Air alone can not spread the compound. The liquid in blisters is harmless and will not spread the rash, but try not to scratch, as the bacteria in your fingernails may cause additional skin infections. Folk wisdom suggests that jewelweed sap will ease the rash, but at least two controlled clinical studies have shown that jewelweed is no more effective than a placebo in poison ivy treatment.

While the toxins in poison ivy are remarkably effective in keeping humans away (Leaves of three, let it be! Hairy vine, no friend of mine!), it doesn’t work as well on avian predators. The many species of birds that are fond of the berries are culprits in spreading the plant as they deposit seeds along their autumn flyways.

A Poisonous Cousin

One of the more mysterious plants in the Northeast, poison sumac (Toxicodendron vernix) is a small tree/large shrub, branched at the base, with compound leaves and grayish-white fruit. Unlike the staghorn sumac in your backyard that has serrated leaflets and hairy bark, poison sumac has smooth leaf edges and smooth bark. The plant exists throughout the region, but because it grows in wetlands, not many people typically see it. (Specifically, it favors fens, a type of stagnant wetland.)

According to Don Cameron, Botanist with the Maine Natural Areas Program, poison sumac is uncommon in northern Maine, both because of the cold temperatures and also because it prefers high pH (calcareous) wetlands. It is dispersed throughout the southern parts of the state, specifically Penobscot, Franklin, Oxford, Knox, Kennebec, Androscoggin, Cumberland, and York Counties. Distribution in New Hampshire is also limited to southern and central portions of the state.

The plant is more common in New York and parts of Vermont where, due to the limestone bedrock, the pH is higher. In Vermont, poison sumac is found in Chittenden, Addison, Rutland, Bennington, Windsor, and Windham Counties and is especially prevalent in the Champlain Valley. According to state botanist Bob Popp, it’s found in shrub swamps, hardwood-cedar swamps, and calcareous red maple/tamarack swamps. In New York, it’s found throughout the state in calcareous wetlands. Rich Ring, botanist for the New York Natural Heritage Program, says that while poison sumac itself isn’t rare, there are many rare plants in New York that are associated with fens. As such, poison sumac is considered an important indicator of fen habitats.

Like poison ivy, poison sumac contains urushiol, so the rash is similar in appearance. In our national consciousness, poison ivy, poison sumac, and poison oak are often lumped together as plants we should watch out for. Northeasterners need not worry about poison oak, though. It struggles to grow north of the mid-Atlantic states.

Danger In The Carrot Family

To the untrained eye, many members of the carrot family look alike. Poison hemlock and spotted water hemlock can be confused with Queen Anne’s lace; wild parsnip can be confused with cow parsnip, which can be confused with giant hogweed, which can be confused with great angelica. Of all the plants just mentioned, though, only wild parsnip and giant hogweed contain sap that will significantly irritate your skin. (The hemlocks are very poisonous, but only if eaten).

Wild Parsnip (Pastinaca sativa), sometimes called poison parsnip, is an aggressive, non-native plant with a habit of rapidly colonizing disturbed areas. The plant has large, pinnately compound leaves that resemble celery leaves. The stem is hollow and grooved, two- to five-feet high. The flowers are small, yellow, five-petaled, and look like a yellow Queen Anne’s lace. Wild parsnip often grows along the edges of highways, which makes highway road crews and roadside mowers especially vulnerable to the poison sap. The rash is characterized by burnlike blisters that often form in splatter formations, or streaks; these patterns relate directly to sap dispersal as the plant is dragged across your skin.

The toxin in parsnip sap is different from the toxin found in poison ivy or poison sumac; the reaction your body has to it has nothing to do with your immune system, and everyone is susceptible. The harmful chemical compounds – specifically, psoralen and its derivatives – are photosensitizing, which means they’re activated by ultraviolet radiation from the sun. (Smear yourself with parsnip sap in a dark closet, and nothing will happen.)

When you get some sap on your skin, the sap absorbs solar energy, then releases it in the form of heat. On a micro level, the psoralen molecule lodges between two strands of DNA, and sunlight fuses the strands together. “You can imagine what this does to an insect that tries to eat the plant,” says Vogelmann.

The chemical reaction damages skin cells and feels very much like a burn. This is a key difference between the phototoxic dermatitis caused by poison parsnip and the allergic contact dermatitis you get from poison ivy. Parsnip burns feel like burns, whereas poison ivy rashes are often more itchy and irritating than painful.

While the pain from a parsnip burn is relatively short-lived, an encounter can leave long-lasting scars. As part of the recovery process, the body produces dark pigmentation that is thought to serve as a protective mechanism against further UV injury (sort of a super-suntan). These dark splotches can linger on the body for years. In an interesting twist, people with psoriasis and similar skin-pigment disorders sometimes turn the toxin to their advantage and use psoralens to help increase their skin’s sensitivity to ultraviolet light.

As its name indicates, wild parsnip is directly related to the edible parsnip growing in your garden. Like its domestic counterpart, the roots of wild parsnip are safe to eat and, by some accounts, quite delicious (especially in late fall or very early spring, after the plant has had time to convert its root starch to sugar). Take the time to properly identify the plant before you eat it, though, since other look-alike members of the carrot family have highly poisonous roots that could kill you.

Giant hogweed (Heracleum mantegazzianum) is wild parsnip on steroids. It’s an enormous plant that reaches heights of 14 feet. Its leaves – sometimes four feet long – are sharply and unevenly lobed. Stems are hollow with purple blotches and coarse hairs. The flowers are white, flat-topped, and can be 20 inches across. Originally brought to the Northeast as a garden plant, it has now spread and is colonizing meadows, streambanks, roadsides, and open wild spaces. The sap in giant hogweed is just as toxic as wild parsnip sap, and there’s more of it. Unlike parsnip, it has yet to become firmly established throughout the Northeast, and state agricultural agencies are trying to keep it that way. If you find giant hogweed growing in the wild, contact your local “hogweed hotline” and report the plant.

The Nettlesome Family

Stinging nettle (Urtica dioica), as its name suggests, stings. The stem and leaves of this plant contain thousands of tiny silicatipped hairs – imagine them as minute hypodermic needles.

When you brush up against the plant, these hairs penetrate your skin, break off, and release an irritating fluid. The toxin was originally thought to be formic acid, similar to what ants emit, but recent research has revealed it to be a combination of different chemical compounds. A nettle sting doesn’t cause a lingering rash like the previously mentioned plants, but it does hurt enough to make you wish you’d worn long pants while hiking.

Stinging nettle is a native plant that grows two to four feet tall. The plant loves nutrient-rich sites, including moist woodlands and river bottoms. It’s often found growing on compost or manure piles, often in shady parts of the garden.

Many people eat nettle leaves. “They’re fantastic,” says Vogelmann, who describes the flavor as pleasing and unique. To pick nettles, Vogelmann looks for plants that are 1–2 inches tall and picks them with gloves or a plastic bag over his hands. At home, he puts them in a large plastic bag, fills the bag with water, and shakes it. This washes the leaves and destroys the stinging hairs. Afterwards, the nettles are painless to handle and eat.

Online recipes abound for nettle pesto, nettles sautéed in bacon fat, and savory nettle soups. The leaves of the plant are high in calcium and iron, and many foragers tout their anti-inflammatory qualities.

Wood nettle (Laportea canadensis) is another native, common species of riparian forests and rich, forested slopes. The plant can form dense carpets in river-bottom floodplains; this serves as an especially nice bit of habitat for small woodland creatures. Wood nettle can be distinguished from stinging nettle by the fact that it has alternate leaves instead of opposite leaves; the leaves are also broader and wider. Wood nettle has fewer stinging hairs, but they can still hurt when you brush up against them.

Keeping It In Perspective

The great risk of profiling hurtful plants in a magazine is that by the end of the story, a reader could be left with a lingering paranoia of anything green. It doesn’t have to be this way. The experienced trout fisherman easily circumvents the patch of wood nettle growing along a mountain stream. The gardener learns to cultivate soil away from the hedgerow full of poison ivy. The landscaper instructs his crew to remove roadside wild parsnip with a sicklebar mower, not a weed-wacker.

Whereas these plants use their poison as a defense mechanism, our human defense mechanism is our mind. Learn to identify these plants and avoid them. In most cases, that’s the only defense you’ll ever need.

Hogweed Hotline

If you find Giant hogweed growing in the wild, let someone know.

Vermont: 802-828-1246
New Hampshire: 877-398-4769
Maine: 207-287-3891
New York: 845-256-3111
Massachusetts: 617-626-1779

Dave Mance III is managing editor of Northern Woodlands.

Photo Gallery

  1. bob
    May 28, 2010

    I got stung by the wood nettle in Louisianna and it left mesquito like bumps wherever it punctured the skin that itch.

  2. Alice Mackay
    Aug 04, 2010

    I brushed up against stinging neddles while fishing. I had a very nasty reaction with large welts on my body. Very painful and itchy. It made me ill and I had to seek medical assistance I was allergic and it has taken 1 month to clearup somewhat. I have scaring and it left black and blue bruising.It is still very irritable and itchy.

  3. Suzie Q
    Feb 03, 2011

    I had never heard of Poison Parsnips until I got a case of it last Spring.  I hadn’t realized that I had gotten into it until one morning I got up and noticed that my chin was getting what I thought was a “zit”.  When my whole chin got red and swelled I thought I had a boil forming.  But then, after about the third day, I got what looked like poison ivy blisters and they oozed, just like ivy - except it never itched.  I had it for 3-4 weeks, with my chin swelling and crusting with oozing liquid.  I couldn’t go anywhere without a large bandaid covering it.  I finally got a prescription for prednisolone pills which turned it around, but I had a red scar for 6-8 months, which finally turned white - but my chin is permanently scarred.  I have never seen my skin react to poisoning like this did - where the blisters sank in and left an indentation in my chin, which never filled in with flesh when it healed.  I have a dent in my chin, now.  Very bad stuff to get.

  4. Nathalie Guilbault
    Jul 28, 2011

    Just thought I’d say that this is one of the best articles I have read on poison ivy, (and others).  I have read many as my husband’s reactions are extreme while I can walk in the exactly the same ares and remain unscathed.  Very informative - thanks.

  5. Walter
    Oct 27, 2011

    Yesterday I found 2 light green stems, maybe 2 feet apart in piney woods, with 3 lily-like buds on each, which are pointy at each end, fat in the middle, growing up maybe 3 feet high. Any help with id?

  6. Meghan
    Oct 31, 2011

    Hi Walter. If you can, email me a photo of the plant you came across, and I’ll see if we can get an ID on it. .(JavaScript must be enabled to view this email address)


  7. Kelly
    May 02, 2012

    My six year old son jumped into a bush that resembled deadly nightshade but the doctors said it wasn’t night shade.  My son had boils and swollen legs within minutes of jumping in.  He was screaming his legs and arms were on fire.  The docs gave him high doses of steroids and soaked his legs in a salt water solution.  I guess I’m wondering if you know of any plants that can cause that? By the time I took our German friend up to look for the bush- the maintenance folks had dug it all up.

  8. Brenda
    May 19, 2012

    Yikes, this stuff is wicked. I am trying to clean out my over run raspberry patch and this weed seems to find the soil perfect there.I was not prepared for the reaction I got when I brushed up against it. It has been an hour and I still have the welts where it touched. I tried vinegar and that seems to have calmed it down. Looks like I will have to dig it out to get rid of it. It is really bad.

  9. scott
    May 30, 2012

    I never even heard of this stuff until my 5 yr old son and I riding in the car.He brings sticks and rocks he finds with him sometimes when we go out in the mountains. Well, he was playing with his stick and broke it over his knee as I was driving. He told me that when he broke his stick, a bug bit him and within minutes he was crying worse than I have ever seen anyone before, and breaking out with bumps all over his legs that went into muscle spasms. I have never seen or heard of anything like it. Once we were at the hospital, they couldn’t find the bite and he only has a scratch. I go and grab this stick and its a petrified peace of this hemlock.  I have never even heard of it before. He is fine now and that is all that matters, but I figured I would write about this so maybe someone out there might beware. Thanks for your time.


  10. Robin
    Jul 08, 2012

    A few days ago I went to my friends’ cabin deep up north in WI. And this morning I woke up with a mysterious rash that was not there a few hours earlier when I went to sleep. My friends say it looks like a bad case of acne and my dad says it looks like poison ivy… but it didn’t start to even show til 2 days later?

    It doesn’t itch, and didn’t show up on contact, it is on both sides of my face and is covering my jaw, and looks like a major breakout of acne. I want to know what it is so I can watch out for it next time. Got any ideas?

  11. Iceni
    Jul 23, 2012

    Hello: I was interested in hearing about stinging nettles. An old folk remedy seems to work.  Usually nearby the nettles are dock plants. Take several of the leaves and crush them in your hands, rub the juice on the sting from the nettles and this can relieve the pain, the itch and the blisters. It has always worked for me.

  12. Bridget
    Aug 08, 2012

    I’m still not sure what plant I came in contact with but after reading this article I’m going to go check for wild parsnip.  I was weed eating and mowing for my in laws.  The next day I woke up and had the “whip” like marks on my legs and the “boil” like blisters. Never did these marks itch but man did they hurt.  I have had shingles in the past so at first I thought maybe I had it again. The blisters hurt to apply any creams so I didn’t. It never spread just hurt.  I still have the scars on my legs and it’s been 6 weeks. Thank you for giving me an idea of what to look for.

  13. Mae
    Jun 23, 2013

    20 June, late afternoon, I cut the grass and there was a section of
    1 1/2 feet buttercups that I cut as well.  Had itchy left cheek and rubbed with the clean bumpy latex glove.  Hours later it was very itchy and had small pus like bumpy rash. Went to Doctor Friday 21st got Fucidin Cream and Antihistine for 2 weeks.  Still itchy 3 days later. Is there a faster cure for the face and does this spread? Read vinegar is good to kill the roots in soil. I put Weed Killer. Will check results in 2 days.

  14. Fred
    Jul 03, 2013

    I have (don’t want) stinging nettle in my vegtable garden and I keep pulling it up.  But it replenishes faster than me!  At least from your article I now know what it is.

  15. Janet Dunn
    Jul 24, 2013

    I know this is a woodland plant discussion, however I have gotten a nasty reaction to something stinging that stuck in my skin at the beach.  It was not an insect but the minute it stuck in my skin it burned and hurt.  I have been on an antihistamine, trying to not itch for days.

    It looks like a bug bite with a large red diameter (size of a dollar coin) and it is swollen a bit.  It was originally on my shirt and I brushed my arm back and a minute thing with cone like bottom stuck in my arm.  I have searched and cannot determine what it is and would appreciate any help. The under side of my arm is so itchy and it hurts to touch.

    Thanks in advance,  Janet

  16. John Collins
    Aug 01, 2013

    Can I cut or burn poison parsnip without harm to myself or others?
    What about spraying a weed killer on it? Thank you.

  17. Pamela H.
    Oct 26, 2013

    I was rescuing a guinea pig near the woods (someones Pet) and I thrust my hand (like an idiot) into a fuzzy bunch of green leaves. A neighbor called it poison ivy but I have to say it was worse. My hand became almost something I couldn’t recognize! It turned bright red and had gloppy bubbles all over the fingers…it was oozing, I called it creepy holly but I don’t know what caused it. blisters on my wrist would pop, the discharge was so gold it looked weird. It wasn’t poison ivy but what the He—was it? My right hand (three fingers) hurt to this day, weeks later…throbs and hurts slightly, I felt I almost lost the 3 fingers ghastly. I’ll be careful after this!!!

  18. John Mc
    Jul 16, 2014

    A good articles on wild parsnip burns: 

    and one comparing human reaction to wild parsnip vs poison ivy:

  19. Randy Fecteau
    Aug 01, 2014

    Are there any good home remedies (for nettle stings) to get rid of the tingly prickly feeling?

  20. Robert
    Aug 22, 2014

    Does anyone know what type of weed or grass irritates the eyes, knows, and throat when mowed? My eyes keep earing, my nose runs, and I keep coughing when I mow the lawn.

  21. Dena
    Oct 25, 2014

    Please HELP! My husband and I were in New Hampshire last week- walked the entire Flume Gorge!

    That’s in Franconia Notch State Park- the next day or maybe it was 2 days- I had what looked like an inch long scratch on the inside of my left leg- about 3 inches above the ankle bone- well it ITCHED- I’m sure I scratched in my sleep and I was using Tecnu Calagel- it became red and angry looking and now I have little raised itchy dots in the area near it and on the back of the other leg in 2 places - smaller area than the first leg though. Now we are back in TX where no one knows ANYTHING about this!! And just fyi- I’ve never been allergic to poison ivy down here- so can’t imagine it being that?

  22. Dave
    Oct 29, 2014

    Sounds an awful lot like poison ivy to me, Dena. And just because you haven’t had a reaction to the plant in the past doesn’t mean it won’t happen in the future.

  23. Ana
    Nov 16, 2014

    Beware, gardening different types of peppers is fun, but the oils in the plants themselves can cause reactions. Late this summer, I came in to contact with a pepper plant, it brushed up against my leg. It was a hot pepper plant branch that had been cut and had been in the sun. I picked it up because there was still a long red pepper on the plant and I wanted to use it to spice up a dish I was preparing.  Little did I know the sap from this plant, just brushing up against the skin can cause blisters and burning itch type rash.

    The blisters appeared days later, were itchy, burst, then the itchy rash spread where the fluid went. Ouch, so itchy.
    Calamine, hydrocorstisone, aloe lotion, basically treat as a wound, wash, apply ointment, cover with bandage, repeat.

    It’s two weeks now and it’s barely starting to fade, but the itching subsided -  thankfully.

  24. Katina
    Aug 19, 2016

    Just ran into some wood nettle today up on Mount Hor. Holy crap did that BUUUURN and then got crazy itchy! At a stream I put mud and wet leaves on and that helped. My bro went off trail with his dogs to avoid a couple with two more dogs and went through a patch with bare legs. He was almost in tears it burned so much. He ran down the rest of the trail (as we had already passed the stream, he wasn’t going back) grabbed the water jug that we bring for the dogs and dumped that over his legs. I could not believe how much that stuff burns! Now I know what to look for on the trail and this is a great article.

  25. Marietta
    Dec 07, 2016

    I had blisters that were itchy and weeping the day after my husband cut and worked with holly putting it around our door for a Christmas decoration. Every where he touched me I broke out. I thought it was poison ivy but don’t have any in the yard. Has anyone ever gotten a dermal reaction like this with holly?

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