Poisonous Plants

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Poison Sumac (Toxicodendron vernix), Anacardiaceae. Detail.
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Poison ivy in fall.
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Poison ivy in summer.
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Poison oak in fall.
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Poison oak in summer.
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A path through the woods with poison ivy growing abundantly.
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Poison sumac in fall.

Europeans encountered many botanical treasures when they settled North America. Especially valuable were new foods they had never before experienced. But it didn’t take long for them to discover some less desirable plants as well.

The first written mention of poison ivy is found in Captain John Smith’s 1624 The Generall Historie of Virginia, New-England, and the Summer Isles. Even back then, “leaves of three, leave it be” was a useful verse to remember when spending time outdoors. It’s even truer today. Poison ivy is benefiting from increased levels of carbon dioxide in the atmosphere, and the plants are more numerous, more vigorous, and more potent than they were even a few decades ago.

Poison ivy and its cousins, poison oak and poison sumac, are known botanically as toxicodendrons. They contain an oil called urushiol that triggers an unpleasant immune reaction in 85 percent of the population. Every one of the lower 48 states is host to at least one of these plant varieties.

Poison ivy can grow as a climbing or trailing vine, or as a shrub, and is found in some form in most states. The West Coast is home to Pacific poison oak, which can grow as a woody shrub or a climbing vine, while Atlantic poison oak grows as a compact shrub throughout the Southeast. Poison sumac, a small tree, is found in wet, swampy areas in every state east of the Mississippi River. More rare than poison ivy and oak, poison sumac is also more toxic.

Recognition & Protection

The first element of prevention is familiarity with the plants’ appearances, which can vary widely. One encounter, or seeing one photo, won’t prepare you to recognize these plants in various settings, so it’s helpful to search online and study multiple photos taken at different times of the year.

For example, poison ivy’s three almond-shaped leaflets are usually glossy, but they range from light to dark green, and their edges can be smooth or notched. Poison ivy is easier to spot in fall, because the leaves are bright red. The leaves are shed by winter, but the off-white berries may remain, and the leafless winter vines may still be recognizable for their hairiness – but smaller offshoots may be hairless. Familiarity with all the possibilities is the best way to prevent exposure.

The next line of defense is protective clothing. Wear long sleeves and pants that minimize exposed skin. Always wear gloves when handling weeds, and exercise caution when using weed whips, which can broadcast urushiol spray over a wide area. Also, be careful when performing tasks like emptying clippings from a mower bag, and never burn weeds unless you’re absolutely certain that no toxicodendrons are present, as inhaling smoke from burnt poison ivy can be dangerous. Inhalation of urushiol-laced smoke is a major occupational hazard for firefighters – during the 2017 California wildfires, hundreds of them were treated for poison oak poisoning.

About Urushiol

Urushiol oil is present in all parts of poison ivy, oak, and sumac, from the leaves to the root, and it’s powerful – it can remain potent for several years after the plant has died, so use caution even around dried-up plants. The smallest amount of urushiol can trigger a severe reaction.

Urushiol is sticky and can cling to anything it touches, including clothing, tools, and even pet fur. So, while you might successfully avoid touching the plant itself, you may inadvertently get urushiol on your skin when you change clothes or pet the dog.

If you suspect there’s even a chance you may have been exposed to a toxicodendron, wash your skin with soap and water as soon as possible, remove and wash your clothing with detergent, and clean your equipment with rubbing alcohol. Timing is important, because urushiol molecules bind with a protein in your skin cells within 15 minutes. However, even after that time, a thorough cleansing with soap and water can lessen the severity of the reaction.

In cases where you don’t have access to running water, alcohol wipes can be effective.

Treating a Rash

If preventive measures fail to keep you from a reaction, the rash usually appears 12 to 48 hours after exposure. The symptoms – redness, swelling, itching, and blisters – are all part of the body’s immune reaction to the toxin.

Itching is often the most difficult part of the ordeal, and there are many home remedies that aim to alleviate the itch, such as cool compresses, calamine lotion, oatmeal baths, and aloe vera gel – anything that cools and soothes will help. Try not to scratch, as that can expose skin tissue to infection. Other home remedies, such as apple cider vinegar mixed with an equal amount of water, or undiluted witch hazel, are meant to dry out the blisters. An oral antihistamine like Benadryl (diphenhydramine) can help to relieve itching, as can topical application of hydrocortisone cream. Some folk treatments use the juices of native plants. A plantain poultice, made by mashing or chewing a handful of the leaves, can cool the burn of the rash, as can a jewelweed poultice made from a mash of the fresh leaves.

Self-help measures are usually enough to handle a poison ivy reaction. However, if the eyes, mouth, or genitals are involved, or when there’s facial swelling or difficulty breathing or swallowing, seek medical attention. A physician can prescribe corticosteroids, such as prednisone, to help reduce the body’s immune response to the urushiol. There is no cure, though, and the symptoms will be completely gone only when the reaction has run its course – as long as three weeks.

And those three weeks can be so aggravating that it’s easy to understand why previous generations developed rhymes to warn against all parts of the poison ivy plant. Alongside the famous “leaves of three” jingle, we can recite “hairy vine, no friend of mine,” and “berries white, run in fright” – all of which remain good advice today.