THE TERRIBLE TOXICODENDRONS - Wicked Plants: The Weed That Killed Lincoln's Mother and Other Botanical Atrocities - Amy Stewart

Wicked Plants: The Weed That Killed Lincoln's Mother and Other Botanical Atrocities - Amy Stewart (2009)


Poison ivy, poison oak, and poison sumac occupy nearly every state in the Union. But most people don’t realize how truly evil toxicodendrons can be.



Toxicodendron radicans


Toxicodendron diversilobum, others


Toxicodendron vernix

Poison ivy is not, technically, an ivy. Poison oak is not an oak. Poison sumac has nothing to do with sumac trees. And by the way, none of them are poisonous.

The irritating oil they produce, urushiol, is not at all toxic, but it does happen to be an oil that most people are highly allergic to. Oddly enough, only humans are bothered by exposure to urushiol. No one knows why the plants have singled out people for their unique form of vitriol. Because urushiol creates an allergic reaction—which is nothing more than the immune system gone haywire, fighting some harmless substance, like Don Quixote charging at windmills—each subsequent exposure is worse than the one before. The immune response gets stronger, so that the reaction gets worse with each repeated exposure.

Someone who has experienced a severe poison ivy outbreak could be very sensitive to the rind of the mango fruit or other parts of the tree.

Roughly 15-25 percent of the population is not at all allergic to toxicodendrons and will never develop a reaction. Another small slice of the population could develop a rash but would need prolonged, intimate contact with the plant to bring it on. But unfortunately, about half of all people will break out if they brush up against the plant, and some of them are so allergic that they may require hospitalization. They are called “exquisitely sensitive” by botanists and physicians.

Those sensitive to poison ivy, poison oak, or poison sumac will break out into an oozing, unbearable rash. Since the oils can persist in sleeping bags, on clothing, and in the fur of adorable little dogs, you may not realize that you’ve been exposed until it’s too late. It can take several days for the rash to appear. Once it does, reactions last two to three weeks. Oatmeal baths may be soothing, and the worst cases may require a shot of steroids, but most victims simply wait it out. Fortunately, reactions are not contagious. Those sores will probably get you banished to the couch, but they will not infect the rest of the family.

Even the most common poison ivies and poison oaks are difficult to recognize. Campers can use a simple trick to identify plants containing urishiol: carefully wrap a piece of white paper around the stem or leaf of the plant in question, crushing the plant without coming into contact with it. If the plant contains urishiol, a brown spot will appear quickly on the paper and turn black within a few hours.

If you’ve had an allergic reaction to poison ivy, oak, or sumac, you’re much more likely to have a reaction to some of their relatives, including:


Anacardium occidentale

The nuts are only safe to eat if they have been steamed open. The oils in the tree, including the fruit from which the nut dangles (called the cashew “apple”), can cause a breakout that looks just like a poison oak reaction.


Mangifera indica

Produces a volatile oil everywhere except the inside of the fruit. Someone who has experienced a severe poison ivy outbreak could be very sensitive to the rind of the fruit or other parts of the tree.


Toxicodendron vernicifluum

Used for centuries to produce lacquer and varnish, but it is extremely difficult to work with and a real hazard to workers. Even lacquer found in ancient tombs has caused a rash.



Sago Palm


Gardeners from Florida to California know the sago palm. It is a very tough, slow-growing tree that is widely used as a feature plant in landscapes. The most common variety, Cycas revoluta, is a popular houseplant and is often found in botanical garden conservatories. What most people don’t realize is that all parts of the plants, especially the leaves and seeds, contain carcinogens and neurotoxins. Pets are routinely poisoned by nibbling on the plant, and it has been responsible for widespread cases of human poisoning as well.


Tropics, some desert environments

Southeast Asia, Pacific Islands, and Australia

False sago, fern palm, cycad

The best-known incident of poisoning occurred in Guam. Locals made a flour from the seeds of the related false sago palm, C. circinalis. The traditional method involved leaching the poison out by soaking the seeds in water, but food shortages during World War II may have forced people to eat the seeds without first treating them properly. The poisonous compounds have also been found in bats, which the people of Guam considered a delicacy. The food shortages during the war, combined with the availability of guns when military personnel were stationed there, meant that bats were also hunted and eaten more frequently during that time.

Today scientists believe that this caused the mysterious variant of ALS, or amyotrophic lateral sclerosis (also known as Lou Gehrig’s disease) that occurred on the island after the war. This peculiar form of ALS included the nerve degeneration common to ALS, the tremors associated with Parkinson’s disease, and some symptoms that were similar to Alzheimer’s. Medical experts named the syndrome Guam disease and watched helplessly as it became the leading cause of death among native adults living on the island. British veterans and POWs who spent time on the island during the war also had exceptionally high rates of Parkinson’s later in life. As the standard of living improved on the island, and people began to eat a more Western-influenced diet, the syndrome all but disappeared.

The American Society for the Prevention of Cruelty to Animals has identified sago palm as one of the most toxic plants that pets may encounter. Just a few seeds can lead to gastrointestinal problems, seizures, permanent liver damage, and death. The palm is especially harmful to dogs that are tempted to nibble leaves and gnaw on its base. In spite of its name, the sago palm is not actually a palm tree. It is a gymnosperm, which means that it produces seed cones similar to those produced by conifers.

Meet the Relatives Cycas is the only genus in this family. Some are rare and sought after by collectors. These plants are extremely ancient; some show up in the fossil record sixty-five million years ago.