18 Poisonous Plants and Flowers

Over the past millennia, people have learned through trial and error which plants are good to eat and which are best to avoid. In our modern, urban world, much of that cultural knowledge has been forgotten. Many gardeners may be surprised to discover that they are growing some of the world's deadliest plants in their own backyards. Here are several plants with lethal tendencies.

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Apples

An apple a day might keep the doctor away, but the same can't be said for apple seeds. The seeds contain cyanogenic glycosides, making them mildly poisonous. If you consume enough of the seeds, by chewing, mashing, or breaking them apart in some way, you could ingest a fatal dose.

But that's a lot of apples. Although the number of seeds varies from apple to apple, as does the exact amount of cyanide-releasing glycosides, the adult of average weight would need to finely chew and eat about 222 apple seeds, or about 26 apple cores, to receive a fatal dose. Much less would be harmful to children. According to one study, seeds of Golden Delicious apple varieties contain more cyanide-releasing glycosides than other types. But no matter the variety, whether you cut up apples for your children or eat them whole, be sure to remove the seeds to be on the safe side.

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Deadly Nightshade

Closeup of a dark berry on a nightshade plant — Melanie Shaw / Flickr / CC BY-ND 2.0

The name says it all—both the foliage and the berries of this plant are extremely toxic. Deadly nightshade has a long, colorful history of use as a poison, but what many people don't realize is that the nightshade family includes common food plants, including potatoes, tomatoes, eggplants and chili peppers.

In fact, all of these plants contain toxins—usually in their foliage—that can be harmful. In particular, humans and pets should avoid potato and tomato foliage and vines in the garden.

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Rosary Pea

This plant may sound pious, but it's actually deadly. Rosary peas got their name from their traditional use as ornamental beads for rosaries. They are used in jewelry around the world, and are also called jequirity beans. The poison contained within the seed is abrin—a close relative of ricin and one of the most fatal toxins on Earth.

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Oleander

Oleander is one of the most toxic, commonly grown garden plants in the world. Ingesting any part of this plant can be deadly, especially for children. Even smoke from burning oleander can be fatal.

The plant's use as a poison is well-known. One study estimates the fatality rate among oleander poisoning cases to be between 6 and 10 percent.

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European Yew

Red Bell Berry growing outside — DM / Flickr / CC BY-ND 2.0

Relatively common in Europe, northwest Africa and the Middle East, nearly all parts of this slow-growing tree can be poisonous. The exception is the red fleshy aril that surrounds the toxic seeds. The aril is frequently eaten by birds.

Ingesting the leaves or the seeds, both of which contain a poison called taxane, can cause death. Symptoms of poisoning can include a fast heart rate, muscle spasms and labored breathing.

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Daffodils

Prized for their beauty, daffodils grow from bulbs that could be mistaken for an edible food, like an onion. Daffodils—also known by their genus name Narcissus—are common ornamental plants with a bright, cheery flower. Most daffodils are deer- and vermin-resistant, but gardeners shouldn't overlook the dark side of this plant. All parts of the daffodil contain a toxin called lycorine. The most poisonous part of the plant is the bulb, as it has the highest concentration of lycorine, but eating any part of a daffodil can cause nausea, vomiting, and other symptoms.

The Greek philosopher Socrates sometimes referred to daffodils as the "Chaplet of the infernal Gods" because of the plant's numbing effect.

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Doll's Eye

White baneberry growing outside — benet2006 / Flickr / CC BY 2.0

It's a good thing the creepy-looking berries of this plant aren't enticing, because consuming the fruit of a doll's eye plant (or white baneberry) could kill you. The berries contain cardiogenic toxins that can have an immediate sedative effect on cardiac muscle tissue.

Symptoms of poisoning include burning of mouth and throat, salivation, severe stomach cramps, headache, diarrhea, dizziness and hallucinations. Ingestion of the berries can eventually lead to cardiac arrest and death.

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Hemlock

This is one of the most famous poisonous plants in history—it's the flora responsible for killing Socrates. All parts of the plant contain the relatively simple alkaloid coniine which causes high blood pressure, vomiting and progressive paralysis of the central nervous system.

Hemlock is also known by several common names, including devil's porridge, beaver poison or poison parsley.

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Stinging Tree

Underside of the Dendrocnide Moroides plant — N. Teerink / Wikimedia Commons / CC BY-SA 4.0

Found in forests in Queensland in Australia and Indonesia, Dendrocnide moroides is one of the most painful and potent stinging nettles in the world. Accidentally brushing past any part of this plant or its stinging relatives can deliver a potent toxin that will cause a painful stinging sensation lasting for days or even months.

A severe sting from this plant will cause a severe allergic reaction and even death in horses and dogs; one human death has been reported. (One researcher documented her time working with Dendrocnide excelsa, which is considered a less dangerous plant, and her increasingly allergic reaction to the plant offers a sense of what's possible.)

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Castor Beans

If you have consumed castor oil before, you might be surprised to learn that castor beans contain one of the most poisonous substances in the world, ricin. Just one castor bean has enough ricin to kill a child within a few minutes.

Despite this grim quality, castor bean plants are frequently grown for decorative purposes, even in parks and public places.

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Angel's Trumpet

Angel's trumpets are woody-stemmed bushes with pendulous flowers that hang like bells. They are prized as decorative additions to the garden because of their elegant flowers. The catch is that all parts of these plants contain dangerous levels of poison and may be fatal if ingested by humans or animals.

Angel's trumpets have occasionally been used to create a recreation drug, but the risk of overdose is so high that these uses often have deadly consequences.

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Monkshood

Monkshood has a long tradition as a deadly plant and was used by ancient warriors to carry out executions. It was also once used as a popular werewolf repellent.

In 2014, a gardener died of multiple organ failure after brushing past this deadly purple flowering plant on the estate where he was working in the U.K.

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White Snakeroot

White snakeroot contains the toxin tremetol, which can be poisonous if consumed directly or second-hand. When snakeroot is eaten by cattle, the animals' beef and milk become contaminated with the toxin, and ingesting those substances can lead to a condition called milk sickness. Abraham Lincoln's mother, Nancy Hanks, reportedly died after swallowing snakeroot-contaminated milk.

Human disease is uncommon today because of current practices of animal husbandry and the pooling of milk from many producers, but milk sickness does still occur.

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Larkspur

The seeds and young plants of the larkspur are toxic to both people and animals. Toxicity decreases as the plant ages. Larkspur has several alkaloids including delphinine, delphineidine, ajacine and others that can cause very unpleasant issues. According to the U.S. Department of Agriculture (USDA) symptoms of poisoning include general weakness and muscle spasms, as well as abdominal pain and nausea. Eventually, it can lead to respiratory distress, paralysis and death.

Larkspur is responsible for heavy livestock losses, according to the USDA, particularly with cattle in Western states when the animals are allowed to graze where the plant is abundant.

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Foxglove

The seeds, stems, flowers and leaves of the foxglove plant are poisonous. They contain digitalis glycosides, which are organic compounds that act on the heart. When someone eats part of this attractive plants or sucks on the flowers, the glycosides affect cardiac function, causing an irregular heartbeat. Symptoms can also include digestive issues, headache, blurred vision and confusion and could lead to death.

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Melia azedarach

In Australia, it's known as white cedar. But this deciduous tree in the mahogany family is also known as a chinaberry tree, the Pride of India, an umbrella tree and the Persian lilac. Its fruits contain a mixture of poisons, including neurotoxins, which can harm humans (as few as 6 berries can kill a person). Birds, however, can tolerate them, so they eat the fruit and spread the seeds.

The flowers on the tree, which is native to Australia and Southeast Asia, are small with light purple and white petals of five, and they often grow in clusters. The fruits are small, spherical and yellow.

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Lantana

These flowers are colorful and bright, but take it as a warning. They're poisonous to humans and animals, and so should not be planted anywhere that you might come into contact with them. Symptoms of poisoning may vary, but gastrointestinal pain and nausea are common, as is shortness of breath. If you do want these flowers, put them in hanging baskets where they can be displayed without risk of contact.

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Lily of the Valley

This exquisite and aromatic little bells harbor a dark secret—toxic compounds called cardiac glycosides that can cause vomiting and diarrhea. The roots are most toxic, but all parts of the plant are, with children most commonly being poisoned by the attractive red berries. Keep it far from the reach of anyone who might feel an inclination to ingest it.