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.

1 of 16 Apples Artotem / Flickr / CC BY 2.0 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, you could ingest a fatal dose. But that's a lot of apples. Healthline did the math: You would need to finely chew and eat about 200 apple seeds, or about 20 apple cores, to receive a fatal dose. If you cut up apples for your children or prefer to eat whole apples down to the core, you may want to remove the seeds, just to be safe.

2 of 16 Deadly nightshade 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.

3 of 16 Rosary pea CanerCakir / Getty Images 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. Many jewelry makers have died after pricking a finger while handling a rosary pea. The poison contained within the seed is abrin — a close relative of ricin and one of the most fatal toxins on Earth.

4 of 16 Oleander Swaminathan / Flickr / CC BY 2.0 Oleander is one of the most toxic, commonly grown garden plants in the world — and oddly enough, it's often found in schoolyards. 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, reports WebMD. Oleander is reportedly a favorite suicide agent in Sri Lanka, where oleander poisonings exceed 150 per 100,000 people each year. About 10 percent of those cases are fatal.

5 of 16 European yew 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. Some people have chosen to commit suicide by ingesting the leaves or the seeds, both of which contain a poison called taxane. Sometimes there are no symptoms of poisoning and a person or animal can die within a few hours of ingesting yew seeds or leaves. If there are symptoms, they can include a fast heart rate, muscle spams and labored breathing.

6 of 16 Daffodils Russell James Smith / Flickr / CC BY 2.0 Prized for their beauty, daffodils grow from bulbs that could be mistaken for an edible food, like an onion. Daffodils — also known by their Latin name Narcissus — are common ornamental plants with a bright, cheery and mostly toxin-free flower. Most daffodils are deer- and vermin-resistant, but gardeners shouldn't overlook the dark side of this plant. The Greek philosopher Socrates sometimes referred to daffodils as the "Chaplet of the infernal Gods" because of the plant's numbing effect.

7 of 16 Doll's Eye 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.

8 of 16 Hemlock Feifei Cui-Paoluzzo / Getty Images 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 stomach pains, 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.

9 of 16 Stinging tree N. Teerink / Wikimedia Commons / CC BY-SA 4.0 Found in forests in Queensland in Australia and Indonesia, Dendrocnide moroides is the deadliest and most potent stinging nettle 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 in pigs, horses, dogs and many other animals, but there's not as much evidence that it has killed humans. (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.)

10 of 16 Castor beans Noppharat05081977 / Getty Images 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 an adult within a few minutes. Despite this grim quality, castor bean plants are frequently grown for decorative purposes, even in parks and public places.

11 of 16 Angel's trumpet Michael Davis / Getty Images 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.

12 of 16 Monkshood typo-graphics / Getty Images Monkshood has a long tradition as a deadly plant and was used by ancient warriors to poison the water of their enemies. It was also once used as a popular werewolf-detection tool. The flower was held near the alleged wolf’s chin, and if a yellow-tinged shadow appeared, that was confirmation that the person was a werewolf. In 2015, 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.

13 of 16 White snakeroot Andrei Stanescu / Getty Images White snakeroot contains the toxin tremetol, which can be poisonous if consumed directly or second-hand. When snakeroot is eated 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.

14 of 16 Larkspur Natasha Wakefield / Getty Images 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.

15 of 16 Foxglove liz west / Flickr / CC BY 2.0 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 can eventually lead to death.