When you glance in a garden, along a roadside or even at the banks of a scenic creek, your eye may land briefly on a colorful or interesting plant. You may find yourself thinking that it is beautiful and would look nice in a bouquet. What you likely don't consider is just how deadly it could be to handle that plant.

Some plants commonly viewed in these seemingly benign contexts are exactly this toxic, with leaves, roots, seeds, stems or flowers that are packed with poisons. Take a look at seven common but lethal plants — including the most deadly plant on the continent.

Water hemlock (Cicuta douglasii)

Water hemlock is considered the most lethal plant in the states.
Water hemlock is considered the most lethal plant in the U.S. (Photo: Dasha Kolesnikova/Shutterstock)

We might as well start with the most insidious of the toxic plants. Water hemlock is considered to be the most deadly plant growing in North America. It is common in moist areas in meadows, marshes, ponds and even roadsides. It can easily be mistaken for a favorite among florists, Queen Anne's lace, so it's good to get to know this plant well so you can spot it even among look-alikes.

Why so dangerous? Cucutoxin contained in the plant causes vomiting and violent convulsions, and is found in the tubers, stems and leaves. Even the green seed heads are poisonous. It is deadly to humans as well as to livestock. It only takes a walnut-sized piece of the tuber to kill a 1,200-pound animal! Mistaking the plant for a similar-looking edible species is sometimes the cause of poisoning.

Another similar plant is the poison hemlock — also horribly toxic, but the effects of the poisons are different than those of water hemlock. Play it safe and stay well away from anything that looks like this plant.

Castor oil plant (Ricinus communis)

The castor oil plant might be beautiful, but it is also quite toxic.
The castor oil plant might be beautiful, but it is also quite toxic. (Photo: InfoFlowersPlants/Shutterstock)

Castor oil is a common substance used in a wide range of products from soaps to paints, inks and even perfumes. Castor oil is also known as a laxative used for medicinal purposes. However, you don't want to just go straight to the source — the castor bean — if you're looking for a home remedy.

Castor beans contain ricin, one of the most toxic known substances. Chewing even a single castor bean will bring on troubling symptoms that will send you to the hospital, and ingestion of four or more seeds can lead to fatal gastroenteritis.

The plant is originally native to Africa, but because of its beauty and usefulness (with the right processing), it has been cultivated around the world.

Belladonna (Atropa belladonna)

Belladonna berries look appetizing, but don't be lured in.
Belladonna berries look appetizing, but don't be fooled. (Photo: kanusommer/Shutterstock)

The berries of belladonna look scrumptious, but avoid the temptation to taste test. This is a very toxic plant that is native to Europe and naturalized in North America.

Like many poisonous plants, belladonna can be helpful when its properties are extracted and used in the right ways at the right dosages. Today, it is used for such illnesses as irritable bowl syndrome, stomach ulcers and even motion sickness. It is also used by optometrists to dilate pupils. The is a use with a long history.

"During the Italian Renaissance, which lasted from the 14th to 16th century, fashionable women drank the juice of belladonna berries to dilate their pupils," writes Medical News Today. "Belladonna owes its name to this practice, as it means 'beautiful woman' in Italian."

This was a risky use. If mishandled, the results are dire. A person who ingests the plant will experience symptoms including "rapid heart beat, dilated pupils, delirium, vomiting, hallucinations, and death due to respiratory failure," according to the USDA Forest Service. Even handling the plant is risky as the toxins can be absorbed through the skin.

Oleander (Nerium oleander)

Oleander is a common garden plant even though it is quite toxic.
Oleander is a common garden plant even though it is quite toxic. (Photo: Iva Vagnerova/Shutterstock)

Oleander is possibly one of the most recognized ornamental plants because it is used widely as a hedge shrub everywhere from freeway dividers to schoolyards. The latter location is a particularly odd choice. As we've reported before, "Ingesting any part of this plant can be deadly, especially for children. Even smoke from burning oleander can be fatal."

Ingesting oleander can cause blurred vision, nausea and vomiting, diarrhea, stomach pain, headache, confusion and serious heart problems. Consuming even a small amount of any part of the plant is highly dangerous. And with these effects, you can bet it is a terrible way to go.

Oleander is so deadly that it is the favored suicide plant in Sri Lanka. Yet, we continue to plant it widely because it is hardy, easy to care for, and grows rapidly into tall and wide shrubs, making it useful as a low-maintenance plant for privacy, noise reduction and yes, looks. It is, after all, quite beautiful when in full bloom.

Manchineel tree (Hippomane mancinella)

This tree is so toxic, just standing under its boughs during a rain will cause harm.
This tree is so toxic, just standing under its boughs during a rain will cause harm. (Photo: Karuna Eberl/Shutterstock)

If there ever was a tree to avoid, it's the manchineel tree. This tree, also known as la manzanilla de la muerte (“the little apple of death”), quite literally oozes toxins!

Science Alert reports:

"Manchineel belongs to the large and diverse Euphorbia genus, which also contains the decorative Christmas poinsettia. The tree produces a thick, milky sap, which oozes out of everything - the bark, the leaves and even the fruit - and can cause severe, burn-like blisters if it comes into contact with the skin. This sap contains a range of toxins, but it's thought that the most serious reactions come from phorbol, an organic compound that belongs to the diterpene family of esters. Because phorbol is highly water-soluble, you don't even want to be standing under a manchineel when it's raining - the raindrops carrying the diluted sap can still severely burn your skin."

Not only should you avoid being under the tree in the rain, you should also avoid being anywhere near it during a fire. Inhaling smoke from the tree causes severe irritation, and even blindness.

In North America, the tree may be encountered in Florida and Mexico. It is also native to the Bahamas, the Caribbean, Central America and northern South America. If you aren't sure how to identify the tree, locals might provide a helping hand. Many of these trees are clearly marked with a red X or ring on the trunk, or a sign is provided. They're important to the ecosystem as a windbreak and erosion barrier, and thus shouldn't be removed. It's best to just leave them be and keep a safe distance away.

Rosary pea (Abrus precatorius)

Rosary pea goes by many, many names, including crab's eye creeper, cock's eyes, love pea, John Crow Bead, Indian licorice, and more.
Rosary pea goes by many, many names, including crab's eye creeper, cock's eyes, love pea, John Crow bead, Indian licorice and more. (Photo: Caner Cakir/Shutterstock)

This lovely little plant is native to India and parts of Asia, and has been introduced in other areas around the globe — including Florida — because of its appealing appearance and usefulness in crafts as beads or for musical percussion instruments.

The problem, however, is that it can quickly become a weed. While that's annoying in itself, it's made all the worse a problem because the seeds are potentially lethal. They contain one of the most toxic substances known, abrin, which is similar in structure to insulin, ricin, botulinum, cholera and diphtheria toxins.

The seed's hard shell typically contains most of the toxin, making its effects on those who handle it mild. But, if the shell is opened or crushed, the effects are disastrous. A single seed, thoroughly chewed and swallowed, is lethal to child and adult alike. Indeed, it is often used in suicide attempts.

Because of its many other uses, this plant is still considered valuable, even if it ranks high at the top of the list of the world's most deadly plants. If you're visiting Florida, perhaps admire the seeds of this plant, but don't harvest.

Monkshood (Aconitum)

Monkshood may be a cheerful resident in any garden, but all parts of it, especially the roots, are laden with toxins.
Monkshood may be a cheerful resident in any garden, but all parts of it, especially the roots, are laden with toxins. (Photo: Peter Turner Photography/Shutterstock)

Monkshood is a beautiful addition to any garden, with those long stalks of richly colored and ornately shaped flowers. The genus Aconitum has 250 species (most of which are highly poisonous). Aconitum napellus is the most commonly grown ornamental variety, and Aconitum columbianum is a species found throughout the western half of the United States. It also goes by Wolfsbane, Wolf’s Bane, Devil's Helmet Flower and even Queen of Poisons.

Not recognizing the plant and being careless while handling it can be enough to send you to the hospital. When the sap comes in contact with any mucus membrane it can lead to cardiac and respiratory failure. Symptoms come on immediately, and if you were to ingest enough of the plant, death may occur as quickly as two to six hours later. In 2014, a 33-year-old gardener died of multiple organ failure after brushing up against this deadly plant while working at an estate in Britain.

With so many species of plant, including those listed here, we walk a delicate line between appreciating a plant for beauty and medicinal use, and courting death by handling it. Is the trade-off worth growing something such as this dangerous flower in your own garden? It's certainly a risky decision.

Jaymi Heimbuch ( @jaymiheimbuch ) focuses on wildlife conservation and animal news from her home base in San Francisco.