Poisonous Plants: Guilty Until Proven Innocent

Just because it’s pretty and deliciously scented doesn’t mean a plant isn’t poisonous. The lily of the valley, very toxic, is a case in point. Photo: plantsam.com

I regularly receive messages from people wanting to know if a given plant is poisonous or even people (often people with young children or pets) who want to banish all poisonous plants from their gardens or home. And that’s a problem, because, in reality, very little is known about poisonous plants. 

That’s because it’s something you simply can’t test for. No one force feeds potentially poisonous plants to humans or pets to see how they react. Toxicologists have to wait for cases to come up and then examine the outcome. Even then, the results are subject to interpretation. If a child or a pet ingests the leaves of a plant, then throws up, that might, for example, mean the plant was poisonous, but it might also mean the leaves were simply not edible. (There is a huge difference between something your body can’t digest properly and something that can actually poison you!) Or the reaction might have been due to stress. If you catch a child eating something they shouldn’t and you panic, they might well react to your fear by vomiting. 

It’s Complicated!

And then there is the problem of misidentification. Most people simply don’t know the plants that surround them. Panicked parents often race their toddlers to the emergency room, convinced the child has consumed a plant known to be poisonous, yet the plant consumed was something quite different and quite innocuous.

To make things more complicated, there is also the problem of which plant part was eaten … and at what time of year. Many plants have toxic parts and nontoxic parts. Roots, for example, are more likely to contain toxins than leaves, shoots or fruits and many plants are more toxic in late summer and fall, when their leaves are mature, than in early spring. 

The potato is the classic example of an edible poisonous plant. Potatoes have edible tubers, but all other parts are toxic. Even green tubers are poisonous. And this is far from rare. Immature fruits of many plants are poisonous, but mature ones are edible. Or the mature fruit is edible, but the seed inside is toxic. 

Of course, individual sensibilities are also a factor. Most people would agree that ripe mangos are not poisonous, but some people, especially those highly allergic to poison ivy and poison oak, react very badly to any contact with its leaves, stems, peels and sap.

Some plants are safe to eat when cooked and poisonous when not (think of cassava, or manioc, which generally needs to be boiled in two waters to before it is considered edible). Just to confuse things even further, others are poisonous only when cooked! 

The Dose Makes the Poison

And then there is the problem of dose. The saying “The dose makes the poison” is very, very true. There are poisonous compounds in most plants—plants use them to make themselves less palatable to predators—, but at such low levels they’re unlikely to cause poisoning … unless you consume excessive amounts. 

People have been poisoned by eating too many carrots or too much spinach (both, as is the case with most vegetables and fruits, contain certain toxins), for example, but it doesn’t happen very often and even then, usually only to individuals with other serious health concerns.

Rhubarb leaves are a good example of the dose making the poison. The whole plant contains oxalic acid, a chemical found in many plants that is safe at low doses, but toxic at high ones. Rhubarb petioles (stalks) contain only low doses of oxalic acid and are edible; the leaf blade, up to 10 times more. Even so, rhubarb leaf blades have only been condemned as poisonous since the 1920s. Before that, they were commonly eaten. But some individuals became ill after consuming them. It turns out most people would need to eat large quantities of rhubarb leaves to make themselves sick. (A lethal dose would be 4 to 8 kg/9 to 18 lb!) Still, the consensus today is that it’s best to eat the petioles only, not the leaf blade. 

If It’s Medicinal, It’s Probably Poisonous

Any plant considered medicinal is likely to be toxic to some degree, because small doses of poisons often have a beneficial effect. In fact, it’s hard to think of a single poisonous plant that doesn’t have a medicinal use.

The best known example is probably foxglove (Digitalis purpurea), source of the heart medicine digitalin. It’s safe when used at a low dose, but highly toxic otherwise.

Many herbs are also medicinal (thyme, sage, rosemary, lavender, etc.) and most of them are toxic to a certain degree. They’re safe in small quantities (a question of dose, of course), but avoid eating large quantities of them.

So, if you’re into growing medicinal plants, you might want to keep children and pets out of your medicinal herb garden.

Proof of Guilt

Toxicologists commonly use actual occurrences to help establish toxicity. People or animals have been known to become ill after consuming said plant or sometimes even just touching it. If that is confirmed, such plants will be declared poisonous. In some cases, though, plants have been declared poisonous based on only one case, often over a century old and without any kind of thorough study. So, even though they are commonly listed as poisonous, even many toxicologists consider their toxicity to be unclear.

Interestingly, although many toxic chemicals found in plants are known to science (solanine in potatoes, urushiol in mangos, etc.), others are not. Lilies (Lilium spp.) are considered highly toxic to cats, for example, but no one knows what exactly makes them poisonous … and to felines only.

Guilt by Association

Quite often, plants are declared poisonous simply based on their family ties. They then appear on lists of poisonous plants, even though no one really knows whether they are toxic or not.

Euphorbias, for example, are known to have toxic sap: a white latex produced from the slightest wound. Many lists of toxic plants simply therefore simply list all euphorbias as poisonous. 

However, the degree of toxicity varies from one species to another. Some species are highly toxic: the popular pencil euphorbia (Euphorbia tirucali) for example. Others are perfectly harmless. The leaves of hedge euphorbia (E. neriifolia, syn. E. edulis), for example, are commonly eaten in India both raw and cooked. 

The poinsettia (E. pulcherrima), also a euphorbia, was long regarded as toxic simply due to its family ties even though there are no confirmed cases of poisoning. It is now considered safe to handle or, at worst, only a minor irritant likely to affect only very sensitive individuals. See Garden Myth: Poisonous Poinsettias for more information.

What to Do?

People who are concerned about toxic plants and want to rid their environment of them are in trouble. That would be almost impossible. Too many plants, both native and introduced, contain toxic compounds. In fact, pretty much every plant contains some toxic compounds, although usually at a level such that actual poisoning is highly unlikely. 

You can try avoiding planting or growing plants appearing on lists of poisonous plants, including one appearing in my own blog. (199 Poisonous Plants to Look Out For), but there are tens of thousands of others. Do get to know your plants by name, though. That way, if a child or pet consumes a potentially poisonous plant, you’ll be able to advise your local poison control center.

Tomorrow, I’ll be publishing a list of over 150 houseplants considered safe around children and pets.

And remember, very few serious poisonings are actually produced by plants: most are caused medications, alcohol, cleaning products, cosmetics, pesticides and other household products. So, put most of your efforts into keeping those products out of the reach of children and pets.