Nasty Plants 2: Some Pretty Plants Will Poison You

oleander: very beautiful, very poisonous

Some plants repel the animals that would eat them with spines (previous post). But other plants are poisonous. If you can keep animals off with spines, why wait until they bite to marshal a defense?
The answers to that will depend on the plant, of course, but one explanation is that not all the animals that would eat a plant are put off by spines. Spines or thorns long enough to hurt a deer may be totally ignored by a small moth that lands between them and lays eggs that develop into voracious plant-eating caterpillars.

The Society for Economic Botany, focused on the study of useful plants, recently estimated that half of the world's 400,000 plants could be eaten  (link). Which lets me suggest that the other half are not edible, many of them protected by toxins that make nibbling them a really bad idea. The link I just cited above goes on to say many of the plants we eat are actually toxic as well. Cassava, for example, requires treatment to be safe to eat, and potatoes and rhubarb have toxic leaves, so we don't eat their leaves.


The world is full of dangerous plants. Mothers teach us not to put all the leaves in the back yard in our mouths. And, in fact, if you don't taste the plants around you, you can navigate safely between some very toxic species.

We all do, daily.

Which ones? As mentioned, potatoes and rhubarb. In potatoes (Solanum tuberosum, nightshade family, Solanaceae) leaves and stems contain the toxin solanine (link) which in sufficient dosage causes serious poisoning. Generally the potato tubers we eat do not contain much solanine, but if the potato skin is green not brown, it likely has enough solanine to upset your stomach. Peel it!

Brown-skinned potatoes: safe to eat

Rhubarb (Rheum rhabarbarum, knotweed family, Polygonaceae) leaves are loaded with crystals of oxalic acid, in concentrations too high to be eaten safely. The stems lack the crystals and so are the part of the rhubarb plant that we eat.

Rhubarb leaves: do not eat!

In your flower garden, foxglove (Digitalis purpurea, plantain family, Plantaginaceae) contains digitalin, the compound used in heart medicines. So it is no surprise that to eat foxgloves in any quantity is asking for trouble. The list of symptoms is chilling (link and link)

Isn't the plant foxglove (Digitalis) beautiful?
Just don't ingest it.

Likewise, everyone is recommending milkweeds be planted in our gardens to help monarch butterflies (earlier post). Milkweeds (Asclepias species, dogbane family Apocynaceae) contain cardiac glycosides. In small amounts they make animals vomit, in large amounts they cause the heart to stop. The amount of toxin varies between different tissues of the plant, between plants, and between plant species. Some species have so little that they make good wild vegetables, others will hospitalize you. Don't casually munch milkweeds. (milkweed diversity, one group)

Swamp milkweed: do not eat.

Lily-of-the-valley (Convallaria majalis, asparagus family, Asparagaceae) is also on the "do not taste" list. All parts of the plant will cause stomach pain, vomiting, blurred vision and dangerous drowsiness.

lily-of-the-valley, Convallaria majallis

Across, say, the United States--and the world--temperature and rainfall vary so much that "common" plants differ between regions. Every area has some ordinary plants that are poisonous. Daffodils (Narcissus pseudonarcissus, amaryllis family, Amaryllidaceae) are very pretty but a piece of the bulb, eaten, can kill you. Beautiful oleander (Nerium oleander dogbane family, Apocynaceae) graces the streets of Los Angeles (picture at the top of this post) but it is toxic enough that a lungful of the smoke will make you sick and eating the foliage will likely be fatal (link).

Dandelions are beautiful but shouldn't be eaten.

Within your house, don't munch on dieffenbachia (Dieffenbachia sp., aroid family, Araceae)! It is called "dumb cane" because the oxalic acid crystals will cause the mouth to swell so you cannot speak. In severe cases, swelling will block the windpipe and emergency medical care is required to prevent suffocation.

Dumb cane, Dieffenbachia

Poinsettias get a bad rap. They aren't poisonous (see snopes.com). Many plants related to poinsettias (Euphorbia pulcherimma, spurge family, Euphorbiaceae) ARE quite toxic, which is why poinsettias have the reputation of being poisonous. People are, rightfully, cautious when it comes to eating things that might be poisonous. Better safe than sorry! Two closely related plants may both be toxic...or may not be. That is what makes the situation tricky and people cautious. Learn the ones to avoid, begin by assuming relatives are also dangerous and then learn about the relatives specifically to find out if they really are poisonous. Carrots (Daucus carota) look a lot like poison hemlock (Conium maculatum) but we eat carrots and poison hemlock will kill you. (same plant family, dill family, Apiaceae). Photos: carrots in flower, poison hemlock flowers.

This post has focused on dangerous leaves. Mild toxins in a leaf give a grazing animal a bad experience--nausea for example. After that, the animal isn't likely to eat a second leaf. Many, many plants are therefore mildly toxic, if all you eat is one bite. Slight toxicity is the chemical equivalent of the spine: BACK OFF!!!  After a nasty experience, not only will you leave that plant alone but you are likely to teach others to avoid it. Most mammals and many birds teach their young what to eat, so learned avoidance is very helpful to plants.

The plants in this post show you that many common plants protect their leaves with toxins. We live next to them safely because we don't eat them.

 If you put a plant into your mouth, be very sure you know that it is safe!

Comments and corrections welcome.

References 
See for example:
Common Poisonous Plants and Plant Parts http://aggie-horticulture.tamu.edu/earthkind/landscape/poisonous-plants-resources/common-poisonous-plants-and-plant-parts/  Accessed 3/10/16
The Poison Garden http://www.thepoisongarden.co.uk/default.htm  Accessed 3/12/16

Kathy Keeler

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