Potentially Poisonous Plants

Almost any substance can be harmful to human health if used improperly. For example, water is essential to our existence, but one can drown in the liquid form, slip and fall on the solid or frozen form, or be scalded by the gaseous form (steam). Plants are as essential as water because they provide for basic human needs such as food, clothing, shelter, and even the air we breathe. And like water, plants also can be hazardous if used improperly. However, our basic knowledge of water hazards is much greater than for plants because water is such a ubiquitous and relatively invariable (water is water, after all) feature of the human experience. Plants, on the other hand, are vastly diverse, and their avoidance of potential hazards is not as deeply ingrained in our psyche. Avoiding the hazards of plants, therefore, requires a practical knowledge of our landscape.

Nightshade, monkshood, foxglove, and poison ivy are among the better known poisonous plants that are justifiably avoided by people who recognize them. Most people are taught at a young age not to touch poison ivy or eat nightshade. But, what makes a plant toxic or poisonous depends on a number of factors. Contact with some plants, such as poison ivy, causes dermatitis or an irritation of the skin. Other plants must be ingested to be toxic. The two most important factors that determine whether a person gets sick from eating a plant are the type of toxin and the amount ingested. Large quantities of almost any food can make us sick. What we don t often realize is that many commonly eaten fruits and vegetables have poisonous parts. It may come as a surprise that apple and cherries have poisonous seeds or that all parts of sunflower plants are "slightly toxic if large quantities are ingested.

It is also important to note that scientists do not typically study the toxicity of landscape plants. Most plants on "poisonous plant lists are there because of strong circumstantial or experimental evidence. Occasionally, however, some plants are erroneously listed as poisonous. One such example is the poinsettia, which is frequently listed as poisonous. However, researchers have found that mice can ingest hundreds of poinsettia leaves or bracts without injury. Although this finding does not preclude human injury from poinsettias, it does propose the poinsettia has been wrongly accused.

Poisonous or toxic plants should be treated with respect. They warrant precaution, not fear. To overcome our fear, we should better understand those plants commonly used in our landscapes and homes. By knowing which plants are potentially dangerous, we can teach our children not to eat them and keep our pets away from them.

Below is a list of common plants that should be appreciated for their beauty, but should not be ingested.

Common Name Scientific Name (Genus) Toxic Plant Part
Woody Plants
Horsechestnut Aesculus All parts
Clematis Clematis All parts
Burning Bush Euonymus Fruit
Privet Ligustrum Fruit and leaves
Hydrangea Hydrangea Leaves, bark, and flower buds
Azalea/Rhododendron Rhododendron All parts
Holly Ilex Fruit
English Ivy Hedera Leaves, stems, and fruits
Daphne Daphne Fruit
Elderberry Sambucus All parts except fruits
Boxwood Buxus Leaves and stems
Black Locust Robinia Bark and seed
Yew Taxus Leaves and seed
Perennial Flowers
Monkshood Aconitum All parts
Delphinium Delphinium Leaves and seed
Bleeding Heart Dicentra All parts
Foxglove Digitalis Flowers, leaves, and seed
False Indigo Baptisia All parts
Iris Iris Underground stems
Jack-in-the-Pulpit Arisaema All parts
Cardinal Flower Lobelia All parts
Lily-of-the-Valley Convallaria All parts
Annual Flowers
Castor Bean Ricinus Seed
Four O'clock Mirabilis Root and seed
Morning Glory Ipomea Root and seed
Sweet Pea Lathyrus Seed
Flowering Tobacco Nicotiana All parts
Angel's Trumpet Datura All parts
Daffodil Narcissus All parts
Snowdrop Galanthus Bulb
Hyacinth Hyacinth Bulb
Squill Scilla All parts
Caladium Caladium All parts
Calla Lily Zantedeschia All parts
Amaryllis Amaryllis Bulb and seed
Dumb Cane Dieffenbachia All parts

Potential problems with plants in the landscape can be avoided by teaching our children a few commonsense rules:

  1. Eat only those plants or plant parts that you know are safe to eat.
  2. Identify the plants in your landscape. Check poisonous plant reference to see which plants may be poisonous.

Unfortunately, no list of this type is comprehensive. There will always be potentially toxic plants that are overlooked, so education of homeowners and their children is important. You don t have to remove these plants from the landscape. Usually, their beauty gives us much pleasure and far outweighs their potential pitfalls.

This article originally appeared in the February 21, 2003 issue, pp. 12-13.


Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Horticulture and Home Pest News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on February 21, 2003. The information contained within may not be the most current and accurate depending on when it is accessed.