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The Garden Scoop

Poisonous Plants

How many times have you been kept up at night worrying about a poisonous plant?  Hopefully, for most of us the answer to that question will be ‘not many’.  For better or worse, humankind has not always had a well-defined separation from the plant world as we often see today. 

With steady advances in agriculture, horticulture and food systems over the past few centuries, most of us worry little about plant toxicity (other than poison ivy) since our food choices are systematically laid out in super markets.  However, in the not too distant past, humans were in much greater daily contact with all the flora that abounds, making toxicity of plants to both people and livestock a near daily concern at times. 

Animals are an important bridge between humans and the plant world, especially when you consider livestock.  Grazing animals consume a wide variety of forage, making pasture plant composition a major focus to ensure animal health and productivity.  Today, livestock producers have a wide variety of tools available for plant identification and management of pasture forage to ensure potentially hazardous plants aren’t creeping into grazing areas. 

In the past, plant toxicity was not as well understood as it is today, forcing early Midwestern pioneers with livestock to take heed and understand the plants in their surroundings or face sometimes dire consequences.

One plant that helps to illustrate this point is a shade-loving Illinois native called white snakeroot (Ageratina altissima).  Unless you spend a lot of time in natural areas, you may not have encountered this particular species.  However, it occurs throughout Illinois and other Midwestern states in a variety of locations on the landscape, but typically flourishes in disturbed areas.

In the 1800’s, a potentially fatal condition known as “milk sickness” was common before we learned it was associated with white snake root.  Grazing animals that consumed white snakeroot in sufficient amounts transferred a toxic compound from the plant to humans via their milk (or meat). Famously, this condition caused the untimely death of Nancy Hanks Lincoln in 1818, the mother of Abe Lincoln. 

There are widespread accounts of milk sickness throughout the 1800’s in both Illinois and Indiana, which were poorly understood at the time.  It was a difficult puzzle to unravel since many grazing animals were allowed access to a wide variety of plant material in less developed or controlled pasture systems than we have today. 

Nowadays, commercial milk is completely safe since milk from any potentially toxic cow is diluted after it’s combined with milk from incalculable other cows during the production process.  In addition, dairy cows typically graze within a pasture systems that have little opportunity for white snakeroot to be present in numbers.

Since the days of ill-understood milk sickness, we have, perhaps unwittingly, surrounded ourselves with a great number of plants that have some level of toxicity to humans or animals.  This is one of the luxuries we can afford in our modern, carefully segregated food system were we rely on very few of the plants immediately around us for daily food or fodder.  So, many of us pay little attention to plant toxicity until a child or pet (who are at greatest risk for inadvertently consuming harmful plants) develops symptoms.

Castor bean (Ricinus communis) is a non-native plant that contains one of the deadliest known plant toxins, called ricin.  However, it is commonly used in Illinois as an ornamental, annual plant for its beautiful foliage and interesting fruits.  The spiky, bright pink fruits and uniquely shaped, green leaves with contrasting reddish-pink venation and stems are unlike any other landscape plant.  A relatively small amount of ricin can be deadly to mammals, making it an interesting anomaly that humans would include this tropical plant in our modern living spaces

Many ornamental woody plants commonly used in the landscape contain toxic compounds, such as holly, azalea, rhododendron, ginko, big leaf hydrangea, arborvitae or English ivy.  Add to the list some very common houseplants as well, including: philodendron, schefflera and mother-in-law’s tongue.

Just like white snakeroot, other native plants have some level of toxicity, so it’s not only exotic plants that are to blame. The following native woody plants have some toxicity: wild black cherry, red oak, black locust, Kentucky coffeetree, red maple, elderberry and black walnut.

Although not all of the plants implicated herein are as toxic as castor bean, it’s worth it to do some research before planting anything into a space where young children or pets have open access to plant materials.  In many cases, a harmful plant may be so unpalatable or inaccessible in our landscape that it presents little risk of poisoning, but you never can be too careful. 

If you are interested in learning more about potentially poisonous plants, the University of Illinois Veterinary School maintains a “Poisonous Plants Garden” where many of the plants discussed in this article can be viewed.  This garden is open to public and accessible off of St Mary’s Road, just north of the College of Veterinary in Urbana.  More information, including a great plant list, can be found at: