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The Dirty Dozen
Gardening Newsletter
Vol. 11, Issue 10 - October 2016
When plants face danger, particularly danger of being eaten, escaping or hiding is not an
option. However, plants are not defenceless. Through millennia of evolution, many members
of the plant kingdom have invented and mastered an effective defense mechanism – poison,
which acts as a deterrent against two-, four- or six-legged herbivores. Some toxic compounds
are deadly if ingested, while others can cause only mild discomfort. Among numerous
poisonous plants, there are some truly potent killers – the dirty dozen of the plant world.
The list opens with the castor bean
plant (Ricinus communis). The castor
bean contains ricin, one of the most
toxic substances known. Deadly ricin is
located in seeds that resemble beans
and is poisonous unless processed to
produce harmless castor oil. If the seed
is swallowed with an undamaged seed
coat, it will most likely pass harmlessly
through the digestive tract. However, if the
seed is chewed and swallowed, the ricin
toxin will be absorbed within minutes with
eventual fatal results. Only one seed is a
deadly dose for a child, about four for an
adult. Surprisingly, the castor bean plant is
commonly grown in the south as a small
tree, and in northern countries as a bushy
ornamental annual.
The deadly castor bean plant is valued for its
ornamental properties.
White snakeroot (Ageratina altissima) is another infamous member of the dirty dozen. It
contains the toxin called tremetol. When eaten by cattle, the cows’ meat and milk become
contaminated with the poison, which is passed to humans via consumption. White snakeroot
was responsible for thousands of deaths in the 19th century. Poisoning occurred by drinking
the milk of cows that had eaten snakeroot. Among the victims, the most famous was Nancy
Hanks Lincoln, Abraham Lincoln’s mother.
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The Dirty Dozen
Gardening Newsletter
Vol. 11, Issue 10 - October 2016
The next member of the dirty dozen is the
well-known yew (Taxus spp.). The entire
shrub is poisonous, except for the red flesh
of the berries. However, the oval, black
seeds within the berries are highly toxic.
Fatality can occur within several hours
after ingesting as few as three seeds.
The primary toxin in yews is taxine, which
is a cardiac depressant. Amazingly, this
lush, dark green shrub is fairly common in
gardens.
Every red berry of yew contains one deadly seed.
Another silent killer is the oleander (Nerium oleander), the parts of which are all toxic.
Just one leaf can kill an adult, and fatal poisonings have resulted from minimal exposure
to its twigs, flowers and seed. Even oleander’s nectar and sap have lethal qualities. The
plant contains numerous toxins that simultaneously affect the nervous and cardiovascular
systems, as well as the digestive tract. However, this evergreen fragrant shrub, valued for its
ornamental qualities, is widely cultivated.
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The Dirty Dozen
Gardening Newsletter
Vol. 11, Issue 10 - October 2016
As is the case with oleander, all fragments of the rhododendron (Rhododendron spp.) are
highly toxic. Even honey produced from rhododendrons is poisonous. Despite these lethal
qualities, the plant earns its place as both West Virginia and Washington’s state flower.
Rhododendron (and azalea) shrubs offer very showy, occasionally fragrant flower clusters in
the spring.
Rhododendron ‘Taurus’ usually blooms at the time of the astrological Taurus, sending spectacular, bell-shaped,
dark red flowers with maroon flecks.
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The Dirty Dozen
Gardening Newsletter
Vol. 11, Issue 10 - October 2016
A close cousin to rhododendrons, mountain laurel (Kalmia latifolia) is a bushy evergreen
shrub. The green parts of the plant and its twigs, blooms and pollen are toxic. Honey may
produce toxicity symptoms but, fortunately, it is bitter enough to discourage most people from
eating it. Symptoms of poisoning appear about six hours following ingestion. The mountain
laurel’s very attractive, round flower heads have earned the plant many admirers, as well as
a place on Connecticut and Pennsylvania emblems as the state flower.
The installations found in Line Garden aim to produce an abstract field.
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International Garden Festival
Gardening Newsletter
Vol. 11, Issue 9 - Septemmber 2016
Some members of the dirty dozen lie
low to the ground, such as the innocentlooking lily-of-the-valley (Convallaria
majalis). The entire plant, particularly the
leaves, is poisonous. Its deadly substance,
called convallatoxin, intensifies the heart’s
contractions. Lily-of-the-valley is valued as
a reliable groundcover in shady, informal
settings. Its white, bell-shaped flowers are
famous for their potent, sweet scent.
Snow-white flowers of the lily-of-the-valley are a symbol
of purity, innocence, simplicity and charm. It is hard
to believe the deadly potential of this woodland plant.
Shown here is the rare variegated lily-of-the-valley
(Convallaria majalis) ‘Albostriata’.
Another sweetly scented killer is the daffodil (Narcissus spp.). The entire plant has deadly
qualities, especially the bulbs, which have potent substances that cause vomiting if ingested.
Cases of poisoning have occurred when daffodil bulbs have been mistaken for onions or
leeks. However, the unquestionable beauty of daffodils makes them one of the most popular
spring flowering bulbs.
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The Dirty Dozen
Gardening Newsletter
Vol. 11, Issue 10 - October 2016
Hoop petticoat daffodils (Narcissus bulbocodium) feature grass-like leaves and unusual deep-yellow, trumpetshaped flowers. Their tiny, delicate bodies are full of toxic alkaloids.
The statuesque monkshood (Aconitum
spp.) is a common garden perennial. All
parts of the plant, particularly the tuberous
roots and seeds, are extremely poisonous.
Ingesting even as little as one gram of the
plant may cause death. The plant’s sap is
also dangerous, and just slight contact with
the flowers can cause fingers to become
numb.
Don’t be deceived by monkshood’s beauty. The whole
plant has poisonous properties.
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International Garden Festival
Gardening Newsletter
Vol. 11, Issue 9 - Septemmber 2016
Monkshood is often confused with another
vertical and showy garden plant, the
larkspur (Delphinium spp.), which is equally
deadly. The entire plant is poisonous,
though the young leaves and the mature
seeds contain the highest concentration
of toxic alkaloids. Six hours is all it takes
for delphinium to cause death. This plant
is highly valued and commonly grown,
anyway.
Delphinium is much admired for its beauty when in full
bloom but if eaten, it can cause fatal poisoning.
Another deadly towering looker is the foxglove (Digitalis purpurea). The entire plant is
toxic, especially the leaves of the upper stem, which are full of chemicals that, while used
medicinally, are deadly in high doses. Depending on the amount ingested, untreated
poisoning leads to death. Foxgloves are still highly regarded as garden plants, since their
bell-shaped blossoms growing in tall spires add drama and height.
Last but not least of the dirty dozen is the water hemlock (Cicuta maculata), also known
as spotted parsley. The whole plant, especially the roots, is poisonous. The poison, called
cicutoxin, wastes no time in attacking the central nervous system, and the symptoms are
usually very violent. Fortunately, the water hemlock is a wildflower common in the meadow
but not in the garden. The poisonous qualities of this plant have long been known. The
ancient Greek philosopher Socrates died after drinking a potent solution of water hemlock.
It is easy to be deceived by plants. Their beauty, fragrance, and color don’t reveal toxic
qualities. Many poisonous plants are common and seemingly innocuous, with their lethal
tendencies well-masked. Our gardens and our houses are populated with many green shady
characters that we should be aware of, but be particularly wary of the dirty dozen.
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The Dirty Dozen
Gardening Newsletter
Vol. 11, Issue 10 - October 2016
Text and photos by Gina Dobrodzicka
Gina Dobrodzicka is a freelance writer and trained horticulturalist living on Vancouver Island.
Her website is www.gdgardendesign.com.
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