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Poisonous Plants

By Buck Abbey, ASLA, Robert Reich School of Landscape Architecture, Louisiana State University

Nicotiana tabacum (pictured), common tobacco, and N. rustica are grown commercially as insecticides.
Photo: Wikipedia
Cost of Wisconsin
Playworld Came America
Playworld Came America

"Often the first evidence of yew toxicosis is unexpected death."
-- Amy Stewart, Wicked Plants, Algonquin Books 2009

There once was an elderly doctor who looked after a walled garden of poisonous plants. The benevolent doctor would daily tend his garden and talk to the plants. He understood their unique ornamental qualities and admired their blooms and fruits.

Euphorbia millii, the "crown of thorns," is an armed and dangerous poisonous plant. Beware!
Photo: Wikipedia

But he held a dark secret about the plants he did not share with others. It was the surreptitious nature and evil ways of these plants he admired the most. He knew them to be deadly, destructive, intoxicating, offensive, illegal and painful. They were bad plants. He knew them to be the villains of the vegetable kingdom and he kept them in servitude to meet his pleasures. His garden was full of the most malignant influences he could find on the planet.

Narcissus spp., daffodil: All Narcissus varieties contain the alkaloid poison lycorine, mostly in the bulb but also in the leaves. It is said that Roman soldiers carried daffodils with them to eat if they should be mortally wounded in battle, hastening their journey to the underworld.

He allowed friends and neighbors to stroll through the garden and admire the plants smell the intoxicating blooms and see the splashes of color on mottled leaves. Visitors were totally unaware of the danger nearby. The kindly old doctor did not reveal the plants concealed identity. They did not know the harm these plants could produce.

Digitalis purpura, Foxglove: The presence of cardiac glycoside digitoxin in the leaves, flowers and seeds of Foxglove make it poisonous. It can be fatal if eaten.
Photo: Wikipedia

Landscape architects can lead people through treacherous gardens as well. If they are not careful, do not understand the hidden nature of plants they can present the public with silent danger. Landscape architects must pay attention to the toxic, lethal, noxious and venomous qualities of some of the most common plants used in landscape design.

The toxicity of Delphinium sp. (Larkspur-buttercup family) has long been recognized. It's considered the most significant of the stock-poisoning plants.
Photo: Wikipedia

Surprisingly, many of these plants show up in gardens designed by landscape architects. We find them on school grounds where little kids play. They also show up in gardens planted by property owners too. People are surrounded by these bad plants yet suspect nothing.

Rhododendron indicum, azalea, has a toxin called grayanotoxin in its pollen and nectar. People have been known to become ill from eating honey made by bees feeding on rhododendron and azaleas.
Photo: Wikipedia

If you look carefully at display advertisements in your local newspaper, you will see photos of these devilish plants mixed in with those being offered for sale. They are commonly sold to home gardeners and commercial landscapers who know nothing of the malicious side of these plants.

Protecting The Public
Plants with bad behavior such as the one that almost blinded Frederick Law Olmsted are serious business.

Landscape architects must be keenly aware of poisonous plants. Ornamental plants can be harmful or lethal to children, adults, pets or livestock. These plants are often found growing around us, wherever we are. Gardens, garden centers, wholesale growing yards and natural areas all contain them. Landscape architects often use these wicked plants in planting design without knowing the plant's deadly nature.

All parts of the Nandina domestica, Heavenly Bamboo, are poisonous (hydrocyanic acid) and can be fatal if ingested. The berries contain nantenine, which is used in scientific research as an antidote to MDMA ("Ecstasy"). Birds are not affected by these toxins and disperse the plant's seeds.
Photo: Auburn University Horticulture

Landscape architects are registered and licensed by each state to protect the health, safety and public welfare of society. Knowledge of poisonous plants may be one of the most potent ways we protect the public. It is important to have some knowledge the plant kingdom's criminal element and which tools they use to assault society. This means a great responsibility comes with licensure and a professional education in landscape architecture. Landscape architects must know the wicked plants in order to warn clients of danger and ward off future lawsuits. This is a prudent 'standard of care' for landscape architects and can be major standard of examination whenever a state licensing law comes up for sunset review. Landscape architects are tested on this knowledge in Section D of the CLARB licensing examination.

The common Louisiana garden plants listed in the recipe given here may be used in landscape design. But their evil character ought to be noted in the plant schedules or specifications as having toxic qualities to protect the liability of the designer. It is always best for landscape architects to point out which part of a plant is poisonous or toxic.

Particularly the flowers, fruits, nuts, seeds, twigs and leaves and those parts most likely to be ingested by people, children and pets.

Nerium oleander is one of the most poisonous of commonly grown garden plants.
Photo: Auburn University Horticulture

Often, the most toxic part of a plant is associated with roots, tubers, rhizomes, corms and bulbs but since these are below ground, they do not pose as great a hazard. In the case of Nicotiana tabacum (common tobacco) it is the leaf.

Some 400,000 people die each year from ingestion the tobacco plant as a result of the plant. This plant was responsible for the death of my two brothers.

Colchicum speciosum resembles a crocus. The leaves, corm and seeds of the Colchicum are poisonous (alkaloid colchicine). Small doses of its roots and seeds were historically thought to have medicinal valude. One 19 century murderess apparently poisoned a number of people this way.
Photo: Wikipedia

It is interesting that public landscape policy does not address plant toxicity in planting design. Community landscape codes should brand the most wicked plants such Aconite napellus, Adenium obesum, Arbus precatorius, Conium maculatum, Dichapetalum cymosum, Ricinus communis, and Taxas baccata as "bad plants." Botanical crime families such as the Anacardiaceae, Apiaceae, Euphorbiaceam, Solanaceae should be listed as outlaws. Set forth in a landscape code, this would act as a form of caution to all who may use them in landscape design.

Plants, seeds and foliage of Consolida orientalis (Larkspur) are all poisonous.
Photo: Wikipedia

Think about the deadly nature of plants. But by all means, do not try my recipe for a toxic salad. It too is a killer.

For a copy of my university course document email me at

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August 24, 2019, 10:44 pm PDT

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