What are Poisonous Plants?

The term poisonous plants refers to many kinds of plants that have a wide range of poisonous or disturbing effects.  This manual mainly deals with plants that cause chemical or physiological disturbances when taken internally.

There are other plants, that while not poisonous in the usual sense, can be dangerous because they cause mechanical irritation when eaten, photosensitization, or disagreeable tastes or odors in meat, milk, or milk products.  These three categories are discussed briefly below.

Mechanical Injury

Some plants cause internal injury from sharp thorns, awns, or spines, or intestinal obstruction due to “hair balls” from various parts of a plant or needle-like crystals in the leaves.  Wounds in and around the mouth may serve as areas of infection and can lead to starvation or weakness.  Some of the important plants and their parts that cause mechanical injury are listed here.

Photosensitization

Animals becomes extremely sensitive to light after eating certain plants.  This can cause a variety of problems from redness and itching of the unpigmented or lightly pigmented areas of the skin to severe necrosis.  Affected animals may die from starvation or other secondary effects.  North Carolina’s most important plant in this category is Hypericum perforatum (common St. Johnswort), which contains hypericin causing primary photosensitivity.  Secondary photosensitization is caused by liver dysfunction, which allows the accumulation of phylloerythrin (a normal breakdown product of chlorophyll) which in turn, causes photosensitivity.  Plants that might cause liver dysfunction are Lantana, Vicia, and Crotolaria.

Disagreeable Tastes and Odors

A number of plants, poisonous and nonpoisonous, produce a disagreeable taste or odor in the milk, milk products, and occasionally meat of cows that graze these plants.  Such tainted milk or meat is difficult or impossible to market.  Some of the more important plants that cause this tainting are listed here. 

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