Poisonous Plants

Top, left to right: star-of-Bethlehem, belladonna, poison ivy, poison oak, bottom: yew, oleander, wisteria, and poison hemlock.



Poisonous Plants, plants containing substances that, taken into the body of humans or animals in small or moderate amounts, provoke a harmful reaction resulting in illness or death. Possibly as many as one out of each 100 species of plants is poisonous, but not all have been recognized as such. Dangerous plants are widely distributed in woods (baneberry) and fields (star-of-Bethlehem), swamps (false hellebore) and dry ranges (scrub oak), roadsides (climbing bittersweet) and parks (kalmia), and may be wild (celandine) or cultivated (wisteria). Many ornamental plants, such as oleander, lily of the valley, and mistletoe, are poisonous.

Botanists have no set rules to determine accurately whether any given plant is poisonous. Toxic species are scattered geographically, in habitat, and in botanical relationship. They contain more than 20 kinds of poisonous principles, primarily alkaloids, glycosides, saponins, resinoids, oxalates, photosensitizing compounds, and mineral compounds such as selenium or nitrates accumulated from the soil. The poisonous compound may be distributed throughout all parts of the plant (poison hemlock), or it may accumulate in one part more than any other, such as the root (water hemlock), berry (daphne), or foliage (wild cherries). A plant may vary in toxicity as it grows, generally becoming more toxic with maturity; certain plants, however, can be highly toxic when young and harmless later (cocklebur).

Some active principles cause skin irritation directly (nettle); others bring about an allergenic reaction (poison ivy). Most poisons, however, must enter the body before they act, and in almost all cases this happens when they are eaten. Usually more than 57 g (2 oz) of the poisonous portion of the plant must be eaten by an average adult before poisoning will result (the amount is proportionately less for children). Some plants, however, are toxic in small amounts; for instance, one or two castor beans, the seeds of the castor-oil plant, may kill a child.

After ingestion, the poison may act immediately on the digestive tract (dumbcane, euphorbia, nightshade), producing severe abdominal pain, vomiting, and possibly internal bleeding, or it may be absorbed into the bloodstream. If so, it passes first to the liver, which may be injured. Oxalates crystallize in the kidneys (rhubarb), rupturing the tubules. Some plants affect the heart (oleander). Small amounts of principles in some of these (digitalis) may be used in medicine. Plants containing alkaloids often produce unpleasant or dangerous reactions in the nervous system. Examples are paralysis (poison hemlock), hallucinations (jimsonweed), or heart block (yew). A few poisons act directly within the cells of the body. The best example is cyanide, released from a glycoside in the plant (wild cherries), which prevents cells of the body from using oxygen. In contrast, unusually high levels of nitrates in plants combine with the hemoglobin of the blood so that it can no longer carry oxygen to the body cells. Some reactions are highly specific. Bracken destroys bone marrow, in which blood cells are formed. Saint John’s wort contains a poison that, when ingested by animals, reacts with sunlight to produce severe sunburn and lesions on exposed skin.

Poisonous plants are too numerous to eradicate, and many are highly prized as houseplants or garden ornamentals. If poisoning is suspected, a physician or the local poison control center should be consulted immediately.