Master Gardeners: Is That Lovely Plant Toxic?

By Master Gardener Julie Segerstrom

Many lovely and common plants in our landscapes are poisonous.

Poisonous plants evolved toxic compounds as protection against predators and disease. If a plant is poisonous, it doesn´t mean you can´t own it—just be aware of and protect others from its possible consequences.

Many plant poisons can affect the central nervous system, paralyzing basic function. Others cause cell and organ damage, while some destroy function slowly over a long period of time.

The amount that causes poisoning is often difficult to specify. In some plants one leaf is fatal and for others it takes an entire cup of seeds. The California Poison website (www.calpoison.org) stresses their slogan: Don´t Guess. Be Sure. When you buy a plant from a nursery, make sure it is labeled or that you know at least the common name.

Keeping plants out of reach, both outside and inside, is necessary for little ones. Children should be taught to never put plants or mushrooms in their mouth. Babies should never be allowed to suck on necklaces made of seeds. Many plants are also poisonous to pets and livestock.

Some parts of a plant can be edible and other parts of the plant poisonous. For examples, we eat the fruit of tomatoes and the tubers of potatoes, but the leaves and vines, if made into tea, are poisonous. In other plants, the berries are poisonous–mistletoe, asparagus and lantana. Seeds are poisonous in some plants, such as apples and peaches. If it seems unlikely to be eating berries and making tea of plants, remember that children playing make-believe outside might innocently decide to use what is around them in the yard.

One interesting and toxic plant family is called Climbing Nightshade. The genus name is derived from the Latin word which means quieting. Nightshade poisons have a quieting or narcotic effect on the respiratory system. A poison derived from the nightshade family might have been what Shakespeare had in mind when Juliet took a drink to give her the semblance of death.

The common morning glory has seeds that contain alkaloids and LSD-like compounds. During the 60´s, potent brews of pulverized seeds (about 50) would produce hallucinations, nausea, convulsions and at times death. The Aztecs used morning glory to induce psychic religious visions.

Another poisonous plant we see in the foothills, mistletoe, contains toxins that have taken the lives of children who have eaten the berries. Tea brewed from the leaves is also toxic. Some pigs and goats can eat mistletoe without injury but cattle have died from eating the leaves. Conversely, birds regularly eat the mistletoe berries. This points out that the old wives tale that says, “if one animal eats it, it is safe for another animal” is simply not true!

The name poison hemlock gives you some idea of its danger. This is a seriously poisonous plant responsible for many deaths, the most famous being the dramatic end of Socrates´ life.

You may have heard that poinsettia is poisonous, but it is considered to be only mildly toxic, causing vomiting, diarrhea and dermatitis. It would require eating a large amount of poinsettia leaves to cause these effects.

Castor bean is another highly poisonous plant. All parts of the plant contain ricin, a severe irritant. As little as one seed can kill a child and three can kill an adult. Naturally, castor oil comes from the castor bean plant. The oil is not toxic because ricin is not soluble in the oil. In addition, heat kills ricin and the seeds are roasted for one hour before pressing the oil from them.

Probably the most well-known poisonous plant is oleander. Using branches of oleander to cook food is enough to cause poisoning. Oleander contains over 50 toxic compounds! It´s even dangerous to burn oleander clippings and prunings because the poison is carried in the smoke.

Some weeds are toxic to livestock as well. Fiddleneck is common in the foothills and causes liver damage. Toxins are metabolized in the liver into highly reactive compounds that cause cell damage. Small amounts eaten over a long period of time, even only seasonally, cause small deteriorating changes often unnoticed until it´s too late—the organ damage is too severe. When clinical signs are overlooked, a large portion of the liver is destroyed.

Another common weed, yellow starthistle causes irreversible necrosis (death) of brain cells in horses.

Poisons are contained in many of the most loved and beautiful plants—azaleas, delphinium, daffodils, iris, foxglove and even oak trees. Naturally adults will not sample their “tea,” but just be prepared for little adventurous ones who might visit you in the future.

See you in the garden.

Julie Segerstrom has been a Master Gardener since 1996. She got much of her information on poisonous plants from the UC Davis Website website and in “Edible and Poisonous Plants of Northern California”, by James Wiltens.