Oleanders have the reputation, but others are more poisonous

By Tony Tomeo

I have been a horticulturist as far back as I can remember. My father can attest to all the geranium cuttings and nasturtium seed I planted in his garden as fast as he could pull them up. One of the first lessons I learned about gardening is to stay away from the oleanders. I was told that they are very poisonous and that I should not even come into contact with their foliage. Consequently, my father never needed to pull up nasturtiums or geraniums from around the oleanders.

Although oleanders are somewhat toxic, they unfortunately do not compare to many other plant species commonly used in gardening. Puppies will get very sick if permitted to chew stems of oleanders. However, most dogs or cats who chew on plants will not consume enough oleander to cause harm. If deer are hungry enough, they can even eat oleander foliage. After learning about several other toxic plant species, I still cannot understand why oleander has the worst reputation.

The various species of Dieffenbachia have earned their common name of "dumb cane." Although they are reliable, attractive house plants, they can be very dangerous to anyone who takes a bite out of them. The sap not only burns the throat and numbs the vocal cords, but also crystallizes when it comes into contact with saliva. One of my doubting colleagues at Cal Poly, San Luis Obispo, was hospitalized to have sharp crystals removed from his throat after trying to disprove this fact.

Foxglove, Digitalis purpurea, is so potent that the medication derived from it is named "Digitalis." In extremely small doses, it is useful for disorders of the heart. Unprocessed or in larger doses, it can be fatal. Other species in the same family (which ironically includes tomatoes, potatoes, cotton and tobacco) are also very toxic. Night blooming jasmine produces small white berries which are rarely in large quantities but very toxic. Also in the same family, angel's trumpet and annual and perennial morning glory more closely resemble "loco weed," another relative which causes cattle to hallucinate and behave "loco" prior to death.

Most garden enthusiasts are familiar with poison oak but would not recognize species of the same family. African sumac, which is actually the same genus as poison oak, may cause milder but similar symptoms to those allergic to it. Those with sensitive skin may even experience an allergic reaction to the sap of either California or Brazilian pepper trees, which are also related. In fact, skin allergies are also common in response to contact with primroses, helleborus, azaleas, rhododendrons, junipers, flowering pines and sap from poinsettias or ficus. Related to poinsettias, castor beans produce poisonous pods. Hulls from walnuts are not very toxic, but will make dogs sick if consumed in large quantities. Although not of the plant kingdom, unfamiliar mushrooms and other fungus should always be assumed to be toxic.

Of course, there are many more toxic plant species than I can adequately describe. Fortunately, most cats and adult dogs have a good sense of what can harm them. If they tend to chew, it is usually done on harmless plants, such as grass. Puppies however, are not as discriminating, so removal of potential harmful plants may be required. If you have children and are unable to confine them in barbed-wire pens like my father eventually did, you should get to know the plant species in your garden that are within their reach. Species which cause skin irritation are usually a problem only after handling plants or sap during gardening procedures and not after casual contact. However, strongly toxic plants--or those that produce toxic fruit that may be attractive to children--should not be planted where children can find them. It is very important to teach children how dangerous some plants can be and that they should never eat any unfamiliar fruit.

Flower of the Week: Hydrangea

I neglected to mention that hydrangea, Hydrangea macrophylla, is also toxic, but I believe that we have heard enough of that! Hydrangeas are bold shrubs in the garden with canes up to 8 feet tall. Smooth, serrated leaves may be 8 inches long, and flower clusters may be 8 inches wide. Hydrangeas prefer rich, loose soil and require regular irrigation. Protection from direct sun exposure is also necessary.

White blooming hydrangeas produce only white flowers, and the foliage tends to be less vibrant than that of the colored hydrangeas. Of the hydrangeas producing colored flowers, there are many cultivars. Some produce better blue flowers, and some produce better pink flowers. Normally, in the alkaline soil of the Santa Clara Valley, they all bloom pink, even the "blue" ones.

Aluminum sulfate may be added to the soil if blue flowers are absolutely necessary. Lime or superphosphate may be used for red flowers. Applications of any of these chemicals must be made early to be effective. Lace cap hydrangeas produce flat-topped flower clusters of small lacy flowers surrounded by large-petaled sterile flowers. Other cultivars of hydrangeas produce more compact clusters or shorter, fuller shrubs as low as 4 feet tall. Hydrangeas normally bloom in summer and autumn, before going dormant for the winter.

Unfortunately, their boldness during summer and autumn is matched by the void left in winter when plants are defoliated and pruned back. Don't worry about pruning too harshly; they come back quickly in the spring. Other less common species of hydrangeas are slowly being introduced into retail nurseries. Although not nearly as popular as they once were when gardens were larger, hydrangeas are still a good choice for autumn blooms.

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Horticulturist Tony Tomeo can be reached at 408.358.2574.

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