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Volume 16, Issue 51 - December 18 - December 24, 2008
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The Bay Gardener by Dr. Frank Gouin

Shining the Light on Poinsettia

These colorful plants are perfectly safe to brighten your holidays


Despite the lingering myth, poinsettia plants are not poisonous to either man or beast. The Department of Horticulture of Kansas State University proved that back in 1970.

            Rats genetically sensitive to toxins were fed small quantities of poinsettia leaves, stems and bracts along with their normal daily diet. As the experiment progressed, larger amounts of poinsettia replaced the regular food. By the end of the experiment, their diet was mostly poinsettia tissue. The rats were monitored for weight changes and changes in activity.

            The conclusion was that poinsettia leaves, stems, bracts and flowers are not poisonous to the rats. Since these rats are used consistently to monitor toxins, the assumption was made that poinsettia plants are not toxic to either man or beast. A recent check with the Poison Control Center confirmed those results. There is no record of an adult or child poisoned from eating poinsettia leaves.

            Even so, I doubt if you or your pets would enjoy a meal of poinsettia because the leaves and the latex that flows from damaged tissues are very bitter. I can only assume that the study rats did not have taste buds or they gradually acquired a taste for the bitter flower.


Poinsettia and Its Odd Cousins

            Poinsettia and chrysanthemum will form flower buds only when daylight is less than 12 hours long. The radish, similarly, will only grow an enlarged root when the days are less than 12 hours. All three species are short-day plants based on their growth response to length of daylight.

            To further complicate understanding the daylight response, chrysanthemums are further subdivided into six-, eight-, 10- and 12-week varieties. A six-week variety will begin flowering six weeks after plants have been exposed to less than 12 hours of light, while a 12-week variety will start flowering 12 weeks after their exposure has been reduced to less than 12 hours.

            This is key to commercial producers of these crops and to hobby gardeners trying to grow their own plants from seed or cuttings. Chrysanthemums and poinsettia are easily propagated and grow rapidly during the months of late May, June, July and early August here in southern Maryland. Near the equator, these plants have to be covered with black cloth to make them flower. Closer to the Arctic circle, they  must be subjected to additional lighting during early spring and early fall to prevent them from flowering.

            These short-day plants grow vegetatively without setting flowering buds during these months. If you hope to propagate poinsettia and chrysanthemum during the fall, winter and early spring in southern Maryland, you’ll have to add growing lights to prevent them from establishing flower buds. Generally a 40-watt bulb will provide sufficient light to extend the light to 12 hours or more.

            You can grow radishes right now, as well as in fall and early spring, if you have a cold frame or unheated greenhouse. In these short days, the radishes will produce enlarged roots with very limited foliage. As the days grow longer, the radish plants will produce a greater amount of foliage. As daylight hours approach 12, the foliage of the radish plants will substantially increase and the size of the root will decrease.

Ask Dr. Gouin your questions at All questions will appear in Bay Weekly. Please include your name and address.

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