Printed in the Mendocino County Observer & aired on KZYX&Z Community News - Tree Tips by Clare Nunamaker
The shortening of the days in the fall means less light, or shorter photoperiods, for plants. Particularly in mild climates, the change in daylight helps plants know when winter is coming, even though temperatures may not be very cold.
It's actually the length of the night, rather than the hours of sunlight, that plants respond to. Through pigments called phytochromes in their leaves, plants measure the time of darkness in a day. Phytochromes in turn can cause plants to drop leaves and become dormant. They can also cause plants to flower, send out new growth, and even set bulbs.
We often think of plants flowering in the spring. Certainly many do, and these are known as "long-day" plants. They bloom in response to lengthening days and shortening nights. But some plants are quite the opposite, known as "short-day" plants. These need long nights - usually over 12 hours -- before they burst into bloom. The Christmas cactus and the poinsettia are well-known examples of winter-flowering plants. Finally, some plants appear not very sensitive at all to the photoperiod and are considered "day-neutral".
It's also interesting that the seeds of some species are sensitive to light. That is, they need light in order to germinate.
Of course the photoperiod is not the only way plants sense the coming of winter or the arrival of spring. Some are sensitive to temperature. When they have measured a certain number of hours of chilling temperatures, they begin to bloom or send out new growth.
How wondrous it is that plants have evolved to develop and depend upon their own internal calendars!
