Long Nights are Getting Shorter

ChiloquinNews article 12/20/2010

Starting on December 21st, winter solstice, the days will start to get longer, and this gardener for one, will enjoy the lengthening days.

Plants also respond to day length, though it is not something the gardener usually considers. At the equator, days and nights are about 12 hours each all through the year, but as the distance from the equator increases, the length of the day increases in summer, and decreases in winter until at the poles, summer is one long day, and winter is one long night. We happily try to grow Southwestern plants in Oregon with never a thought about the different day lengths here. No wonder some of those plants grow so miserably!

The response of plants to changing day length was first measured in the 1920’s and was thought to be just that—a response to the length of the day—and it was termed photoperiodism. However, it is actually the length of the night that the plants measure. And it turns out that most of the plant’s life is regulated in part by the length of the night—when to break dormancy, when to flower, when to become dormant again. The blossom that breaks forth in spring before the leaves ever appear, is from flower buds that formed in response to the lengthening nights of the previous fall.

We started off this winter with a drop to subzero temperatures and then it warmed up. Is this going to be dangerous for our plants? It is important that a dormant plant not awaken before spring since any new growth in winter will be frozen. So there are safeguards in place and two things need to happen—as well as warmer days, the plant also needs longer days before it can break dormancy. Depending on which part of the globe the plant originated, so the length of the day needed can be longer or shorter than that required by the local plants. No doubt this is why in spring, when the days are getting warmer, the same plants always break dormancy first, and then get frozen back by a late freeze, while others delay opening their leaves for so long we begin to wonder whether they ever will.

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