How Plants Sense Longer Days after the Solstice

By Laura Huenneke

Short-day plants like Christmas cactus and relatives form flowering buds only after days grow longer in the fall. (Photo by L Huenneke)

We recently observed the winter solstice, an annual event recognized and celebrated in cultures around the world for millennia. The solstice marks the shortest day – and longest night – of the year, when the earth reaches the point in its annual revolution when our northern hemisphere is no longer pointing straight away from the Sun. From this date forward, until late June, days will lengthen and we see longer and more direct sunlight.

Plants, too, can see changes in daylength. Several of the plants with which we celebrate winter holidays provide excellent examples of sensitivity to daylength. Plants such as Christmas cactus and poinsettias will start producing flower buds and then open their flowers as the days shorten and nights lengthen, in late fall. Commercial growers take great care to keep these short-day plants in lengthening periods of darkness leading up to the holiday sales of poinsettias each year. We can manage our indoor environment to provide that same long, dark night experience (for example, by placing a Christmas cactus in a dark closet each night) if we want to trigger flowering in late fall.

Some favorite garden plants and crops, too, are choosy about daylength. For onions, it is the initiation of energy being stored in the bulb that is triggered by the change in daylength. There are so-called short-day onions, long-day onions, and intermediate-day onions. Short-day onions begin diverting energy from leaves to growing bulbs as soon as days reach 10 – 12 hours of sunlight; they are generally planted in the fall in mild winter areas (like southern Arizona) so that they start forming bulbs very early in spring. Long-day onions don’t switch from growing leaves to growing bulbs until there’s 14 to 16 hours of daylight – as in the northern Great Plains. Northern Arizona doesn’t really match either of those situations, so if we want to grow large onion bulbs we need to choose intermediate day-length onions.

Another garden favorite that is highly sensitive to daylength is the strawberry. Many of the best-known cultivars are so-called June-bearing strawberries. These start producing flower buds in the fall as days become shorter. Those flowers bloom in early spring and the single crop of fruit is produced in early summer. In Flagstaff, this one annual crop might be damaged by late freezes. Other varieties are called ever-bearing; these produce a spring crop like the June-bearers, but also flower in mid-summer for a late-season crop. Most recently, breeders have developed so-called day-neutral strawberries, which produce flower buds all season while growing. These varieties don’t produce heavy yields like the June-bearers, but do manage to put out a few berries all summer long if it doesn’t get too hot.

How do plants sense daylength? Many plants with a sensitivity to photoperiod (the balance between daylight and darkness over a day) rely on a substance called phytochrome, which absorbs red wavelengths of light during the day and changes shape to an altered form. Having that altered form present in high abundance for most of the night triggers the formation of flowering in short-day plants like poinsettia or Christmas cactus. However, even a brief interruption during the night (supposedly, even someone walking through a dark greenhouse with a flashlight in the middle of the night) can reset all the phytochrome to its original form and prevent the formation of flower buds.

The action of the altered phytochrome through the long night means it is more accurate to call a poinsettia or Christmas cactus a long-night plant than a short-day plant. The first scientists working on photoperiod, though, were focused on daylength, so their historical terms have won out over improved scientific understanding.

Sadly, many of us live in cultures and societies that no longer attune themselves to the natural cycle of daylength. Beyond the dates passing on a calendar, our plant companions take quiet note of the environment outside the window and remind us of our connection to the changing of the seasons.

Laura Huenneke is a retired plant ecologist, a certified Master Gardener, and one of three co-editors of this column. Gardening Etcetera is written for the community by certified Master Gardeners of the University of Arizona's Coconino County Cooperative Extension Master Gardener Program. To learn more, visit https://extension.arizona.edu/programs/coconino-county-master-gardener.