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Short day plants

Posted 10/14/2015


Now that the days are noticeably shorter and cooler, some people are delighted because it means the intense summer heat of the Mojave Desert has passed. Shorter days can also cause a spurt of melancholy for those who dread the coming long nights. What we might want to remember is that for many plants, shorter days mean more flowers.

Many plants cannot flower when they experience long days. We humans consider a long day to be one with more than 12 hours of light and a short day to have fewer hours, but plants do not necessarily follow our clocks. Plants contain specialized light receptors, and these cells are able to respond to the varying amounts of light they experience. When something is a “short day plant”, it requires a certain number of hours of darkness to do some action such as blooming. It could also be called a “long night plant”. The plant itself determines how long that period of dark needs to be — it may be more than 12 hours, but it could also be fewer. The phenomenon is called “photoperiodicity”, an impressive word to describe how sensitive a plant is to length of light or dark.

Important crop plants have this sensitivity to length of nights. Cotton, for instance, will only flower and produce a boll when they experience a certain number of hours in the dark. Soy, an important component of much of our food supply, will only bloom and set seed when nights are long enough. Rice, coffee and tobacco, as well as the ornamental cosmos, all share the same darkness requirement.

However many hours of nighttime are required for blooming, having flowers appear as we approach winter is a welcome event. Those plants that we closely associate with the holiday season tend to need short days.

It only makes sense; who would want a poinsettia that is not in flower? If it were merely green, it would probably not be adorning most households, stores and other business during the month of December. Likewise, if Halloween, Thanksgiving or Christmas cacti were only green, with no blossoms during the holidays, they might not be quite so popular. Sales would likely drop.

Even though it is possible to purchase chrysanthemums virtually any time of year, they are truly short day plants. In nature, they flower in the autumn. They would not be in bloom for Easter, Mother’s Day or the 4th of July, if they were left to themselves.

Of course, most of our favorite plants have not been left to themselves. Human beings have been growing and hybridizing plants since agriculture began more than 12,000 years ago. For most of the time since then, we have adapted our horticulture techniques to maximize yields.

When it comes to light or dark requirements, we have learned that it is possible to trick plants into perceiving whether it is time to flower The discovery of electricity permitted us to go even further, since it is possible to grow plants under lights whose on/off cycles are carefully managed.

When trying to force last year’s poinsettia back in to bloom, people have learned that it needed a fixed number of dark hours for this to happen. Sadly, some people thought that meant placing the defenseless plant in a paper bag, putting the bag in a closet, and taking the dead thing out six weeks later. That does not work.

It is simply that these short day plants should be kept in the dark for a fixed number of hours each day. Even though it is not, strictly speaking, accurate, using a 12-hour night will force many short day plants to flower.

Some are so sensitive to photoperiod that even a brief interruption with light is enough to stop flower induction. Others can tolerate a short burst of light, and will flower as long as the light does not last too long.

Not all short day plants are welcome in our landscapes. Some very problematic weeds also respond to day or night length.

Nutsedge, a weed that plagues Southern Nevada, appears in lawns, golf courses and agricultural lands. It is also called “nutgrass” since the leaves do resemble grass until you look at them closely. When days are long, it produces green, triangular rough-edged leaves, but when days become shorter, a single plant can create 7,000 little tubers (the “nuts”). A tuber contains everything needed for a new plant, including enough carbohydrate to keep the youngster alive until it has a full complement of leaves. If one waits until days are short before pulling out this weed, it is likely that there will soon be an enormous flush of new nutsedges, as the parent plant had kept its tubers from growing.

Knowing a plant’s photoperiod needs can help the gardener not only with flowers, but also with weed control.

Email Angela O’Callaghan, Social Horticulture Specialist, or call at 702-257-5581.

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