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Are you Photoperiodic? Poinsettias Are

Updated December 04, 2018

David Graper

Professor Emeritus of Horticulture Science

Closeup of a bright red Poinsettia (Euphorbia pulcherrima) plant.
It is important to remember the poinsettia needs enough light so the green leaves can continue to photosynthesize and produce the carbohydrates that the plant needs to grow.

Are you photoperiodic? I certainly am. I especially don’t like seeing the sun set earlier and earlier each evening as we approach winter. Then, when we switch to Daylight Savings Time, I know that my available daylight to get those last, fall gardening tasks taken care of is pretty much gone until spring. It is even worse when we get several days in a row where the weather is cloudy and we don’t even get to see the sun.

But are people really photoperiodic? I don’t know that much about human physiology but basically this means that you are attuned to the light and dark cycles that we all experience every day. Most of us are used to getting up when it gets light outside and start thinking about going to bed after dark. Our bodies certainly get set to a regular schedule that coincides with daily changes in light and dark. But, as people we can adjust the light and dark cycles so the natural light/dark cycles are often not as important. However, some people who suffer from Seasonal Effective Disorder are much more closely attuned to the natural rhythm of night and day. If you would like more information on SED, there is a lot of information online and you can certainly discuss this with your doctor too.

Many plants are photoperiodic. They rely on cues from nature, like the shortening day or night to make changes in how they grow. They may switch from producing leaf buds to producing flower buds. Many of our deciduous trees use the shortening day lengths to trigger the development of fall leaf color and leaf drop. Some vegetable crops, like onions, use our long day lengths in the summer to stimulate the development of the bulb of an onion.

We just got done with the Christmas holiday where many people commemorated that special time by purchasing poinsettias, a plant that is very strongly photoperiodic. It needs long, daily dark periods to initiate flower buds and the coloring of the upper leaves or bracts. Many commercial growers use black-out cloths to cover benches or whole greenhouses of plants to assure that the plants get the required number of hours of dark for the required number of weeks to get their poinsettias ready to sell at the right time. Commercial greenhouses also have to deal with encroaching residential and commercial areas that are often well-lit at night. This stray light can delay a crop of poinsettias for several weeks; disaster when you have a holiday dependent plant like the poinsettia. Just walking down the aisle of a greenhouse at night with a flashlight can be enough to throw off the timing of a crop.

Plants use specialized processes to sense the length of the light or more importantly the length of the dark period. Phytochrome, a plant pigment that is affected by the length of the dark period, plays a key role in this process. When it is exposed to a long dark cycle it is changed into a different form that then interacts with other plant processes to stimulate a response within the plant. Greenhouse growers can also use lights that turn on in the middle of the night or at the end of the day to shorten the dark period. This makes the phytochrome molecule change back to its other form. By manipulating the length of the dark and light cycle they manipulate the phytochrome to facilitate whichever response they need in their greenhouse crop. Commercial greenhouse research made a huge leap forward when photoperiodicity in plants was discovered. It made it possible to get such common greenhouse crops like chrysanthemums to come into flower at any time of the year, instead of just in the fall when they would normally come into bloom.

Photoperiodicity in plants continues to be a common topic of research in greenhouse and other horticultural crops, particularly as new plant species are being commercially produced. It turns out that many plants have a much more complicated photoperiodic response than the common poinsettia or chrysanthemum. Temperature often modulates the photoperiodic response. The photoperiodic response can be cumulative such that the plant keeps track of how many light and dark cycles it perceives before it will elicit a response. Photoperiodicity can also be bypassed in some plants by just waiting; eventually the plant will flower anyway but it will flower faster under the proper photoperiod.

Plant breeders also look at photoperiodicity in plants and often use it as a selection criteria in developing new varieties. Take the onion example that I mentioned earlier. Here in the “north” we typically grow long day onions, meaning that these varieties are stimulated to produce bulbs during the long days of the summer. If you lived and gardened in the southern United States, you would likely grow short day onions because they often plant their onions in the fall of the year to get bulb stimulation during the shorter days of late winter and early spring. If you planted those same short day onions up here in the north, your onions would probably produce onion bulbs that are much smaller than the better adapted long day varieties. But, now gardeners can buy day-neutral onion varieties that will grow and produce onion bulbs in short or long days. So, take note of the type of onions and other vegetable seed you select as you begin scanning through all those garden catalogs that will soon be appearing in your mailbox. At least now that we have made it past the winter solstice, the day length is gradually increasing again so you will have a little more time to read about those vegetables by natural sunlight.

Keeping that Poinsettia Alive for Next Christmas

So let’s get back to the poinsettia, which is hopefully still looking great on your dining room table or whereever it is being displayed in your home or office. If you want to keep it going and try to get it to bloom again for next Christmas here are some tips for you.

  • Poinsettias need bright light! They can actually probably handle full sun at this time of the year when the sun angle is low in the sky and we often have lots of cloudy weather. The important thing to remember is that if you want to keep your plant healthy, you need to give it enough light so that the green leaves can continue to photosynthesize and produce the carbohydrates that the plant needs to grow. Unlike us who have to eat carbohydrates and other foods to get the energy we need to live, plants make their own food, but they have to have enough light to do it. So, move the plant closer to a window or some artificial lights, so they can keep growing. Otherwise the plant will slowly start deteriorating and eventually die.
  • Proper watering is critical. I would suggest that you cut a hole in the foil pot wrapper or just remove it completely so that when you water the plant, the excess water can drain out of the bottom of the pot. If you have the pot sitting in a pot saucer or some other type if container, dump out the left-over water so that the plant does not sit in water which can kill the roots and encourage root-rot. Poinsettias also need some fertilizer so use a regular houseplant fertilizer at full strength about once a month from now until the spring.
  • A little pruning can help improve the appearance of your plant. This is especially true if you might have let your plant get too dry and lost some of the leaves or the tops of the plant. Just remove any dried or yellowed leaves and snip off the stems that have gotten to look rather ugly. Later in the spring you can do a more thorough pruning to encourage new stems to develop and eventually form your new and larger plant for next year.
  • Repotting your plant can be helpful, particularly if you notice that your plant is drying out too quickly between waterings. If that hasn’t been a problem, you could wait until spring or early summer to move it to a larger pot. Remember to always use potting soil and not just regular garden soil for potting houseplants. Garden soil will generally be too heavy and not have enough drainage or aeration, particularly for a poinsettia.
  • Once it warms up outside and there is no danger of frost, you can move your poinsettia outside. If you have a shady area on your deck or patio, that will work well. You can also dig a hole and sink the whole pot in the ground. Keep in mind that this should be a shady spot. Our summer sun will be too much for a poinsettia. A site protected from the wind is important too. Poinsettias are pretty fragile, especially when they get a little larger, so avoid a windy spot. Be sure to keep up on the watering during the summer. One long, hot weekend without water can easily kill your plant. Increase the fertilizing to once every week or two, fertilizing more if the plant is growing rapidly.
  • By mid-summer, take a look at your plant. Is it getting too tall or would you like it to get bushier? Now is the time to do some more pruning if you want to. Try to leave at least 4-6 nodes or buds on each stem. There is usually a bud at the base of each leaf. If you cut the top or tip off a branch, this will encourage the next lowest few buds to grow out and produce a new stem, and eventually a new flower head.

Now we get back to the photoperiodic response of poinsettia. At our latitude, the natural day length gets short enough to begin stimulating the flowering process by about the first of September. This is also the same time that we need to be thinking about the chance of frost and colder weather. Move your poinsettia plant back inside your home. Then, to simulate the longer nights of the fall, find a place where you can keep your plant in the dark from about 5:30 PM until 7:30 AM, or at least 14 hours of darkness. A spare room will work fine as long as it really gets dark in there at night and someone is not likely to go in there and mistakenly turn on some lights. Some people will cover their plant with a cardboard box. Use duct tape or some other dark tape to cover up any holes or cracks so that light won’t get in. Be careful to not damage the leaves while you are putting on or taking off the box, the leaves are prone to bruising. Keep up this light dark ritual every day for 6-8 weeks and your poinsettia should be blooming and brightly colored for Christmas.

Good luck, you will need it! If all else fails, there is still the compost pile option with plenty of new plants to choose from in your local garden center, florist shop or just about any other kind of store next Christmas.

Related Topics

Plant, Flower