No Night Light for Short-Day Plants

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The Thanksgiving cactus is a short-day plant that blooms about a month before Christmas cactus. It needs short days in order to bloom.

The vast majority of plants we grow in our homes are more or less indifferent to daylength (they are day-neutral) and are able to bloom at any time of the year. Since light is more abundant in the spring and summer, however, they do tend to bloom best at those seasons.

However, there is a small group of houseplants that need short days in order to flower. These plants will not flower under the long days of summer, but rather begin to bloom in the fall or winter, when days are less than 12 hours long. They are called, logically enough, short-day plants.

In nature, these plants usually come not from the equatorial zone, but from areas north or south of the Equator where there is at least a small difference in daylength in what is locally the fall. When days are shorter than 12 hours, they will naturally start to get ready to bloom.

Just a Few Choice Plants

The list of short-day houseplants is not very long. It includes is the poinsettia (Euphorbia pulcherrima), Thanksgiving and Christmas cactus (Schlumbergera spp.),  Christmas kalanchoe, including the popular double Calandiva varieties (Kalanchoe blossfeldiana and its hybrids), most rhizomatous begonias (Begonia spp.), at least one bromeliad, queen’s tears (Billbergia nutans), and quite a few orchids. Many but not all cultivars of Phalaenopsis and Cattleya, for example, will bloom in late winter or early spring, but their bloom is stimulated by shorter days the previous fall.

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The florist’s chrysanthemum is not a good houseplant, but is more a temporary indoor decoration. It blooms under short days.

The florist’s chrysanthemum (Chrysanthemum x morifolium) is often included on lists of short-day houseplants and is indeed a short-day plant, but you could hardly call it a true houseplant! Instead it is more a garden plant or cold greenhouse plant you buy already in bloom to use to decorate your home. It dies pretty quickly in most indoor environments.

If you want to bloom a chrysanthemum to decorate your home, pot one up in the fall and leave it outside under the naturally short days found there, then bring it indoors to enjoy when the first flowers start to open. Have no fear: it is highly tolerant of cold and won’t mind being outdoors in the often chilly weather of October and November.

Getting a Short-Day Plant to Bloom Indoors

To bloom a short-day houseplant, place it in a sunny window in a spot that receives no artificial light at night, starting at the beginning of autumn (day length begins to decrease to less than 12 hours a day beginning at the fall equinox, that is September 21st or 22nd).

You can also grow them in a room that is illuminated at night as long as their location doesn’t receive too much artificial lighting. For example, you can place a panel of some sort between the plant and the source of artificial light or set it behind another plant that blocks any evening lighting.

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The poinsettia is extremely sensitive to any interruption in its short-day regime. Even headlights from passing cars can cause its blooms to abort.

The degree of sensitivity to artificial light does however vary according to the type of plant. The poinsettia is highly sensitive to the slightest amount of artificial light and just a few seconds of nighttime light at the wrong time can cause its blooms to abort. Thanksgiving and Christmas cactus and the various rhizomatous begonias, on the other hand, are much more accommodating and often bloom well in a room lit at night as long as only a modest amount of artificial light reaches them… or may bloom only on the side of the plant that wasn’t exposed to evening illumination!

Blooming Twice?

Sometimes short-day plants bloom a second time during the winter, towards spring, at least in northern regions. That’s because the days in high latitude regions remain short for months on end and the plant, used to only a limited period of short days in the wild, sometimes reacts to these extra-long periods of short days with a second wave of flowers.

How Not to Get a Short-Day Plant to Bloom Indoors

The Internet if full of false or misleading information about gardening. One example you sometimes see is the suggestion that you can get short-day houseplants to bloom by putting them in a closet for 2 or 3 months in the fall.

True, this does ensure short days (I mean, you can’t get shorter days than 0 hours of light a day!) and if the plant survives, it may bloom, but such a radical treatment is very harmful to the plant, eventually killing it or at least weakening it terribly. Which would you rather have: a half-dead plant with a few flowers (closet treatment) or a healthy plant in full bloom (the no-light-at-night treatment)?

Time for Action

Each fall, place your short-day plants in a brightly lit spot with no extraneous artificial light starting in late September or early October, keep watering it normally and you’ll be rewarded with a beautiful blooming few months later, often in time for Christmas. Yes, it is that simple!20160924a

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3 thoughts on “No Night Light for Short-Day Plants

  1. Barry Langille

    The internet is wonderful for getting things wrong, isn’t it? I remember the Poinsettia-in-a-closet thing from many years ago, maybe 1970’s, anyway pre-computer. The idea was to put the plant in a closet in late afternoon, and to remember to take it out every morning. I see a lot of things that started out sensible and which someone got wrong through lack of experience (maybe not really paying attention?)

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