A-Z of botany: short-day plants


Some plants are very precise in when they bloom. There are lots of possible reasons for this. I have recently written a piece for The English Garden about Hamamelis and one of the fascinating things about these gorgeous shrubs is the fact that they bloom in midwinter and yet, unlike most winter flowers, the blooms are showy and bright and obviously designed for insect pollination. So why bloom in winter and not summer? Obviously plants don’t ‘decide’ to do anything – it just happens. But of it turns out to be a hindrance to survival then that particular idea will disappear – nature is less forgiving of ridiculous ideas than Alan Sugar! Lots of plants flower in spring and that makes lots of sense because it gives lots of time for seeds to form before cold weather sets in in northern temperate climates. Of course there are then problems if the seeds are shed and germinate too quickly and the seedlings are tiny when winter comes. So some seeds have systems to prevent germination until they have ‘sensed’ winter is over.

Flowering at the end of the year means that, as long as there is time to ripen the seeds, these can just sit there until warmth returns in spring. There is also a theory that some plants bloom when there are fewer other plants in bloom because there is a reduction in competition for pollinators. This sounds odd but one of the reasons given for exterminating the introduced and invasive Impatiens glandulifera (Himalayan balsam) is that it is so attractive to bees and so productive of nectar that the bees prefer to visit it than native plants and so these do not get pollinated so effectively. So maybe flowering at the end of the year when there are few flowers, does ensure they receive full attention from the bees (or flies, in the case of hamamelis).

The control of flowering by day length is called Photoperiodism and although some plants are not affected by day length, some only bloom when the days are lengthening and others only when daylight hours are decreasing. These are generally called short-day plants even though it has been shown that it is the length of darkness not light, that does the trick. The best example is chrysanthemums which only form flower buds when the hours of darkness are longer than the hours of light. Once buds have been initiated then day length is not important. Chrysanthemums are important commercial plants as cut flowers and as potted plants. Most potted plants, which are sold, in bloom, at about 15cm high, are what fanciers would classify as ‘Late Sprays’ and can potentially reach 1.5m or more high. To produce these compact potfuls the cuttings are rooted and when enough growth has been made, the greenhouses are shaded at the same time each day to give the plants the trigger to bloom. It is very important that while this is happening there is no flash of bright light or the trigger is lost. Once buds appear there is no need to curtail the daylight hours and increased light ensures sturdy growth.

Now that pot chrysanths are less popular I doubt that many people plant them in the garden after the flowers fade but I remember people asking me if they can be put out and yes they can, but without controlling the light the plants keep on growing until buds are initiated in late summer so your cute little plant will be a straggly mess before it flowers in the garden in November. And because these varieties are late bloomers, bred to bloom under glass, the flowers often get damaged by damp, cold weather in the garden


Exactly the same principles apply to that Christmas favourite the poinsettia. If you can keep yours alive it can be grown to bloom again the following year but you need to control daylength and that means no artificial light at night or it will not produce more bracts. And to get bracts before Christmas you need to artificially extend nights in September and October till you see bracts forming. Below is a poinsettia growing outside in Tenerife as a large shrub.


Kalenchoes are similar and if put in the house where you have artificial light at night they will not bloom again after the buds that were there when you bought it.




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2 Comments on “A-Z of botany: short-day plants”

  1. derrickjknight
    December 14, 2016 at 9:17 am #

    More fascinating information. It is only recently that the Head Gardener explained that leaves still fall in a mild autumn because that is governed by the day length.

  2. Sheryl @ Flowery Prose
    December 23, 2016 at 10:59 pm #

    I finally had a chance to stop in and catch this post – such fantastic info and your photos are wonderful! I am delighted to see the one of the poinsettia in shrub form!

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