Photoperiodism: When plants count the hours

A flower meadow
A flower meadow

How beautiful, the lilies of the valley are blooming again. But how do they actually know that now is their flowering time and not only at Pentecost, when again the peonies miraculously get a start signal to unfold their flowers? Behind this is a phenomenon called photoperiodism.

The fact is that our plants shape the change of seasons and make the garden year so exciting for us: snowdrops open the round in January, spring anemones delight us in March, gladioli bloom at the beginning of summer, sunflowers shine in midsummer, and asters herald the arrival of fall. How boring it would be if everything would bloom at the same time. Fortunately, we have to thank the sun for the fact that this is not the case.

The length of the day is the all-determining factor, it influences growing, flowering and wilting. This dependence of the development of plants on the daily light-dark period is called photoperiodism. The beginning of the flowering period is also influenced by the length of the day. Strictly speaking, plants do not measure the length of the brightness period, but that of the dark period. The night thus determines the time of flowering, even a bright full moon can delay the flowering period of sensitive plants.

Long-day plants such as delphiniums flower when the day length exceeds 14 hours, while short-day plants such as dahlias open their flowers when the day length is below these values. What exactly triggers flower formation has been researched on long-day plants: depending on the day length, the plant hormone florigen is produced in the leaves and transported to the shoot axis to initiate flower formation.

Why lettuce bolts

The high pyramids of lettuce look impressive, but they are still an unpopular sight in the vegetable patch: in this state, the leaves taste bitter and are inedible. As a long-day plant, lettuce forms flowers from a day length of 12 hours and bolts to do so. Therefore, there are day-neutral varieties for the summer months that prevent this.

Which group a plant belongs to is genetically determined. To distinguish between spring and fall, two successive light-dark periods of different lengths are required. There are also day-neutral plants, such as cyclamen, in which the length of the day or night has no influence.

Asters, chrysanthemums and Christ’s thorn belong to the short-day plants. Incidentally, day-neutral and short-day plants are widespread at the equator, while long-day plants are more common in the far north. This presumably has the advantage that these plants can precisely match the relatively short vegetation period in summer with long days and short nights and make optimal use of it for their flowering period and propagation.

Simulate short days

Christmas star needs 12 to 14 hours of darkness over a long period of time. To ensure that it delights us with red bracts at Christmas time, you should cover your poinsettia with a cardboard box every day from October onwards, for example from 6pm to 7am. The cover must be opaque, because even the smallest ray of light is enough to interrupt the dark period and destroy all efforts.

In addition, of course, temperature and weather also determine the exact time of flowering. So, despite research into the highly complicated processes, nature does not completely let itself be seen in the cards. And so every year we can be surprised anew by the blossoms of our lily of the valley.

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