December 21, 2025, 12:15 pm | Read time: 4 minutes
In summer, the day seems endless, while in winter, the light disappears by afternoon. For us, this mainly affects our mood. For plants, however, the length of day and night determines when they grow, bloom, and reproduce.
It’s no coincidence that lettuce bolts in midsummer, dahlias start blooming in earnest in the second half of the year, and poinsettias turn red just in time for December. Plants don’t develop based on the calendar date but rather on the duration of darkness. The length of the nights dictates their developmental rhythm. This principle is called photoperiodism, which refers to the ability of plants to use the regular alternation of light and dark as a timekeeper.
Why the Night Is More Important Than the Day
Even when we talk about long-day and short-day plants, plants don’t measure the length of the day but the duration of the dark phase. Sensitive light sensors in their leaves detect when it truly gets dark and when the night ends. Only when the appropriate sequence is achieved over several days does the plant begin to form flowers. Even short light impulses can disrupt this rhythm.
Long-Day Plants Love Short Nights
Long-day plants start blooming when the nights are short, meaning in spring and summer. Many well-known vegetables belong to this group, but they react differently to day length depending on the variety. Lettuce, spinach, radishes, peas, and dill exhibit this behavior particularly clearly. Grains like wheat, rye, and barley are also part of this group.
This explains a classic gardening problem. When lettuce grows during the long summer days, the plant switches from leaf formation to flower formation. It shoots up, and the leaves become bitter and inedible. For the summer months, there are varieties that react less strongly to long days, as well as truly day-neutral varieties.
What Seed Packets Really Reveal
Seed packets usually don’t indicate whether a variety is a long-day or short-day plant. Instead, they list recommended sowing times. These recommendations are nothing more than a practical translation of the light requirements of the respective variety.
Short-Day Plants Wait for Long Nights
Short-day plants work the opposite way. They need long, uninterrupted nights to form flowers, which usually occur in late summer or fall. Dahlias, chrysanthemums, and begonias are among the most well-known examples. Strawberries and certain lettuce and spinach varieties are also included. For these plants, even a brief light impulse at night can interrupt the necessary dark phase. If darkness is regularly shortened, flowering is delayed or doesn’t occur at all.
Day-Neutral Plants Are More Relaxed
There are also plants that are hardly affected by day length. They bloom once they reach a certain developmental stage. Tomatoes, peppers, chili, and cucumbers belong to this group. As a result, these crops can be grown in greenhouses or on windowsills almost year-round, provided that temperature, water, and nutrient supply are adequate.
Why Origin and Variety Are Crucial
How strongly a plant reacts to night length is genetically determined and closely linked to its origin. Plants from near the equator often react particularly sensitively to deviations because the day length there hardly changes throughout the year. Even small differences in night length can determine whether they bloom or not. Species from northern latitudes, on the other hand, are adapted to significant fluctuations and use the long summer days specifically to grow and reproduce quickly.
Why the Poinsettia Loses Its Color
The Colorful Leaves of the Poinsettia Are Not Flowers
The Poinsettia as a Classic Example
The poinsettia vividly demonstrates how strongly plants react to the night. Its bracts only turn red when the nights reach a certain length over several weeks. In Central Europe, this phase occurs in winter, which is why the poinsettia has become a typical Advent plant here.
If you want to keep the plant for years, you must ensure long nights in the fall. Turning on a light occasionally in the evening is enough to prevent the bracts from turning red.
An Often Overlooked Connection
If plants don’t bloom or bloom at the wrong time, it’s not always due to care or location. When the night is artificially brightened by lamps, patio lights, or streetlights, it’s called light pollution. It can disrupt the blooming impulse of sensitive species.