Photoperiodism
Photoperiodism describes plants' ability to respond to the length of day and night. It governs key developmental processes such as flowering, bud formation, sprouting and dormancy.
What matters here is not temperature, but the ratio of light to dark phases within 24 hours. Many plant species use this signal to align their life cycle with the seasons.
How does photoperiodism work?
Plants do not measure day length directly, but the duration of uninterrupted darkness. Special photoreceptors - in particular the pigment phytochrome - detect changes in light and trigger hormonal processes that ultimately lead to flowering or other developmental stages.
Forms of photoperiodism
- Long-day plants flower when days are long, i.e. nights are short
- Short-day plants require long nights to flower
- Day-neutral plants show little response to day length
Examples from the garden
Lettuce, spinach and winter radish are typical long-day plants and tend towards bolting during long summer days. Chrysanthemums and some bean varieties, on the other hand, are short-day plants.
Why does photoperiodism matter?
The response to day length is an evolutionary adaptation mechanism. It ensures that plants flower at a time when pollinators are active, leaving enough time for seeds to ripen. This safeguards reproduction.