Long-day Plants
Long-day plants are species that only begin to flower once day length exceeds a certain minimum threshold. What matters here is not the temperature, but the length of the daily light phase.
Typically, these plants respond to the long days of late spring and early summer. Once the critical day length is reached, they switch from vegetative growth to the generative phase and begin to flower.
What does "critical day length" mean?
Every long-day plant has a species-specific threshold of daylight hours. Once this is exceeded - usually between 12 and 16 hours of light per day - flowering begins. Rather than measuring brightness directly, the plant tracks the length of the uninterrupted dark period.
Typical long-day plants in the vegetable garden
- Lettuce
- Spinach
- Mooli (winter radish)
- Radish
- Coriander
- Many cereal crops
In some crops, flowering in summer leads to what is known as bolting, which can impair harvest quality.
Why do plants respond to day length?
The day-length response - also known as photoperiodism - is an evolutionary adaptation mechanism. It ensures that plants flower at a favourable time, for example when pollinators are active or when there is enough time left for seeds to ripen.
Distinction from short-day plants
Unlike long-day plants, short-day plants require a longer dark period to form flowers. There are also day-neutral plants, whose onset of flowering is independent of day length.