Short-day plants

Short-day plants are species that only initiate flowering once the nights exceed a certain minimum length. What matters, then, is not day length alone, but above all the length of the uninterrupted period of darkness.

Short-day plants typically respond to the shortening days of late summer and autumn. Once the species-specific threshold of darkness is reached, they switch from vegetative growth to the reproductive phase and begin to flower.

What does "critical night length" mean?

Every short-day plant has an individual threshold for the amount of darkness it requires. Once this is exceeded - often more than 12 hours of uninterrupted darkness - flowering is triggered. Even brief interruptions of light during the night can disrupt this process.

Typical short-day plants

• Chrysanthemums
• Poinsettia
• Some bean species
• Sorghum
• Tobacco (certain varieties)

Why do plants respond to long nights?

The response to day length - also known as photoperiodism - is an adaptive mechanism to seasonal change. Short-day plants generally time their flowering so that their seeds can ripen before winter sets in.

Distinction from other plant types

Unlike short-day plants, long-day plants require shorter nights to initiate flowering. There are also day-neutral species, whose onset of flowering is largely independent of day length.