# Short-day plant

> A plant that flowers when the night exceeds a critical length (days are short enough); photoperiod cannabis is a short-day plant.

- Canonical: https://genetixbay.com/knowledge/glossary/short-day-plant.html
- Also written: short day plant, short-day plants, SDP, long-night plant, short-day flowering
- Glossary category: Plant biology · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

A short-day plant is one that flowers when the daily dark period is longer than a critical threshold. The name is a historical accident: experiments in the 1920s varied [day length](https://genetixbay.com/knowledge/glossary/photoperiod.md), but it later became clear that the plant measures the uninterrupted night. "Long-night plant" is the more accurate description. Chrysanthemums, poinsettias, soybeans and photoperiod cannabis are all short-day plants.

## In practice

Outdoors, photoperiod cannabis flowers in late summer as nights lengthen, earlier at higher latitudes and in cultivars from northern origins. Indoors, growers trigger it by giving a long, dark night: classically [12/12](https://genetixbay.com/knowledge/glossary/12-12.md), and for many cultivars [13/11](https://genetixbay.com/knowledge/glossary/13-11.md) or more light still works.

The critical day length varies widely by cultivar; [hemp](https://genetixbay.com/knowledge/glossary/hemp.md) studies found flowering triggered anywhere from about 13 to 15 hours of light depending on [genotype](https://genetixbay.com/knowledge/glossary/genotype.md) (Zhang et al. 2021). That's why GenetixBay encourages growers to test their cultivar.

A short night break (light in the middle of the dark period) can hold short-day plants vegetative. That's how mums are kept in veg commercially, and why [light leaks](https://genetixbay.com/knowledge/glossary/light-leak.md) matter.

## Don't confuse with

- **Long-day plant**: flowers when nights are short (e.g. spinach).
- **Day-neutral plant**: flowers by age, like [autoflower](https://genetixbay.com/knowledge/glossary/autoflower.md) cannabis.
- **Photoperiod**: the general day-length response, not the type.

## Related terms

[Photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md), [Phytochrome](https://genetixbay.com/knowledge/glossary/phytochrome.md), [Autoflower](https://genetixbay.com/knowledge/glossary/autoflower.md), [Flowering stage](https://genetixbay.com/knowledge/glossary/flowering-stage.md)

## Read more

- [Photoperiod vs autoflower, and how autoflowering is inherited](https://genetixbay.com/knowledge/basics/photoperiod-vs-autoflower.md)
- [Photoperiod and light leaks: 18/6, 12/12 and the dark period](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md)

## Sources

1. Zhang M, Anderson SL, Brym ZT, Pearson BJ. 2021. Photoperiodic flowering response of essential oil, grain, and fiber hemp (Cannabis sativa L.) cultivars. Frontiers in Plant Science 12:694153. https://doi.org/10.3389/fpls.2021.694153
2. Taiz L, Zeiger E, Møller IM, Murphy A. 2015. Plant Physiology and Development, 6th edition. Sinauer Associates.