# Light deprivation

> Light deprivation (light dep) covers greenhouse or outdoor plants with blackout material to give long nights, forcing photoperiod plants to flower earlier than the natural season.

- Canonical: https://genetixbay.com/knowledge/glossary/light-deprivation.html
- Also written: light dep, light-dep, dep, blackout, forced flowering
- Glossary category: Cultivation and training · 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"

Light deprivation is the practice of shortening the day artificially for sun-grown plants by pulling an opaque cover over them each evening and removing it in the morning, so they get roughly 12 hours of uninterrupted darkness. [Photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md) cannabis responds by flowering, even in midsummer when natural nights are short.

## In practice

Light dep lets greenhouse and outdoor growers harvest in summer, before autumn rain and mould, and run two or more crops per year. Covers must be truly lightproof and pulled at the same time every day; inconsistency or leaks can cause [re-veg](https://genetixbay.com/knowledge/glossary/re-veg.md) or stress. Under covers, heat and humidity build quickly in warm weather, so ventilation and [fans](https://genetixbay.com/knowledge/glossary/fan-leaf.md) are important.

Automated blackout curtains do this on a timer in commercial greenhouses; small growers use tarps on frames.

## Don't confuse with

- **Darkness before harvest**: a separate idea (24-72 h of dark before chopping) that GenetixBay classes as a myth.
- **Indoor flip**: the same switch done with a light timer.

## Related terms

[Greenhouse growing](https://genetixbay.com/knowledge/glossary/greenhouse-growing.md), [Outdoor growing](https://genetixbay.com/knowledge/glossary/outdoor-growing.md), [Flip](https://genetixbay.com/knowledge/glossary/flip.md), [Photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md)

## Read more

- [Photoperiod and light leaks: 18/6, 12/12 and the dark period](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md)