# Cold nights

> Cold nights are deliberately low night temperatures in late flower; they can bring out purple colour in cultivars that carry it, but don't raise terpenes or resin.

- Canonical: https://genetixbay.com/knowledge/glossary/cold-nights.html
- Also written: cold night temperatures, night temperature drop, cold snap, cooling for colour, temperature drop for purple
- Glossary category: Environment and light · Evidence: Myth
- 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"

"Cold nights" refers to dropping night temperatures well below day temperatures in the last weeks of flower, often to 12–16 °C (54–61 °F) against days in the mid-20s °C. Growers do it to bring out purple, red or blue colour. The colour comes from [anthocyanins](https://genetixbay.com/knowledge/glossary/anthocyanin.md), pigments whose production rises in low temperatures in plants that have the genes for them.

## In practice

It only works on cultivars that carry the colour genetics. A green line will not turn purple in the cold, and some purple lines colour up at normal temperatures anyway. Keep it to late flower, avoid going much below about 10 °C (50 °F), and manage humidity carefully: cold nights push [RH](https://genetixbay.com/knowledge/glossary/relative-humidity.md) up and dense buds near the [dew point](https://genetixbay.com/knowledge/glossary/dew-point.md) are an open invitation to botrytis.

The founder admits to being "a sucker for any red strain", but [bag appeal](https://genetixbay.com/knowledge/glossary/bag-appeal.md) is not quality. Colour says nothing about potency or aroma.

## Don't confuse with

- **Purple stems from deficiency**: purple [petioles](https://genetixbay.com/knowledge/glossary/petiole.md) in veg often point to cold roots, phosphorus uptake problems or genetics, not a desired colour.
- **DIF**: the general concept of [day-night temperature difference](https://genetixbay.com/knowledge/glossary/dif.md).

## Myth check

GenetixBay house position: **colour from cold is real** (anthocyanins; *Established*). Claims that cold nights increase **[terpenes](https://genetixbay.com/knowledge/glossary/terpene.md), flavour or resin are a Myth** unless peer-reviewed data show otherwise, and none do so far. Members can test it in a [Mythbusters](https://genetixbay.com/knowledge/mythbusters.md) side-by-side.

## Related terms

[DIF](https://genetixbay.com/knowledge/glossary/dif.md), [Temperature](https://genetixbay.com/knowledge/glossary/temperature.md), [Anthocyanin](https://genetixbay.com/knowledge/glossary/anthocyanin.md), [Dew point](https://genetixbay.com/knowledge/glossary/dew-point.md)

## Read more

- [Temperature by stage: day, night and root-zone targets](https://genetixbay.com/knowledge/environment/temperature-by-stage.md)
- [Purple or red stems and leaves on cannabis: a differential](https://genetixbay.com/knowledge/diagnosis/purple-stems-leaves.md)