"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, 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 up and dense buds near the dew point are an open invitation to botrytis.
The founder admits to being "a sucker for any red strain", but bag appeal is not quality. Colour says nothing about potency or aroma.
Don't confuse with
- Purple stems from deficiency: purple petioles 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.
Myth check
GenetixBay house position: colour from cold is real (anthocyanins; Established). Claims that cold nights increase terpenes, flavour or resin are a Myth unless peer-reviewed data show otherwise, and none do so far. Members can test it in a Mythbusters side-by-side.
A water-soluble flavonoid pigment stored in cell vacuoles that colours leaves, stems and flowers red, purple or blue, set by genetics and enhanced by cool temperatures.Relative humidity is the water vapour in air as a percentage of what it could hold at that temperature, so the same RH dries plants differently at different temperatures.Dew point is the temperature at which air becomes saturated and water starts condensing; when leaves or buds cool to it, free water forms and mould risk jumps.Bag appeal is how good flower looks in the bag or jar, frost, colour, density and trim, which sells cannabis but does not measure its quality.A nutrient deficiency is a shortage of an essential element in the plant tissue, causing characteristic symptoms; the cause is often pH or uptake rather than an empty feed.The stalk that attaches a leaf to the stem at a node; in cannabis it carries all the leaflets of a palmate leaf.DIF is day temperature minus night temperature; a larger positive DIF lengthens stems, while zero or negative DIF tends to keep plants shorter.Terpenes are volatile hydrocarbon compounds built from five-carbon isoprene units; in cannabis they are made in the trichome heads and are a major part of aroma and flavour.In growing, temperature usually means grow-room air temperature; cannabis grows best around 24–28 °C (75–82 °F) by day and a few degrees cooler at night.
