DIF (from "difference") is the day temperature minus the night temperature. A day of 26 °C and a night of 20 °C is a DIF of +6 °C (about +11 °F). Research on greenhouse ornamentals showed that stem elongation responds to DIF more than to average temperature: positive DIF increases internode length, zero DIF reduces it, and negative DIF (cooler days than nights) reduces it further. A short temperature drop at dawn gives part of the effect.
In practice
Growers use DIF mainly to manage stretch in early flower. Keeping the gap small, around 0–4 °C (0–7 °F), may help restrain height in stretchy cultivars; a larger gap of 6–10 °C (11–18 °F) late in flower is often used for colour. Evidence in cannabis specifically is thin; the effect is well documented in other crops. Supported by extrapolation.
Watch the humidity side. When lights go off and the room cools quickly, RH climbs and may approach the dew point on leaves and buds, inviting botrytis. Ramp temperature down, run dehumidification at lights-off, and keep air moving.
DIF = day temperature − night temperature (°C or °F).
Don't confuse with
- Cold nights: a large positive DIF used for colour; see that entry for what it does and does not do.
- VPD: humidity-driven, though DIF changes VPD at night.
The length of stem between two adjacent nodes; short internodes mean compact plants and tight bud stacking, long ones mean stretch.The stretch is the rapid increase in height in the first two to three weeks after switching to flower, when many cultivars grow 1.5 to 3 times taller.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.Botrytis cinerea, the fungus behind bud rot or grey mould, which rots dense colas from the inside in cool, humid conditions late in flower.A dehumidifier removes water vapour from grow-room air, usually by condensing it on a cold coil; it is the main tool for controlling humidity in flower.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.Vapour pressure deficit: how much more water vapour the air could hold at its temperature, in kPa; it is the main environmental driver of transpiration.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.
