A microclimate is the local environment in one small part of the grow space, which can be very different from what the room sensor reports. Common examples: the humid, still air inside a dense canopy, a hot spot directly under the centre of a fixture, a cold corner near an intake, or the air right next to the medium surface.
In practice
A sensor at mid-height in an open spot might read 24 °C (75 °F) and 50% RH, while the air inside a thick cola near the bottom of the canopy sits several percent higher in RH with almost no air movement. That is where botrytis and powdery mildew start. Dense, unpruned lower canopies are the classic problem.
How to manage microclimates: move the sensor to canopy height and check several locations; run fans above and below the canopy; thin lower larf that will never ripen; space plants so air can pass between them. Check the coldest surfaces after lights-off for condensation.
Don't confuse with
- Boundary layer: the still air film directly on a leaf surface, millimetres thick.
- Room average: what the controller sees; microclimates are why it can mislead.
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.The dense, elongated cluster of flowers that forms at the end of a stem or branch; the main cola tops the leader.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 fungal disease, on cannabis caused mainly by Golovinomyces ambrosiae, that grows as white powdery patches on leaves and buds and thrives in moderate humidity.A large palmate cannabis leaf with several long leaflets on a long petiole; fan leaves do most of the plant's photosynthesis and carry few trichomes.Airflow is the movement of air through and around the canopy; it thins the still boundary layer on leaves, evens out temperature and humidity, and lowers mould risk.An oscillating fan sweeps back and forth to circulate air over and through the canopy, mixing the room and preventing still, humid pockets.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.
