The working band
General
Plants do well between 70 and 85 F (21 to 29 C)
Fastest growth
Lights-on around 72 to 77 F (22 to 26 C) when CO2 is ambient
With extra CO2
Prefer a few degrees warmer, about 79 to 85 F (26 to 29 C)
Strain spread
Varieties differ by a few degrees, so watch your cultivar rather than one universal number
Stage targets
Germination: 70 to 78 F (21 to 25 C).
Vegetative: 68 to 82 F (20 to 27 C).
Flowering: 68 to 80 F (20 to 26 C).
Cloning: 75 to 85 F (24 to 29 C).
Light and day-night gap
More light, more heat
Stronger light raises the temperature the plant can use well
Moderate lamps
At 40 to 50 watts per square foot, stay on the cool side of the range
High-intensity lamps
At 60 watts per square foot and up, stay on the warm side
Night drop
Up to about 10 F (5 C) cooler in the dark is fine. A wider swing stretches stems and slows ripening
Steady rooms
A small day-night gap makes stouter stems and denser flowers
Cold limits
Below 60 F (15 C)
Photosynthesis and metabolism slow and growth waits
Below 40 F (4 C)
Tissue damage, then about a day of warmth before growth resumes
Seedlings outdoors
Young plants can push through snow, but ripening buds hate even one cold night
Equatorial cultivars
A few nights under 40 F (4 C) can stop growth altogether