Typical indoor PPF
HID rooms
Most MH or HPS gardens land near 1,000 µmol/m²/s (about 5,000 fc / 55,000 lux) at the tops
Fluorescent rooms
Seldom more than about 300-500 µmol/m²/s (about 1,500 fc)
Veg target
Most varieties do well at 2,000-2,500 fc (21,500-27,000 lux, 400-500 µmol/m²/s)
Headroom
Plants can use 5,000 fc (54,000 lux, 1,000 µmol/m²/s) or more if the rest of the room keeps up
Payoff
Brighter light with other needs met raises yield and quality and can shorten ripening
Variety ranking
Most light
Sativas, then sativa-leaning hybrids, then indica-leaning hybrids, then indicas
Sativas
Evolved below about the 30th parallel and expect long, intense sun
Sativa-indica hybrids
Still want the high end, but less than pure sativas
Indica-sativa hybrids
More forgiving; they can work in the mid-to-low indoor range
Indicas
Northern-latitude plants and the best bet for a dimmer room
Watts per square foot
Equatorial sativas
About 70-80 W/sq ft (750-860 W/m²). Less light makes loose, lanky buds
Sativa-indica hybrids
Lush buds from as little as 60 W/sq ft (640 W/m²)
Indica-sativa hybrids
About 50-60 W/sq ft (535-640 W/m²)
Indicas
Some work from about 40 W/sq ft (430 W/m²); others need about 50 (535 W/m²) for tight buds
More on indica
60 W/sq ft (640 W/m²) is not harmful and tends to make larger, tighter, stronger flowers
Low-light stretch
Cause
Low light, a leafy canopy, or tree shade raises the far-red share and lengthens internodes
Who suffers most
Equatorial sativa genetics stretch harder in shade
Veg benefit
More light in vegetative growth speeds growth and thickens stems
Whole-plant rule
Extra light only pays if water, nutrition, CO2, and temperature are not the new limit