Light is plant food. Too little and you get stretchy plants and airy buds; too much and leaves bleach, canopies fry, and growth stalls. The good news: modern LEDs plus three rules of thumb get you 95 % of the way.
The only jargon you need
PPFD = how much usable light actually lands on your leaves (measured in µmol/m²/s). Every stage has a comfort zone:
| Stage | PPFD target | Feels like |
|---|---|---|
| Seedling | 100–300 | Gentle morning light |
| Veg | 400–600 | Strong daylight |
| Flower | 600–900 | Full summer sun |
No quantum sensor? A free phone app (Photone and similar) gets close enough for a home grow, and your light's manual almost always publishes a height/dimmer chart — the manufacturer already did this math.
Hang heights that just work (typical quantum-board LED)
| Stage | Distance | Dimmer |
|---|---|---|
| Seedling | 24–30" | 25–40 % |
| Veg | 20–24" | 50–75 % |
| Flower | 16–20" | 90–100 % |
Move in steps: change height or dimmer, never both at once, then watch the plants for 48 hours.
Read the plant, not just the numbers
- Stretching / big gaps between nodes → light is too weak or too far. Lower it.
- Leaves praying upward → happy. Leave it alone.
- Taco-folded leaves, bleached tips at the top → light stress. Raise it or dim 10 %.
- Canopy hot to your hand — if the back of your hand is uncomfortable at canopy level for 30 seconds, it's too hot for leaves too.
Light schedule quick reference
- Photoperiods: 18/6 in veg → flip to 12/12 for flower. The 12-hour dark period must be pitch black and uninterrupted.
- Autos: run 18/6 the whole way (some run 20/4; more light = more food, more heat, more water use).
- Consistency beats cleverness: a $10 timer removes the single most common human error.
Coverage honesty
A light's "flowering footprint" is smaller than its "veg footprint" on the box. A 100W board flowers about 2x2 ft well; a 240W does 2x4. Spreading a small light over a big tent doesn't make more bud — it makes fluffier bud everywhere. Match wattage to your canopy: 30–40 real watts per square foot.
Upgrade order forever: light first, then everything else. The light IS the yield.