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.
For the daily-total version of PPFD, see the DLI optimization guide — DLI tells you whether your light schedule and intensity combine to give enough total light per day.
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.
The inverse-square reality: light intensity drops dramatically with distance. Moving a light from 18" to 24" doesn't reduce intensity by a third — it roughly halves it. This is why height adjustments are powerful and need to be small.
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%. See the light burn guide for diagnosis
- 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
For the details on spectrum, see the light spectrum science guide.
LED vs other grow lights
| Type | Pros | Cons |
|---|---|---|
| Quantum board LED | Efficient, cool, long-lived | Higher upfront cost |
| Bar-style LED | Even coverage, low profile | Expensive for quality |
| HPS (high-pressure sodium) | Cheap, proven | Hot, inefficient, high bills |
| CMH (ceramic metal halide) | Good spectrum, moderate heat | Bulb replacements |
| T5 fluorescent | Great for seedlings/clones | Not enough for flower |
For home grows in 2024+, a quantum-board LED is the correct answer in almost every case. The efficiency per watt means lower electricity bills, less heat to manage, and longer lifespan. The upfront premium pays for itself within two grows in electricity savings.
Coverage honesty
A light's "flowering footprint" is smaller than its "veg footprint" on the box. A 100W board flowers about 2×2 ft well; a 240W does 2×4. 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. This is the single most reliable predictor of yield quality. Everything else — terpene profile, density, trichome production — improves when you hit this number.
| Tent size | Minimum wattage | Recommended |
|---|---|---|
| 2×2 ft | 100W | 120–150W |
| 2×4 ft | 200W | 240–320W |
| 4×4 ft | 400W | 480–600W |
| 5×5 ft | 600W | 750W+ |
Common lighting mistakes
Leaving the light at one height all grow. As the plant grows, the canopy closes the gap. Check your distance weekly and adjust — a few inches of drift causes light burn on top colas while lower buds starve.
Running 24/0 for photoperiods. Plants need rest. The 6 hours of dark isn't wasted — root growth and hormone production happen during the night cycle. 18/6 outperforms 24/0 for photoperiod genetics.
Upgrading everything except the light. A $300 tent with a $50 light grows worse than a $50 tent with a $300 light. The light IS the yield.
Upgrade order forever: light first, then everything else.
For the deep dive on LED features and PPFD mapping, see LED grow light mastery. For help sizing your first setup, see the first grow equipment guide.