Cannabis plants in the flowering stage need three things from their light: a strict 12 hours on and 12 hours off photoperiod, roughly 600 to 1,000 PPFD at the canopy, and a red-dominant spectrum with peaks near 630 and 660 nm. High-pressure sodium lamps and full-spectrum LEDs with boosted red diodes both deliver that, and either can produce dense, resinous buds when distance, heat, and airflow are managed. The 12 hours of uninterrupted darkness is what actually triggers and sustains flower development, so the schedule matters as much as the fixture.
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What light schedule do flowering cannabis plants need?
Flip to 12 hours of light and 12 hours of total darkness on the first day of flower, and hold that schedule until harvest. A single light leak during the dark period can stall bud development, trigger male flowers on female plants, or push a plant back into vegetative growth. Run the lights on a timer, seal zippers and vents, and cover any LED indicators inside the tent.
Cannabis Grow Lights for Autoflowering Strains: A Practical Guide
How much light intensity does flowering cannabis need?
Flowering plants want more intensity than plants in veg, but the ceiling depends on your setup. Use a quantum sensor or a phone-based PPFD app at canopy height and adjust the fixture or its dimmer until you land in range.
cannabis grow lights for flowering stage
- Low-intensity setup without CO2: 500 to 700 PPFD
- Standard indoor garden without CO2: 700 to 900 PPFD
- Sealed room with CO2 enrichment: 900 to 1,200 PPFD
- Daily light integral (DLI) target: 30 to 45 mol/m²/day
Pushing past 1,000 PPFD without added CO2, water, and nutrients usually causes bleaching, foxtailing, and leaf burn instead of bigger flowers.
How do you calculate DLI for flowering?
Multiply PPFD by 3,600, multiply again by the number of light hours, then divide by 1,000,000. A flowering room at 700 PPFD on a 12-hour cycle delivers about 30 mol/m²/day, which sits at the low end of the flowering target. If your DLI is short, raise intensity before lengthening the photoperiod, since the dark cycle must stay at 12 hours.
What spectrum works best for flowering cannabis?
Flower development responds most to red and deep red wavelengths, with blue needed for structure and overall plant health. A practical flowering spectrum keeps 60 to 80 percent of output in the red band, adds 400 to 450 nm blue, and may include far-red at 730 nm for stretch control and finishing. Full-spectrum white LEDs with extra 660 nm diodes cover this well, and HPS already peaks in the right region.
HPS vs LED for flowering: which is better?
- HPS: proven yields, low upfront cost, high heat output, bulbs need replacing every 2 to 3 harvests, and efficiency sits near 1.0 to 1.5 µmol/J.
- LED: 2.3 to 3.0 µmol/J efficiency, less heat, tunable spectrum and dimming, higher upfront cost, longer service life.
In a cold basement, HPS heat is useful. In a warm room or a small tent, LED makes temperature control far easier and cuts electricity per gram harvested.
How far should grow lights be from flowering plants?
Distance depends on wattage and lens type, so intensity readings beat tape measures. As a starting point, keep LED fixtures 12 to 24 inches above the canopy and air-cooled HPS lamps 18 to 30 inches above it. Hold your hand at canopy height for 30 seconds: if the back of your hand feels uncomfortably warm, raise the light.
Common flowering light mistakes
- Leaving the photoperiod at 18/6 into week three of flower
- Hanging lights too high and losing 40 percent or more of usable intensity
- Ignoring canopy hotspots where colas stretch toward the fixture
- Running full blue-heavy veg spectrum for the entire flower cycle
- Skipping dimming at the flip, which shocks plants moved from low light
Track PPFD at the canopy once a week, adjust height as plants stretch, and keep daytime temperatures near 75 to 82 F with lights on. Timers, quantum sensors, and replacement bulbs are easy to grab at a local licensed retailer such as Pop's Cannabis before the flip.