Light and Spectrum
Photons are the fuel. Everything else is support.
Educational use only. Cannabis cultivation is subject to federal, state/provincial, and local laws. Verify your local laws before proceeding. Nothing here constitutes legal or medical advice. For adults 21+ (18+ in medical jurisdictions).
Light is the only input cannabis cannot get from the soil or the nutrient solution — it is the primary driver of photosynthesis and therefore of yield. But not all light is equal, and more is not always better. Understanding PPFD, DLI, photoperiods, and spectrum gives you real control over your grow, regardless of what fixture type you are using. Growers who understand light usually outperform those with more expensive equipment who do not.
PPFD and DLI: measuring light correctly
Lux and lumens measure light as the human eye perceives it — but plants perceive light differently. The relevant measurement for plant growth is PPFD: photosynthetic photon flux density, measured in micromoles of photons per square meter per second (µmol/m²/s). A PAR (photosynthetically active radiation) meter or quantum sensor measures this directly.
DLI — daily light integral — takes PPFD one step further: it multiplies the average PPFD by the number of seconds of light per day to give the total photon dose. This is the most useful number for comparing grows with different light schedules. A 600W HPS running 18 hours at 600 µmol/m²/s delivers the same DLI as a more efficient LED running at 720 µmol/m²/s for 15 hours.
Most indoor cannabis growers target a DLI of 40–55 mol/m²/day during vegetative growth and 45–65 mol/m²/day during flowering. Premium commercial operations push 60–70+ with supplemental CO2, but this is beyond diminishing returns without elevated CO2.
PPFD targets by stage and CO2 level
| Stage | Ambient CO2 (~400ppm) | Elevated CO2 (1000–1500ppm) |
|---|---|---|
| Seedling | 200–400 µmol/m²/s | Not applicable |
| Early vegetative | 400–600 µmol/m²/s | 600–800 µmol/m²/s |
| Late vegetative | 600–900 µmol/m²/s | 900–1,200 µmol/m²/s |
| Flowering | 800–1,100 µmol/m²/s | 1,200–1,500 µmol/m²/s |
| Late flower | 700–900 µmol/m²/s | 1,000–1,300 µmol/m²/s |
Photoperiods and the flowering trigger
Photoperiod cannabis varieties are short-day plants: they remain vegetative under long light periods (18 hours light / 6 hours dark is standard) and flower when darkness reaches a critical threshold, typically 12 hours. This is controlled by phytochrome proteins in the plant that detect the duration of uninterrupted darkness.
The practical implication: any light leak during the dark period — even a brief phone screen or a timer malfunction — can delay or disrupt flowering. Inspect your dark period setup carefully. Light leaks are one of the most common causes of hermaphroditism in photoperiod strains.
Autoflowering varieties (Cannabis ruderalis hybrids) bypass the photoperiod trigger entirely and flower based on age rather than light schedule. Most autoflowers begin flowering at 3–5 weeks regardless of light schedule. They are typically run at 18–20 hours of light throughout the entire grow to maximize DLI and speed.
Tip
A 18/6 schedule (18 hours light, 6 hours dark) is the standard vegetative photoperiod, but some growers run 20/4 for faster vegetative growth. The plant does not require the dark period during veg — some run 24/0 successfully. During the dark period in flower, even 5 seconds of light interruption can delay flowering by days to weeks.
LED vs HPS vs CMH: choosing a fixture
HPS (High Pressure Sodium) was the standard for decades and still produces excellent results. The 600W HPS is one of the most proven grow lights ever made. Its main limitations are heat output (significant HVAC requirements) and relatively low efficiency compared to modern LEDs.
LED quantum board fixtures (Samsung LM301B/H chips, driver brands like Meanwell) have become the dominant choice for home growing over the last five years. They run significantly cooler, use 20–35% less electricity than equivalent HPS for the same PPFD output, and typically last 50,000+ hours. The gap in canopy penetration between HPS and LED (a common concern) has largely closed with higher-powered LED fixtures.
CMH (Ceramic Metal Halide, also called LEC) offers a full spectrum that is closer to natural sunlight than HPS and produces exceptional terpene and resin profiles in many growers' experience. It runs warmer than LED but cooler than HPS. Many premium craft growers use CMH for flower despite the lower efficiency, specifically for the spectrum quality.
Light type comparison
| Type | Efficiency (µmol/J) | Heat | Lifespan | Best for |
|---|---|---|---|---|
| HPS 600W | 1.4–1.7 | High | 10,000–24,000 hrs | Proven performance, lower upfront cost |
| LED QB (quality) | 2.3–2.8+ | Low–medium | 50,000+ hrs | Efficiency, home growing, reduced HVAC |
| CMH 315W | 1.6–1.9 | Medium | 20,000 hrs | Spectrum quality, terpenes, craft growing |
| Blurple LED (old) | 0.8–1.2 | Medium | Variable | Not recommended |
Spectrum, red ratio, and far-red light
Cannabis, like most plants, uses light primarily in the red (620–700nm) and blue (400–500nm) ranges for photosynthesis. Blue light promotes compact, bushy vegetative growth with shorter internodal spacing. Red light drives elongation and flowering. Most full-spectrum LEDs and HPS lights provide an appropriate balance for general growing.
Far-red light (700–800nm) was long considered outside the photosynthetically active range, but research has shown that far-red significantly accelerates photosynthesis when combined with red light — a phenomenon called the Emerson effect. Far-red at the end of the photoperiod (a 10–15 minute "end-of-day" far-red treatment) can also accelerate the flowering transition. Some premium LED fixtures now include far-red LEDs specifically for this purpose.
UV light (280–400nm), particularly UVB, stimulates trichome production as a plant defense mechanism. Some growers add UVB supplementation in late flower for the final 2–4 weeks. The evidence is promising but not as established as it is for PPFD targets.
Pro tips
- →Distance from canopy determines PPFD — manufacturers' recommended hanging heights are a starting point, not a final setting
- →Use a PAR meter app on your phone as an approximation (they are not as accurate as a quantum sensor but far better than guessing)
- →Dimming your light and raising it is better than full power at max height — you get a more even footprint
- →CO2 supplementation only benefits growth when PPFD is above ~900 µmol/m²/s. At lower intensities, the plant cannot use the extra CO2
- →Light burn first appears as bleaching on the top cola closest to the light — looks like a nutrient deficiency but is position-specific