Cannabis is a high-light plant that needs between 100 and 1,000 µmol/m²/s of photosynthetically active light depending on its growth stage. That metric, called PPFD (photosynthetic photon flux density), measures usable light hitting the canopy each second. But intensity is only one piece. How many hours you leave the lights on, what spectrum you use, and how much CO2 is available all determine whether your plants thrive or stall.
Light Intensity by Growth Stage
Cannabis needs dramatically different light levels as it matures. Seedlings are fragile and photosynthesize slowly, so they do best at 100 to 300 µmol/m²/s. Blasting young plants with more than that risks bleaching and stunted growth before they’ve built the root system to support rapid photosynthesis.
During vegetative growth, plants can handle 400 to 600 µmol/m²/s. This is the stage where stems thicken, leaves multiply, and the plant builds the framework that will later support flowers. More light means faster, bushier growth, but only up to the point where the plant can actually use it.
Flowering is where cannabis is hungriest for light. Target 600 to 1,000 µmol/m²/s, with the higher end reserved for the final weeks when buds are filling out. Without supplemental CO2, though, pushing past about 900 µmol/m²/s leads to diminishing returns. The plant simply can’t process more photons without more carbon dioxide, and the excess light causes bleaching, where leaves and even bud tips turn white or pale yellow.
How Many Hours of Light Per Day
For photoperiod strains (the majority of cannabis genetics), growers keep lights on for 18 hours and off for 6 during the vegetative stage. This long-day schedule tells the plant to keep growing rather than flowering. Some growers run 20/4 or even 24/0 during veg, but 18/6 is the standard because it balances strong growth with energy savings.
Flowering is triggered by switching to a 12-hour light, 12-hour dark schedule. This equal split mimics the shortening days of autumn and signals the plant to begin producing flowers. The dark period is critical. Even brief light interruptions during the 12 hours of darkness can confuse the plant and delay or disrupt flowering. Research has tested slightly longer photoperiods like 14 hours of light during bloom, but 12/12 remains the long-held standard in cannabis production because it reliably initiates and sustains flower development.
Autoflowering Strains Are Different
Autoflowers flower based on age rather than light schedule, which gives you more flexibility. Most growers run 18/6 from seed to harvest. Some push to 20/4 or a full 24 hours of light, arguing it maximizes vegetative growth in a strain that already has a short life cycle. There’s no consensus on which is best, and experienced growers get good results with both 18 and 24 hours. The one thing to avoid is dropping below 18 hours, since harvesting autoflowers under a 12-hour cycle produces noticeably smaller buds.
Light Spectrum: Blue vs. Red
Cannabis responds to the color of light, not just the quantity. Blue wavelengths (roughly 400 to 500 nanometers) keep plants compact by shortening the distance between nodes and reducing leaf stretching. This is useful during vegetative growth when you want a stocky, bushy plant rather than a tall, lanky one.
Red wavelengths (600 to 700 nanometers) drive flowering and overall photosynthesis more efficiently. Far-red light, just beyond visible red, triggers a shade-avoidance response: stems elongate and leaves stretch as the plant tries to outgrow competitors. Traditional high-pressure sodium (HPS) lights emit a lot of red and far-red, which is one reason plants grown under them tend to grow taller. Research comparing HPS to LED fixtures found that LED-grown plants were shorter and more compact. In one study, plant height averaged 58 cm under LEDs versus 67 cm under HPS. However, the HPS plants produced slightly higher flower yields (26.6 grams versus about 23 grams per plant under LED), likely due to the higher total light output of those particular HPS fixtures rather than an inherent spectral advantage.
Full-spectrum LED panels that blend blue, red, and white diodes are the current standard for home growers because they cover the plant’s needs across all stages without swapping bulbs.
CO2 and the Light Ceiling
Light and carbon dioxide work together. Plants use light energy to convert CO2 into sugars, so when one runs out, more of the other is wasted. In a normal indoor environment, CO2 sits around 400 ppm, and that limits how much light a cannabis plant can productively absorb. Without supplemental CO2, your effective ceiling is roughly 900 µmol/m²/s. Beyond that, you’re paying for electricity that doesn’t translate to growth.
With CO2 enrichment to 1,000 to 1,500 ppm, plants can use PPFD levels well above 1,000 µmol/m²/s, which accelerates growth and increases yields. The ideal range during flowering is 1,200 to 1,500 ppm. Going above 1,500 ppm is technically tolerable for the plants (up to about 2,000 ppm), but the extra CO2 produces diminishing returns and wastes gas. CO2 enrichment only makes sense in sealed grow rooms where you can control the concentration, and it’s typically paired with high-output LED or HPS lighting to justify the cost.
How Far to Hang Your Lights
The distance between your light and the canopy controls how much PPFD actually reaches the leaves. Every fixture is different, but general guidelines based on LED wattage give you a starting point.
For low-wattage LEDs (100 to 300 watts), start at 18 to 24 inches for seedlings, drop to 12 to 18 inches during veg, and bring the light to 8 to 12 inches for flowering. Medium-wattage panels (300 to 500 watts) need more distance: 24 to 30 inches for seedlings, 18 to 24 for veg, and 12 to 18 for flower. High-output LEDs above 600 watts should hang 30 to 36 inches above seedlings, 24 to 30 inches during veg, and 18 to 24 inches during flowering.
LEDs run cooler than HPS lights, which typically need at least 24 to 36 inches of clearance to avoid heat damage. That heat difference is one reason LEDs have largely replaced HPS in home grows: you can position them closer and deliver more usable light per watt of electricity.
Signs You’re Giving Too Much Light
Cannabis tells you when it’s getting more light than it can handle. The most obvious sign is bleaching, where upper leaves or bud tips lose their color and turn white or pale yellow. This isn’t a nutrient deficiency; it’s the destruction of chlorophyll from photon overload.
Other warning signs to watch for:
- “Taco-ing”: leaf edges curl upward, making each leaf look like a taco shell. This is the plant trying to reduce its surface area exposed to light.
- Claw-like curling: leaf tips curl downward instead of upward, creating a hooked appearance.
- Drooping and dehydration: excessive light drives rapid water loss through the leaves faster than the roots can replace it.
- Stunted growth: instead of stretching, the plant compresses its nodes and stays unusually short as a stress response.
If you notice any of these, raise your light a few inches and check again in 24 to 48 hours. Heat stress from HPS fixtures causes similar symptoms (drooping, yellowing, brittle leaves), so rule that out by checking the temperature at canopy level. Aim to keep it below 85°F (30°C).
Measuring Light Without Expensive Tools
PPFD meters designed for horticulture cost $100 to $500, which is overkill for many home growers. A cheaper option is to use a lux meter or a smartphone light meter app and convert the reading. Lux measures visible brightness, while PPFD measures photons useful for photosynthesis, so the conversion depends on your light source’s spectrum. Under white LEDs, dividing lux by roughly 65 gets you a ballpark PPFD. Under HPS lights, the conversion factor is different (closer to dividing by 50) because of the heavier red output.
These conversions are approximations, not precise measurements. But for a home grower trying to decide whether the light is close enough or too far away, an approximate PPFD reading is far more useful than guessing. Several smartphone apps offer this conversion automatically if you select your light type.