Light is the single most powerful tool a cannabis grower has for controlling plant behavior. By manipulating your cannabis light schedule, you determine when plants grow vegetatively, when they flower, and ultimately how much you harvest. This guide covers everything from standard photoperiod schedules to autoflower timing and the science of DLI (Daily Light Integral).
The Basics: How Cannabis Responds to Light
Cannabis is a short-day plant (technically a long-night plant). Photoperiod cannabis varieties use the duration of uninterrupted darkness to determine when to transition from vegetative growth to flowering. This is controlled by phytochrome proteins — photoreceptors that measure the ratio of red to far-red light, which shifts during darkness.
Key concepts:
- Photoperiod cannabis: Requires specific light/dark ratios to trigger flowering
- Autoflowering cannabis: Flowers based on age, regardless of light schedule (thanks to Cannabis ruderalis genetics)
- Critical dark period: Photoperiod cannabis needs 10–12+ hours of uninterrupted darkness to initiate flowering
Vegetative Light Schedule: 18/6 and Alternatives
During the vegetative phase, your goal is maximum growth — big, healthy plants with strong branching and root development. The most common vegetative light schedule is 18 hours on / 6 hours off (18/6).
Vegetative Schedule Options
| Schedule | Light / Dark | Pros | Cons |
|---|---|---|---|
| 18/6 | 18h on / 6h off | Industry standard, good growth, plants get rest period, lower electricity cost | Slightly slower growth than 20/4 |
| 20/4 | 20h on / 4h off | Faster vegetative growth, more DLI | Higher electricity cost, minimal rest period |
| 24/0 | 24h on / 0h off | Maximum light exposure | No rest period (debatable benefits), highest electricity cost, more heat |
| 16/8 | 16h on / 8h off | Energy savings, some strains thrive | Risk of premature flowering in sensitive strains |
Our recommendation: 18/6 is the gold standard for photoperiod vegetative growth. It provides ample light for vigorous growth while giving plants a dark period for respiration and root development. Research suggests that the dark period is when plants process and transport sugars produced during photosynthesis, making the 6-hour rest period beneficial rather than wasted time.
How Long to Veg?
Vegetative duration depends on your goals, space, and growing method:
- SOG (Sea of Green): 1–2 weeks veg, many small plants
- Standard indoor: 4–8 weeks veg
- SCROG/mainlining: 6–10+ weeks veg to fill the screen
- Mother plants: Indefinite veg (kept under 18/6 permanently)
Flowering Light Schedule: The 12/12 Trigger
When you’re ready to flower your photoperiod cannabis, you switch to 12 hours on / 12 hours off (12/12). This provides the critical dark period that triggers the flowering hormone response.
Critical Rules for 12/12 Flowering
- Absolute darkness is essential: Even brief light interruptions during the dark period can cause stress, hermaphroditism, or reversion to vegetative growth. Light leaks are the #1 enemy of flowering cannabis.
- Consistency matters: Use a reliable timer. Irregular schedules stress plants and can cause hermies. Digital timers are more reliable than mechanical ones.
- The transition period: After flipping to 12/12, expect 1–2 weeks before you see visible flower development. During this “stretch” period, plants may double in height.
- Don’t change the schedule mid-flower: Once flowering starts, maintain 12/12 until harvest. Some growers drop to 11/13 or 10/14 in the final 2 weeks, but this is strain-dependent and risky for beginners.
Flowering Duration by Strain Type
| Strain Type | Typical Flowering Time | Examples |
|---|---|---|
| Fast Indica | 7–8 weeks | Northern Lights, Hindu Kush |
| Standard Indica | 8–9 weeks | OG Kush, Granddaddy Purple |
| Hybrid | 8–10 weeks | Blue Dream, Girl Scout Cookies |
| Sativa-dominant | 10–14 weeks | Haze varieties, Thai strains |
For a deeper dive into photoperiod science, read our comprehensive Guide to Cannabis Light Cycles and Photoperiods.
Autoflower Light Schedules: 20/4 vs 18/6
Autoflowering cannabis doesn’t rely on light schedule changes to initiate flowering — it flowers automatically after a genetically determined vegetative period (usually 2–4 weeks from seed). This means you can run virtually any light schedule throughout the plant’s entire life cycle.
Autoflower Schedule Comparison
| Schedule | DLI Potential | Electricity Cost | Growth Rate | Recommendation |
|---|---|---|---|---|
| 24/0 | Maximum | Highest | Fastest (debated) | Not recommended — no rest, diminishing returns |
| 20/4 | Very High | High | Excellent | ⭐ Best for maximizing yield |
| 18/6 | High | Moderate | Great | ⭐ Best balance of cost and yield |
| 16/8 | Moderate | Lower | Good | Good for warm climates (heat management) |
| 12/12 | Lower | Lowest | Reduced | Only if running autos alongside photos in flower |
The debate: The 20/4 vs 18/6 debate for autoflowers is one of the most discussed topics in cannabis cultivation. Here’s what the evidence suggests:
- 20/4 provides approximately 11% more daily light than 18/6, which can translate to marginally larger yields
- 18/6 gives plants a meaningful rest period, which some growers believe improves terpene production and overall plant health
- The difference in yield between 20/4 and 18/6 is typically 5–15% — meaningful but not dramatic
- Electricity costs with 20/4 are roughly 11% higher
Our recommendation: Run 20/4 for autoflowers if you want maximum yield and your environment can handle the extra heat. Use 18/6 if you’re in a warm climate, want to save on electricity, or prefer giving plants a rest period.
Light Deprivation (Light Dep) Techniques
Light deprivation is the practice of artificially reducing daylight hours to trigger flowering in outdoor or greenhouse cannabis. This technique allows outdoor growers to harvest earlier than the natural season would permit.
How Light Dep Works
- Cover your greenhouse or hoop house with blackout tarps to simulate 12/12 conditions
- Begin covering when plants reach desired vegetative size (typically June–July for Northern Hemisphere)
- Maintain strict timing: Cover at the same time every day and uncover at the same time every morning
- Ensure complete darkness: Any light leaks will disrupt flowering, just like indoor grows
Benefits of light dep:
- Harvest in July–August instead of October–November
- Avoid fall rain and mold pressure
- Run multiple harvest cycles per outdoor season
- Control plant size (shorter veg = smaller, more manageable plants)
Supplemental Lighting
Supplemental lighting is used to extend the photoperiod in greenhouses or to boost light intensity in areas of the canopy that receive less direct light.
When to Use Supplemental Lighting
- Greenhouse growing: During short winter days or cloudy periods, supplemental LEDs maintain vegetative schedules or boost DLI during flowering
- Canopy penetration: Side lighting or under-canopy lighting can increase yields in dense canopies by illuminating lower bud sites
- UV supplementation: Adding UV-A/UV-B during late flowering may increase trichome production (research is ongoing)
- Far-red supplementation: Brief far-red exposure at end of day can accelerate flowering initiation (the “Emerson effect”)
Understanding DLI (Daily Light Integral)
DLI measures the total amount of PAR light delivered to plants over a 24-hour period, expressed in mol/m²/day. While PPFD tells you instantaneous light intensity, DLI tells you total daily light — which is what actually drives plant growth.
DLI Formula
DLI = PPFD × hours of light × 0.0036
Target DLI by Growth Stage
| Growth Stage | Target DLI (mol/m²/day) | PPFD @ 18h | PPFD @ 12h |
|---|---|---|---|
| Seedlings | 12–18 | 185–278 | 278–417 |
| Vegetative | 25–40 | 386–617 | N/A (use 18h) |
| Flowering | 35–50 | N/A (use 12h) | 810–1,157 |
| Flowering + CO₂ | 50–65 | N/A | 1,157–1,504 |
DLI is the most accurate way to compare lighting across different schedules. For example, an autoflower running at 500 PPFD under 20/4 receives a DLI of 36 mol/m²/day — comparable to a photoperiod at 833 PPFD under 12/12.
For a complete deep-dive into light measurement science, read our guide on Understanding PAR, PPFD & DLI.
Common Light Schedule Mistakes
- Light leaks during flowering: Even a tiny LED indicator light can cause hermaphroditism. Use tape to cover any lights in your grow space, and check for light leaks during the dark period.
- Inconsistent schedules: Plants are sensitive to timing changes. Invest in a quality digital timer and don’t manually adjust schedules.
- Too much light too early: Seedlings don’t need — and can be harmed by — full-intensity light. Start at 200–300 PPFD and increase gradually.
- Running 24/0 for photoperiods: While it won’t trigger flowering, 24-hour light provides diminishing returns and increases stress. 18/6 is more efficient.
- Flipping to flower too early: Remember the flowering stretch! Plants may double in height after the 12/12 switch. Plan accordingly.
Quick Reference: Light Schedules at a Glance
| Scenario | Recommended Schedule | Duration |
|---|---|---|
| Photoperiod Veg | 18/6 | 4–8 weeks (or as needed) |
| Photoperiod Flower | 12/12 | 7–14 weeks (strain dependent) |
| Autoflower (full cycle) | 20/4 or 18/6 | Seed to harvest: 8–12 weeks |
| Mother Plants | 18/6 | Indefinite |
| Light Dep (outdoor) | 12/12 forced | Until harvest |
Final Thoughts
Your light schedule is one of the most powerful levers you have as a cannabis grower. Whether you’re running photoperiod strains that need precise 12/12 triggering or autoflowers that thrive under 20/4, understanding the why behind your timing decisions makes you a better cultivator.
Pair your schedule knowledge with the right hardware — check out our LED Grow Light Buyer’s Guide for fixture recommendations, and visit our Grow Lights Equipment page for specific product picks.