Grow Light Timer Settings: Complete Schedule Guide by Plant Type
Master photoperiod schedules, understand 18/6 vs 24/0 light cycles, and automate your grow room for maximum yields.
Key Takeaways
- ✓ Most plants thrive on 16-18 hours of light during vegetative growth
- ✓ 12/12 light schedules trigger flowering in photoperiod plants
- ✓ Seedlings and clones can handle 18-24 hours, but 18/6 is usually optimal
- ✓ Invest in a reliable digital timer - inconsistent schedules stress plants
- ✓ Use our DLI Calculator to optimize light intensity + duration
Table of Contents
Getting your grow light timer settings right is one of the most important factors in indoor growing success. Unlike outdoor plants that follow the sun's natural cycle, indoor plants depend entirely on you to provide the right amount of light at the right times. This works together with proper wattage to determine your growing success.
The good news? Once you understand photoperiods and set up a reliable timer, your grow room essentially runs itself. This guide covers everything from basic light schedules to advanced automation strategies that professional growers use.
Pro tip: Light duration (photoperiod) works together with light intensity (PPFD) to determine your Daily Light Integral (DLI) - the total amount of photosynthetically active light your plants receive each day. Use our grow light calculator to dial in both settings for optimal results.
1. Understanding Photoperiods and DLI
A photoperiod is simply the duration of light exposure in a 24-hour cycle. Plants have evolved to use photoperiod as a signal for seasonal changes, triggering different growth phases based on day length.
The DLI Connection
Daily Light Integral (DLI) measures the total photons your plants receive per day. It's calculated by multiplying light intensity (PPFD) by the number of hours of light:
This means you can achieve the same DLI with either:
- High intensity for fewer hours (e.g., 600 PPFD for 12 hours)
- Lower intensity for more hours (e.g., 400 PPFD for 18 hours)
Important: There's a limit to how much light plants can use at once. Above ~1000-1200 PPFD, most plants experience photoinhibition (diminishing returns). So you can't simply blast plants with extreme light for 6 hours - spreading light over 12-18 hours is more efficient.
Three Types of Photoperiod Response
| Type | Response | Examples |
|---|---|---|
| Short-Day Plants | Flower when nights are long (>12 hours darkness) | Cannabis, chrysanthemums, poinsettias |
| Long-Day Plants | Flower when days are long (>14 hours light) | Spinach, lettuce, wheat, barley |
| Day-Neutral Plants | Flower based on age/size, not light duration | Tomatoes, peppers, autoflowers, most houseplants |
2. Common Light Schedules Explained
Here are the most common light schedules used by indoor growers, with the pros and cons of each:
18/6 (18 Hours On, 6 Hours Off)
The most popular vegetative growth schedule
Pros:
- Excellent for vegetative growth
- Allows dark period for plant recovery
- Lower electricity costs than 24/0
- Mimics long summer days naturally
Cons:
- Slightly slower than 20/4 or 24/0
- Temperature swings during dark period
Best for: Vegetative cannabis, leafy greens, herbs, most indoor plants. See our microgreens guide for specific recommendations.
20/4 (20 Hours On, 4 Hours Off)
Extended light for faster vegetative growth
Pros:
- Faster growth than 18/6
- Still provides dark period
- Good for autoflowering plants
Cons:
- Higher electricity costs
- More heat management needed
- Some plants may show stress
Best for: Autoflowers, fast vegetative cycles, commercial operations
24/0 (24 Hours On, No Dark Period)
Continuous light - controversial but sometimes used
Pros:
- Maximum photosynthesis potential
- Good for seedlings/clones short-term
- Consistent temperature (no night drop)
Cons:
- Highest electricity costs
- Plants may stress from no rest
- Research shows diminishing returns
- Can cause nutrient deficiencies
Best for: Seedlings/clones only (short-term), speed-focused commercial grows
12/12 (12 Hours On, 12 Hours Off)
Standard flowering schedule for photoperiod plants
Pros:
- Triggers flowering in short-day plants
- Lowest electricity usage
- Required for flower development
Cons:
- Slower vegetative growth
- Requires light-proof dark period
- Not suitable for all plant types
Best for: Flowering cannabis, photoperiod flowering plants
3. Timer Settings by Growth Stage
| Growth Stage | Recommended Schedule | Target DLI | Duration |
|---|---|---|---|
| Germination | 16-18 hours (low intensity) | 4-8 mol/m2/day | 3-7 days |
| Seedling | 18/6 (medium intensity) | 10-15 mol/m2/day | 2-3 weeks |
| Clone/Cutting | 18-24 hours (low intensity) | 6-10 mol/m2/day | 1-3 weeks |
| Vegetative | 18/6 or 20/4 | 20-40 mol/m2/day | 3-8 weeks |
| Pre-Flower | 12/12 (transition) | 30-45 mol/m2/day | 1-2 weeks |
| Flowering | 12/12 (strict) | 40-65 mol/m2/day | 6-12 weeks |
Calculate your exact DLI: Use our grow light calculator to input your light's PPFD and desired photoperiod. The calculator will show if you're hitting optimal DLI for each growth stage.
4. Schedules by Plant Type
Houseplants & Tropicals
| Recommended: | 12-16 hours |
| DLI Target: | 4-12 mol/m2/day (varies by species) |
| Notes: | Most are day-neutral; schedule for convenience |
Common plants: Pothos, Monstera, Philodendron, Ficus, Peace Lily
Succulents & Cacti
| Recommended: | 12-14 hours |
| DLI Target: | 15-25 mol/m2/day (high light lovers) |
| Notes: | Need higher intensity, not longer duration |
See our succulent grow light guide for specific recommendations.
Herbs & Microgreens
| Recommended: | 14-18 hours |
| DLI Target: | 12-20 mol/m2/day |
| Notes: | Longer light = faster growth and more compact plants |
Common plants: Basil, mint, cilantro, parsley, microgreens
Leafy Greens & Lettuce
| Recommended: | 16-18 hours |
| DLI Target: | 12-17 mol/m2/day |
| Notes: | Avoid <10 hours or lettuce may bolt |
Common plants: Lettuce, spinach, kale, arugula, chard
Fruiting Vegetables
| Recommended: | 14-18 hours (they're day-neutral) |
| DLI Target: | 20-40 mol/m2/day (high light) |
| Notes: | Flower based on age/nutrients, not photoperiod |
Common plants: Tomatoes, peppers, cucumbers, strawberries
Cannabis (Photoperiod)
| Vegetative: | 18/6 or 20/4 |
| Flowering: | 12/12 (strict - no light leaks!) |
| DLI Target: | Veg: 25-40, Flower: 40-65 mol/m2/day |
See our 4x4 tent guide for optimal setups.
Autoflowering Plants
| Recommended: | 18/6 or 20/4 (entire lifecycle) |
| DLI Target: | 35-50 mol/m2/day |
| Notes: | Day-neutral genetics; no schedule change needed |
Some growers run 24/0, but 20/4 is more cost-effective with similar results.
5. Best Timers for Grow Lights
Investing in a quality timer is one of the most important (and affordable) upgrades you can make. Here are the top options for different budgets and needs:
Century 7-Day Digital Timer
- 7-day programmable with up to 8 on/off settings per day
- Battery backup saves programs during power outages
- 15A/1875W capacity (handles most LED lights)
- Simple LCD display and intuitive programming
BN-LINK 7-Day Heavy Duty Timer
- Dual-outlet design (run 2 devices on same schedule)
- 20 on/off programs - more flexibility
- 15A/1875W per outlet
- Random and countdown modes (bonus features)
- UL listed for safety
Kasa Smart Plug (HS103)
- WiFi control via app - adjust schedules from anywhere
- Voice control with Alexa/Google Home
- Sunrise/sunset scheduling (auto-adjusts seasonally)
- Energy monitoring to track electricity costs
- 15A/1800W capacity
VIVOSUN 7-Day Dual Outlet Timer
- Designed specifically for grow rooms
- 240V compatible for high-wattage HPS/MH lights
- 15A/1875W capacity
- 8 on/off cycles per day
- Grounded 3-prong outlets
AC Infinity CONTROLLER 67
- Complete environmental controller with 4 outlet zones
- Schedule lights, fans, and other equipment
- Temperature and humidity triggers
- WiFi app control with alerts
- Data logging and graphing
Amp rating tip: Always check your timer's amp rating against your light's power draw. A 1000W HPS light draws ~8.3 amps at 120V. For high-wattage setups, use a 20A+ timer or a relay/contactor to handle the load safely.
6. Advanced Automation Tips
Beyond basic on/off scheduling, here are advanced techniques that professional growers use:
Sunrise/Sunset Simulation
Instead of instant on/off, gradually ramp lights up over 15-30 minutes to simulate natural sunrise. This reduces plant stress and allows stomata to open gradually. Some LED controllers and smart dimmers support this natively.
How to implement: Use a dimmable LED driver with a smart dimmer, or chain multiple light bars to separate timers with staggered start times.
Temperature-Based Scheduling
Run lights during cooler nighttime hours to reduce HVAC costs. In hot climates, a "lights out" period during peak afternoon heat (2-6 PM) can save significantly on cooling.
Example schedule: Lights on 6 PM - 12 PM (18 hours), lights off during hottest 6-hour window.
Utility Peak Avoidance
Many utility companies charge higher rates during peak hours (typically 2-7 PM). Schedule your dark period during peak pricing to reduce electricity costs by 20-40%.
Check: Contact your utility or check your bill for "time of use" rate schedules.
Staggered Lighting Zones
For large grows, start lights in different zones 15-30 minutes apart. This prevents massive power spikes that can trip breakers and reduces stress on your electrical system.
Example: Zone 1 at 6:00 AM, Zone 2 at 6:15 AM, Zone 3 at 6:30 AM.
Mid-Day Siesta (Gas Lantern Routine)
Some growers use a "gas lantern routine" (GLR) with a mid-day dark period: 12 hours on / 5.5 hours off / 1 hour on / 5.5 hours off. This can reduce power usage while maintaining vegetative growth, though results are debated.
Note: Not recommended for beginners - stick with proven 18/6 or 12/12 schedules first.
7. Common Timer Mistakes to Avoid
Light Leaks During Dark Period
Even small amounts of light during flowering can cause plants to revert to vegetative growth or develop hermaphrodite traits. Seal all light leaks with tape, use light-proof ducting, and turn off indicator LEDs on equipment.
Inconsistent Scheduling
Manually switching lights leads to inconsistency. Even a few hours of variation can stress plants and reduce yields. Always use a timer - the $12 investment pays for itself many times over.
Overloading Timer Capacity
Running a 1500W load on a 10A timer is a fire hazard. Always check the amp rating (divide watts by voltage: 1500W / 120V = 12.5A) and leave 20% headroom.
Ignoring Daylight Savings Time
Timer schedules don't auto-adjust for DST. A 1-hour shift can interrupt flowering and cause issues. Update timers manually after each DST change, or use smart plugs that adjust automatically.
No Backup Power Plan
Power outages can reset cheap mechanical timers. Use digital timers with battery backup, or note your schedule so you can reprogram quickly after outages.
Quick Reference: Optimal Timer Settings
Vegetative Growth
18/6 schedule (18 hours light, 6 hours dark)
Flowering (Photoperiod)
12/12 schedule (strict, no light leaks)
Seedlings & Clones
18-24 hours (lower intensity)
Autoflowers
18/6 or 20/4 (entire lifecycle)
Next step: Use our DLI Calculator to dial in the perfect combination of light intensity and duration for your specific plants.
Related Articles
Understanding DLI for Plants
Learn how Daily Light Integral affects plant growth and how to calculate optimal levels.
Grow Light Distance & Height Guide
Find the optimal hanging height for your grow lights at each growth stage.
LED vs HPS Grow Lights
Compare efficiency, heat output, and running costs of different light technologies.
Best Grow Lights for 4x4 Tents
Top LED recommendations for the most popular tent size, with PPFD maps.
Frequently Asked Questions
What is the best grow light schedule for seedlings?
Seedlings do best with 16-18 hours of light per day. This mimics long spring days and promotes healthy growth without stressing young plants. Run lights from 6 AM to midnight, or use a timer for consistent 18/6 scheduling.
Is 24 hours of light better than 18/6?
For most plants, 18/6 (18 hours on, 6 hours off) is better than 24/0 continuous light. Plants use the dark period for important metabolic processes. 24/0 can work for seedlings and clones short-term, but extended use may cause stress and actually slow growth.
How do I trigger flowering with grow lights?
To trigger flowering in photoperiod plants, switch to a 12/12 light schedule (12 hours on, 12 hours off). This mimics fall conditions and signals the plant to transition from vegetative growth to flowering. Maintain complete darkness during the off period.
What timer should I use for grow lights?
Digital timers with 7-day programming are ideal for grow lights. Top picks include the Century 7-Day Digital Timer ($12), BN-LINK 7-Day Heavy Duty Timer ($15), and smart plugs like Kasa or Wemo for remote control. For high-wattage lights (1000W+), use heavy-duty timers rated for your amperage.
Can inconsistent light schedules hurt my plants?
Yes, inconsistent light schedules can stress plants significantly. Irregular timing can cause hermaphroditism in flowering plants, stunted growth, and even revert flowering plants back to vegetative state. Always use a timer rather than manual switching for consistent results.