Light Schedule Generator
Generate a full grow-cycle light schedule with real calendar dates — flip date, transition stretch window, pre-harvest flush start, and harvest window. Enter your start date, lights-on time, and grow plan to get timer programming times for every phase plus an .ics calendar file to import all milestones at once. Covers photoperiod and autoflower.
Build your grow light schedule
Most growers set lights-on at 6 AM or midnight. Lights on during the day = cheaper off-peak electricity in some areas. Lights on at night = cooler room temps during lights-on in hot climates.
Timer settings
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Set start date and lights-on time to see timer settings.
Common light schedule mistakes
- Light leaks during the dark period. Any light exposure during the 12-hour dark period disrupts the photoperiod response. Check your space for cracks, indicator lights, and timer LEDs. Even brief ambient light from opening the door during dark period is enough to cause re-vegging or hermaphroditism in sensitive strains.
- Not accounting for transition stretch. Plants can double in height in the first 2–3 weeks after flipping to 12/12. If you flip with plants at 60% of your light-to-ceiling clearance, you may end up with canopy touching the light by week 3. Flip when plants are at 40–50% of available height.
- Timer drift after power outages. Mechanical timers and some digital timers lose their schedule after power outages. Verify timer schedule after any power interruption — especially critical during flower when the 12-hour dark period must be consistent.
- Running autoflowers on 12/12. Autoflowers don't need 12/12 — they flower based on age. Running them on 12/12 gives them 6 fewer hours of light per day than 18/6, which directly reduces yield. Autoflowers always benefit from more light hours, not fewer.
Flip to 12/12
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Harvest window
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Phase timeline
Key milestones
Calendar export
Download all milestones as a .ics file for Google Calendar, Apple Calendar, or Outlook.
Set up a light schedule that holds all grow
Choose photoperiod or autoflower
Photoperiod plants flower only when the light schedule drops to 12/12 — everything else keeps them in veg. Autoflowers flower based on age regardless of light schedule; they never need a flip. For photoperiods, you'll get three phases (seedling, veg, flower) with a specific flip date. For autoflowers, you get one phase at your chosen schedule throughout, with an estimated auto-flower-start milestone.
Set start date and lights-on time
Start date is the day seeds sprout or clones are transplanted from dome. If starting from clone, set seedling weeks to 0. Lights-on time determines when the timer turns on each day — all other times (lights-off per phase) calculate from this single setting. Most growers use 6 AM (lights on during the day) or midnight (lights on at night, cooler tent temperatures during the active grow period in hot climates).
Program your timer from the ON/OFF times shown
Each phase card shows the lights-on and lights-off times for that phase. For a two-timer setup (one for veg, one for flower): program your timer with the veg OFF time now, and update it to the flower OFF time on the flip date. The Timer Settings box in the left column shows the settings at a glance. For single-timer smart controllers, program both phases in advance and set the switch-over to the flip date.
Download the .ics file in Pro mode to import all milestones
Enable Pro mode and download the .ics calendar file to import flip date, transition stretch-end, pre-flush start, and harvest window directly into Google Calendar, Apple Calendar, or Outlook. All milestones appear as all-day calendar events on their actual dates. This is the fastest way to make sure flip day doesn't sneak up on you. The Training Timeline Planner generates complementary training milestones on the same timeline.
Why midnight lights-on makes sense in hot climates
Lights generate heat. If your grow space runs warm, having lights on during the hottest part of the day (noon to 6 PM) compounds the cooling challenge. Setting lights-on at midnight means lights-off at noon — the tent's hottest period is during dark, when the plant isn't actively growing and the room is being cooled passively. The grow room runs coolest during the 6 AM–noon window, which is when lights are on and plants are actively transpiring. This is why many experienced growers run "lights on at night" — not aesthetics, but temperature management.
Phytochrome, photoperiodism, and why dark period interruption matters more than light duration
Cannabis photoperiodism is driven by a pigment system, not a clock. Understanding how it works explains why a single night of light exposure undoes weeks of 12/12, why autoflowers are immune to this, and why 18/6 and 20/4 both work for veg. See the Environment & Climate Guide for more on light and plant responses.
12h
Uninterrupted dark triggers flowering
Cannabis flowers when it detects 12+ consecutive hours of darkness. The light period length matters less than the dark period length — this is why "12/12" is really "12 hours of uninterrupted dark."
50–100%
Transition stretch range
Plants stretch in the first 2–3 weeks after flip. A plant at 24" at flip may reach 36–48" by week 3. Account for this when deciding when to flip relative to your light height.
Pfr → Pr
The phytochrome switch
During darkness, the active form (Pfr) converts to the inactive form (Pr). After 12+ hours of darkness, Pr levels are high enough to trigger the flowering signal. Any light resets the conversion.
Why dark period interruption matters more than light hours
Phytochrome B (PhyB) exists in two interconvertible forms: Pfr (active, produced by red light) and Pr (inactive, accumulates in darkness). During the light period, red wavelengths from your grow light continuously convert Pr to Pfr. During the dark period, Pfr slowly reverts to Pr without light driving the conversion. After approximately 10–12 consecutive hours of darkness, Pr accumulation crosses a threshold that triggers the flowering hormone cascade.
This is why a single light interruption during the dark period — even 2–3 minutes of dim red light — can reset the process. The light converts accumulated Pr back to Pfr, and the plant has to start the dark-period accumulation over. Done repeatedly, light leaks prevent flowering or cause hermaphroditism. For this reason, "12/12" is more accurately described as "12 hours of completely uninterrupted darkness" rather than "12 hours of dark."
Why autoflowers are immune to photoperiod manipulation
Autoflowering cannabis (Cannabis ruderalis genetics) carries a mutation in the phytochrome signaling pathway that decouples flowering from the photoperiod response. The flowering trigger in autoflowers is age-based rather than light-period-based — an internal hormone timer triggers the transition regardless of dark period length. Running autoflowers on 12/12 doesn't prevent flowering, but it does reduce yield: autoflowers produce more under 18/6 or 20/4 simply because they receive more total light energy per day for photosynthesis.
| Schedule | Daily light (h) | Best for | Notes |
|---|---|---|---|
| 16/8 | 16 | Seedlings, clones | Enough for healthy seedling growth without stressing young plants |
| 18/6 | 18 | Veg (standard), auto full cycle | Most common veg schedule. Good balance of growth rate and electricity cost |
| 20/4 | 20 | Veg (aggressive), auto for max yield | ~11% more light than 18/6. Faster veg growth but higher electricity cost. Diminishing returns above 20h for most strains |
| 12/12 | 12 | Photoperiod flower | Triggers and maintains flowering. Dark period must be uninterrupted. Do not run autoflowers on 12/12 |
Common light schedule questions
When should I flip to 12/12?
When plants are at 40–50% of your available vertical space. Photoperiod strains stretch 50–100% after flipping — a plant at 24" at flip may reach 36–48" by mid-flower. If you flip when plants are at 60–70% of your height, you'll be light-burning the tops before week 3 of flower. The calendar date matters less than canopy height relative to your setup. The tool's flip date is an estimate based on your veg weeks — verify against actual canopy height before flipping.
What happens if I miss a flip day?
Nothing permanent — flip the next day. One extra day of 18/6 doesn't hurt anything; plants are still in veg. Missing a flip day just shifts your harvest date by one day. What does matter: once you've flipped to 12/12, you can't go back to 18/6 without triggering a re-veg (which takes 2–4 weeks and wastes significant time). Flip when you're ready, not on a specific calendar date if the plants aren't where you want them.
Can I run autoflowers and photoperiods in the same room?
Only if the room stays on 18/6 throughout. Autoflowers run on 18/6 all grow. If you also have photoperiods, you can't flip to 12/12 without halting the autoflower's optimal light intake. Running both in one room means the photoperiods never get to flip — they stay in veg indefinitely. Separate spaces, or run one type at a time. The only exception: autoflowers finishing their cycle while photoperiods are already in 12/12 flower — this works if the auto was started early enough to be near harvest when the room flipped.
How do I check for light leaks?
During your scheduled lights-off period, sit inside the grow space with the door closed for 5 minutes and let your eyes fully adjust to darkness. Any light source — no matter how faint — is a potential flowering problem. Common culprits: timer LED indicators (cover with electrical tape), digital thermometer/hygrometer displays, fan LED rings, power strip indicator lights, gaps at the tent zipper or base, and light from nearby rooms coming under doors. In flower, err on the side of over-sealing. An inline fan power indicator covered with tape costs nothing; losing a crop to hermaphroditism costs weeks.
Should I run 24/0 for autoflowers?
Debated. Some growers report no yield difference between 20/4 and 24/0; others report slight stress symptoms from continuous light (leaf curl, slower growth in some strains). Most evidence points to autoflowers benefiting from at least a minimal dark period (4–6 hours) for root development and metabolic rest. 18/6 is the most broadly supported schedule. 20/4 provides marginally faster growth at higher electricity cost. 24/0 is probably not worth the risk of stress for most strains. The marginal yield gain from the extra 4 hours of light in 24/0 vs 20/4 is small; the potential stress effect is real for some genetics.
Timers and light controllers
A reliable timer is the single most important piece of equipment for maintaining a consistent light schedule. The light itself can be excellent — but if the timer drifts or fails after a power outage, you're risking re-veg or hermaphroditism at the worst possible moment.
Digital timers
Digital outlet timers ($15–30), battery-backup digital timers ($20–40), 7-day programmable timers ($25–50). Digital timers hold their program after power outages (battery backup models). Mechanical timers are cheap but lose their setting on power loss and drift over time. For flower, use a digital timer with battery backup — it's $10–15 more than a mechanical and eliminates the single most common light-schedule failure mode.
Smart light controllers
Smart outlet controllers ($20–45), AC Infinity CLOUDLINE controller ($60–120), multi-channel environment controllers ($80–200). Smart controllers let you pre-program schedule changes (like the veg-to-flower flip) weeks in advance and manage everything from an app. Eliminates the need to manually update timer settings on flip day. Some controllers also adjust light schedule automatically based on temperature readings.
LED grow lights
Full-spectrum LED panels ($150–600+). LED lights with onboard dimmer and scheduling features let you ramp light intensity up through veg and into flower without changing fixtures. Some high-end LEDs integrate with smart controllers to automatically dim during the last hours of the light period, simulating natural light cycles. Use the PPFD/DLI Calculator to verify intensity at your specific hang height.
Data loggers
Temperature/humidity data loggers ($25–60), Wi-Fi remote monitors ($30–80). Data loggers record environment over time — including during dark periods when you're not watching. If your timer is drifting or misfiring, a data logger showing unusual temperature spikes (lights on at wrong time = heat spike) will catch it before it damages plants. Worth having in flower especially.
Seeds & genetics
Feminized photoperiod seeds, autoflower seeds, clones. The light schedule you build is specific to your genetics — an 8-week flower schedule is right for most indica/hybrid strains but too short for sativa-dominant strains that may need 10–12 weeks. Know your strain's expected flower time before setting the schedule, or add buffer weeks and harvest by trichomes rather than calendar.
Light-leak prevention
Black electrical tape ($3–6), tent light-proof patches ($5–12), blackout fabric ($8–20/yd). Covering indicator lights and sealing tent gaps costs almost nothing and eliminates one of the most common flower problems. Do a light-leak check at the start of every flower phase — not just once when you set up. Tents develop new gaps as zippers wear.