A cannabis light schedule splits the 24 hours into light and darkness: 18/6 (18 hours of light, 6 of darkness) for seedlings and the vegetative stage, and 12/12 to trigger and sustain flowering in photoperiod strains. Autoflowers flower according to age and stay on 18/6 or 20/4 for the whole cycle. During flowering, the 12 hours of darkness must be continuous and complete.
In brief
- Seedlings, cuttings and vegetative stage: 18/6; 20/4 is an alternative, while 24/0 costs more with no clear benefit.
- Flowering in photoperiod strains: 12/12 in total darkness; the first pistils appear after 1–2 weeks.
- Autoflowers: a constant 18/6 or 20/4 from seed to harvest.
- A reliable timer is essential; powerful lights need a timer or controller with a suitable relay.
- Outdoors in Ticino, flowering starts by itself between late July and mid-August as the days shorten.
How many hours of light for each stage
| Stage | Light/dark | Typical duration | Intensity (PPFD) |
|---|---|---|---|
| Seedlings | 18/6 | 2–3 weeks | 100–300 µmol/m²/s |
| Cuttings rooting | 18/6 | 7–14 days | 80–200 µmol/m²/s |
| Vegetative stage | 18/6 or 20/4 | 3–6 weeks (5–8 with SCROG) | 300–600 µmol/m²/s |
| Flowering (photoperiod) | 12/12 | 8–10 weeks | 600–900 µmol/m²/s |
| Autoflowers | 18/6 or 20/4 | 8–12 weeks in total | rising from 200 to 700 µmol/m²/s |
Hours and intensity go together: the daily light integral (DLI) is PPFD × hours × 0.0036. At 18/6 with 450 µmol/m²/s you get 29 mol/m²/day; at 12/12 you need about 800 µmol/m²/s to reach 35 mol/m²/day. That is why the dimmer goes up in flowering as the hours go down. Choosing the light itself is covered in the guide to the best LED grow light.
Running costs change with the hours too. With a 200 W LED and 0.30 CHF/kWh: 18/6 = 0.2 kW × 18 h × 7 days = 25.2 kWh, i.e. 7.56 CHF a week; 20/4 = 28 kWh, 8.40 CHF; 24/0 = 33.6 kWh, 10.08 CHF. 24/0 costs a third more than 18/6 and takes away the plant's dark hours.
Switching photoperiod plants to 12/12
Photoperiod strains stay in the vegetative stage while nights are short. In nature many start flowering once daylight drops below about 14–15 hours; indoors you switch to 12 hours of uninterrupted darkness (12/12), which reliably triggers flowering in every strain. You decide when that happens.
- Switch to 12/12 when the plant has reached about half the final height your tent allows: in the first 2–3 weeks of flowering it keeps stretching, often by 50–100% (up to 1.5 to 2 times its height at the switch).
- Change the timer in one go, from 18/6 to 12/12: there is no need for a gradual transition.
- Before switching, make sure the tent is light-tight (see below).
- Wait for the first white pistils at the nodes: they usually appear after 7–14 days.
What happens after the switch, week by week, is covered in the guide to flowering week by week.
Autoflowers: a constant cycle
Autoflowers move into flowering on their own about 3–5 weeks after germination, whatever the light schedule. Keep them on 18/6 or 20/4 from start to finish: on 12/12 they get less light and yield less, without flowering any sooner. If autoflowers and flowering photoperiod plants share one tent, 12/12 is the compromise, at the autoflowers' expense. The differences are explained in the guide to autoflowering cannabis.
Total darkness in flowering: light leaks
During the 12 dark hours the plant measures the length of the night. A single brief opening of the tent rarely does harm, but repeated or long light leaks can delay flowering, send the plant back into vegetative growth or encourage male flowers on plants that are already stressed: how to spot them and what to do is explained in the guide to hermaphrodite cannabis.
- With the light off, go into the dark room, close the tent from the inside or put your face close to the seams and wait 5 minutes for your eyes to adjust.
- Look for bright spots: zips, seams, cable ports, the sleeves around the air ducts.
- Cover the indicator lights on timers, controllers, humidifiers and power strips inside the tent with black tape as well.
- If you must work in the dark, use a dim green torch and only for a few minutes.
Mechanical and digital timers, controllers
- Mechanical timer: cheap, 15-minute steps; over time it can drift by a few minutes and it stops during a power cut.
- Digital timer: accurate to the minute, with a battery that keeps the time during a power cut; still check the clock after an outage.
- Controller: runs the light, extractor fan and sometimes the dimmer, gradual sunrise and sunset and a maximum temperature from one unit.
Check the timer's maximum load in watts or amps. LED drivers draw an inrush current when they switch on: with several lights or high wattages, use a model with a suitable relay or a controller, and do not plug everything into one cheap power strip. Timers, sockets and controllers are in the electrical and controllers sections.
Shifting the schedule without stress
Running the light at night can pay off: in summer the tent stays cooler and, if your supplier has a night tariff (check your bill), electricity is cheaper.
- In the vegetative stage: shift the schedule in one day by lengthening the light or the dark period once by as many hours as needed.
- In flowering: lengthen a single night only, for example from 12 to 16 hours of darkness, then resume 12/12 at the new time. Never extend the light period beyond 12 hours.
The natural light cycle outdoors in Ticino
Outdoors the light cycle changes every day. In Locarno and Lugano there are about 15 h 45 min between sunrise and sunset at the solstice on 21 June, 14 h 50 min on 1 August, 13 h 20 min on 1 September and 12 h around 25 September, a few days after the equinox. In the valleys the mountains cut the hours of direct sun. Photoperiod strains usually begin flowering between late July and mid-August, once daylight drops below about 14–15 hours, depending on the variety. Street lamps or garden lights left on at night can disturb flowering: choose a spot that is truly dark.
Frequently asked questions
How many hours of light during flowering?
Photoperiod strains need 12 hours of light and 12 hours of continuous darkness. Autoflowers stay on 18/6 or 20/4 even in flowering.
What happens if the dark period is interrupted?
A brief, one-off interruption usually does no harm. If it happens repeatedly it can delay flowering, send the plant back into veg or encourage male flowers: restore the darkness and find the leak.
Can you grow 12/12 from seed?
Yes: a photoperiod plant starts flowering as soon as it is mature, after about 3–5 weeks. It stays small and yields less, but it is a solution for low tents.
What light schedule for seedlings?
18/6 with gentle light, about 100–300 µmol/m²/s. 24/0 brings no benefit and uses a third more electricity.
Dimmable lights for every stage are available in the LED section at Gro.w.LaB.
Updated on 13 October 2026.