For photoperiod cannabis grown indoors, the baseline schedule is 18 hours of light and 6 hours of dark during vegetative growth, then 12 hours of light and 12 hours of dark to trigger flowering. Autoflowering plants flower by age rather than by photoperiod, so most growers keep them on 18/6 or 20/4 from seed to harvest. The single most useful purchase for either path is a 24-hour timer that holds the cycle without drift. Consistency matters more than optimization. A stable schedule keeps plants in the intended phase, while random changes cause stress and, in photoperiod plants, can trigger hermaphroditism.
cannabis grow light schedule for indoor
What to look for in a light schedule
- Phase length: photoperiod plants need a dark period long enough to trigger and sustain flowering. Twelve hours is the common standard; shorter dark periods can delay or prevent flowering.
- Uninterrupted dark: a dark period that is truly dark is a buying requirement for tents, zippers, and ducting. Any light leak during the dark phase can stall flowering or cause revegetation.
- Timer reliability: digital timers hold a cycle within seconds, while cheap mechanical models can drift with brownouts or physical bumps.
- Heat and airflow: running 24 hours of light raises leaf temperature and can stretch internodes. Match the light schedule to the room's ability to remove heat.
- Stage flexibility: you may want 20/4 for rapid vegetative growth, 18/6 for lower power use, and 12/12 for flowering. A timer that supports multiple on and off periods helps.
Parameter bands by stage
- Seedling and clone: 18/6 to 24/0 for photoperiod plants; 18/6 to 20/4 for autoflowers. Lower intensity matters more than a long photoperiod here.
- Vegetative photoperiod: 18/6 is the default. Some growers use 20/4 or 24/0, but 18/6 keeps electricity use and heat lower with little yield penalty.
- Flowering photoperiod: 12/12 is the standard. A few cultivators try 11/13 to finish faster, but that can reduce yield.
- Autoflower: 18/6 to 24/0 throughout the life cycle. Most home growers settle on 20/4 as a balance of growth and cost.
Pitfalls to avoid
- Light leaks during the dark period. A pin hole in a tent seam can be enough to disrupt flowering.
- Switching to 12/12 before plants are mature. Immature plants may flower poorly or take longer to form buds.
- Assuming 24/0 is always better. Dark periods support respiration and root health; continuous light can stress some cultivars.
- Using a timer that loses its schedule after a power outage. Choose one with a battery backup if outages are common.
- Changing the schedule mid-flower. Once flowering starts, keep the cycle steady until harvest.
FAQ
Can I run photoperiod plants at 24/0?
No. Photoperiod cannabis needs a long dark period to flower, and continuous light can cause stress, leaf discoloration, and poor bud development. Use 18/6 or 20/4 for vegetative growth.
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Do autoflowers need 12/12?
No. Autoflowers flower based on age. They perform well under 18/6, 20/4, or 24/0. The 12/12 schedule is not required and can reduce total light exposure.
Is 13/11 close enough to 12/12?
Some cultivars will flower under 13 hours of light, but the response is inconsistent. A true 12/12 schedule is the safer choice for reliable flowering and better yields.
Cheap Cannabis Grow Lights That Work: Buying Guide
What timer should I buy?
A digital 24-hour timer with battery backup and at least one programmable on and off pair is enough for most indoor grows. Look for a model rated for the wattage of your light.