Cannabis Light Schedule Guide for Indoor Growing

Light is the engine that drives cannabis growth. Indoors, you are the sun, and that means you need to understand exactly how much light your plants need at every stage.

Understanding Photoperiodism

Cannabis uses the length of the dark period to determine when to start flowering. Indoor growers control the light timer to mimic seasons. More light hours means vegetative growth. A reduction in light hours triggers flowering.

Autoflowering strains bypass this system and flower based on age.

Seedling Stage (0-2 Weeks)

Light schedule: 18 hours on, 6 hours off (18/6)

Seedlings need gentle, consistent light. Some growers run 20/4 or 24/0, but 18/6 works well and saves electricity. If using a powerful LED, keep it higher above the canopy or dim it to avoid light stress.

Vegetative Stage (2-8+ Weeks)

Light schedule: 18 hours on, 6 hours off (18/6)

This is when your plant builds its structure.

The vegetative phase length is entirely under your control with photoperiod plants. The plant will stay in veg as long as it gets more than 14 hours of light per day. During veg, plants respond well to high light intensity.

Transitioning to Flower

Switch from 18/6 to 12 hours on, 12 hours off (12/12)

This is the single most important light change. After the flip, plants typically spend 1 to 2 weeks in a "stretch" phase where they can double in height.

Plan for this stretch when deciding when to flip.

The 12 hours of darkness must be truly dark and uninterrupted. Even brief light leaks can cause the plant to revert to vegetative growth or develop hermaphrodite traits. Check your grow space carefully.

Flowering Stage (8-12 Weeks)

Light schedule: 12 hours on, 12 hours off (12/12)

Maintain 12/12 from the flip until harvest.

Do not change it. During early flowering (weeks 1-3), the plant stretches and forms bud sites. During mid-flowering (weeks 4-6), buds fill in. During late flowering (weeks 7 onward), buds ripen and trichomes develop.

Autoflower Light Schedules

Autoflowers do not require a light change. Common schedules:

18/6: Most popular. Plenty of light with a rest period.

20/4: Slightly more light. Marginal difference from 18/6.

24/0: Maximum growth potential but plants generally benefit from some dark period.

For a first autoflower grow, 18/6 from start to finish is simplest.

Common Light Schedule Mistakes

Light leaks during flowering. A tiny LED on a power strip, a zipper that does not fully close, or light leaking under a door can all disrupt the dark cycle.

Inconsistent timing. Use a reliable timer, not your memory.

Not matching intensity to stage. Seedlings need gentle light. Vegetative plants handle full intensity. Adjust height and dimmer settings as the plant progresses.

Keeping the same schedule hoping photoperiod plants will flower. They will veg indefinitely under 18/6. You must flip to 12/12 to trigger flowering.

Quick Reference

For photoperiod plants: 18/6 for seedling and veg, then switch to 12/12 for flowering.

For autoflower plants: 18/6 from seed to harvest.

Set it correctly, use a timer, check for light leaks, and your plants will respond with healthy growth and solid flower production.