Photoperiod

Photoperiod refers to the duration of light and darkness a plant receives in a 24-hour cycle. In cannabis cultivation, the photoperiod is the primary trigger that tells Cannabis Sativa and Cannabis Indica plants when to transition from vegetative growth to flowering. Plants that depend on this light cycle to flower are called photoperiod plants, in contrast to autoflower varieties which flower based on age.

How It Works

Cannabis is a short-day plant, meaning it flowers when the nights become long enough. In nature, this happens as summer transitions to autumn and daylight hours decrease. The plant detects the length of the uninterrupted dark period — not the light period — through photoreceptor proteins called phytochromes.

When the dark period reaches a critical length (roughly 12 hours), the plant receives the hormonal signal to begin producing flowers.

Light Schedules

Vegetative Phase

During vegetative growth, growers provide long days to keep the plant growing leaves and stems:

• 18/6 — 18 hours light, 6 hours dark (most common)
• 20/4 — 20 hours light, 4 hours dark
• 24/0 — Continuous light (debated; some growers prefer a rest period)

As long as the dark period stays below the critical threshold, the plant will continue vegetating. This means the grower controls how large the plant grows before triggering flower.

Flowering Phase

To initiate flowering, the light schedule is switched to:

• 12/12 — 12 hours light, 12 hours dark

This simulates the shorter days of autumn. Most plants will show signs of flowering within 1–2 weeks of the switch. The flowering phase typically lasts 8–12 weeks depending on the strain, with indica-leaning varieties finishing faster than sativa-dominant ones.

Light Leaks

Photoperiod plants are sensitive to interruptions in the dark period. Even brief exposure to light during the dark phase — known as a light leak — can:

• Delay or prevent flowering
• Cause the plant to revert to vegetative growth
• Stress the plant into producing hermaphrodite flowers (both male and female)

Common sources of light leaks include indicator LEDs, gaps in grow tent zippers, and light from adjacent rooms. Ensuring complete, uninterrupted darkness during the dark period is critical for photoperiod grows.

Photoperiod vs. Autoflower

| | Photoperiod | Autoflower

Outdoor Growing

Outdoors, the photoperiod is controlled by the seasons. In the Northern Hemisphere, most photoperiod cannabis strains begin flowering in late July to August as daylight hours decrease, and are harvested in September to October. The exact timing depends on latitude and strain.

Growers at extreme latitudes (far north or south) may face challenges — either too many daylight hours in summer to trigger flowering, or frost arriving before the plant finishes. This is one reason autoflower varieties are popular in northern climates.