Every light cycle from seedling to harvest: when to switch, how to prevent light leaks, sunrise simulation, dark period rules, and hermaphrodite prevention.
Light is the single most powerful environmental variable in cannabis cultivation. More than nutrients, temperature, or any other input, light duration directly controls when your cannabis plant transitions from vegetative growth into flowering. Understanding and managing light schedules is the difference between a perpetual vegetative plant and a heavy-yielding harvest on schedule.
Before diving into specific schedules, understanding the distinction between photoperiod and autoflowering genetics is essential because they respond to light fundamentally differently.
| Property | Photoperiod Cannabis | Autoflowering Cannabis |
|---|---|---|
| Flowering trigger | 12+ consecutive dark hours | Age (typically 2–4 weeks) |
| Veg duration control | Grower controlled (18/6 kept indefinitely) | Fixed by genetics |
| Light schedule flexibility | Must switch to 12/12 for flower | Any schedule works (18/6 optimal) |
| Sensitivity to light leaks | Very high | Very low |
| Yield potential | Very high (can veg indefinitely) | Medium (fast but fixed size) |
Cannabis seedlings are in their most delicate phase of development. The cotyledons (seed leaves) and first true leaves are still small, and the root system is not yet established. The light schedule goal during this phase is to provide adequate light for photosynthesis without causing heat or light stress to fragile young tissue.
Most growers run 18 hours of light and 6 hours of darkness from germination through the seedling stage. This is consistent with veg-stage settings and avoids the need to switch timers. The 6-hour dark period is generally considered beneficial for seedlings, giving them a rest period for cellular processes.
Some growers use 24 hours of continuous light during the seedling stage, arguing that maximum photosynthesis accelerates early development. This approach is more controversial — some strains show slower development under 24/0 due to lack of dark-cycle metabolic rest. For most growers, 18/6 is the safer and equally effective choice.
During the vegetative phase, cannabis grows leaves, branches, and root mass in preparation for flowering. The goal is maximum canopy development within your available grow space and timeline. Photoperiod plants will remain in vegetative growth indefinitely as long as the light period exceeds roughly 16–18 hours.
18 hours light / 6 hours dark is the industry standard for photoperiod vegetative growth. It provides 75% of the day in light — more than enough for vigorous growth — while maintaining a consistent dark period. The 6-hour dark period allows temperature drops, carbon dioxide uptake cycles, and general metabolic recovery.
Autoflowering strains thrive on 20 hours of light with 4 hours of darkness. Because autos are not triggered by photoperiod, longer light periods simply equal more photosynthesis and faster growth. The 4-hour dark period is maintained to allow basic physiological rest processes. Running 20/4 with autoflowers typically produces 10–15% more growth compared to 18/6.
Running lights 24 hours with no dark period is used by some growers for fast veg growth. However, several issues emerge: electricity consumption increases significantly, plants may develop calcium deficiencies (calcium mobility is linked to transpiration during dark periods), and some strains (particularly indica-dominant varieties) show reduced vigour without a dark period. Not recommended for most growers.
Photoperiod cannabis plants flower when they experience 12 consecutive hours of uninterrupted darkness per 24-hour cycle. In nature, this occurs as days shorten in late summer. Indoors, the grower controls this by switching the timer from the vegetative schedule (18/6) to 12 hours light and 12 hours dark.
Light leaks are one of the most underestimated problems in indoor cannabis cultivation. A single unplanned light source penetrating the grow room during the dark period can disrupt the plant’s photoperiodic response, causing re-vegging, delayed flowering, or hermaphroditism.
The standard method for checking a grow room for light leaks is the "midnight inspection": enter your dark grow room 30–60 minutes after lights-off and allow your eyes to fully adjust. Stand in multiple corners of the room and look for any visible light sources. Even a pinhole leak is visible after proper dark adaptation. Use black gaffer tape or Panda film to seal any detected leaks.
High-end LED grow lights and some commercial controllers offer sunrise and sunset simulation — a gradual ramp-up of light intensity at the start of the photoperiod and a gradual fade at the end. While not essential for successful cultivation, sunrise/sunset simulation offers several potential benefits:
The dark period is not simply "lights off." It is a biologically active phase during which cannabis plants undergo critical physiological processes. Understanding why the dark period matters helps growers treat it with appropriate respect.
Cannabis is a dioecious plant — individual plants are typically either male or female. However, under environmental stress, female plants can develop male characteristics (pollen sacs) — a condition called hermaphroditism or herming. Light stress is the number one environmental cause of hermaphroditism in indoor grows.
| Growth Stage | Photoperiod Schedule | Autoflower Schedule | Duration | Notes |
|---|---|---|---|---|
| Germination | 18/6 or 24/0 | 18/6 or 20/4 | 2–5 days | Low intensity, warm |
| Seedling | 18/6 | 18/6 or 20/4 | 1–2 weeks | Low intensity, avoid heat |
| Vegetative | 18/6 | 18/6 or 20/4 | 4–8 weeks | Full intensity, train plants |
| Flowering (photoperiod) | 12/12 | 18/6 or 20/4 | 8–12 weeks | Zero light leaks, high intensity |
| Late Flower / Pre-Harvest | 12/12 | 18/6 | 1–2 weeks | Some growers reduce to 10/14 final week |
The most widely used light schedule for cannabis in veg is 18 hours of light and 6 hours of darkness (18/6). This provides sufficient light energy for fast vegetative growth while allowing a short dark period that benefits metabolic recovery. Running 24/0 continuous light is viable but can stress certain photoperiod strains without proportional growth benefits.
Photoperiod cannabis plants flower when the dark period reaches 12 consecutive hours. Switching your timer from 18/6 to 12/12 is the standard method to trigger flowering indoors. The dark period must be completely uninterrupted — even a brief light leak during the dark period can delay flowering and cause hermaphroditism.
Light interruption during the dark period is one of the most common causes of hermaphroditism in cannabis. Even a few minutes of light exposure from a phone screen, timer malfunction, or door gap can stress photoperiod plants significantly. Always inspect grow rooms for light leaks before beginning the flowering cycle.
Autoflowering cannabis flowers based on age rather than light cycle, so it does not require 12/12 to trigger flowering. Most autoflower growers run 18/6 or 20/4 from seed to harvest. The 18/6 schedule is widely recommended as it balances growth speed, energy efficiency, and plant health.