The light cycle plays a critical role in cannabis cultivation, directly influencing plant growth, development, and flowering. Cannabis is a photoperiod-sensitive plant, meaning its biological processes are controlled by the duration of light and darkness it receives. Managing the light cycle correctly is essential for successful cultivation, whether growing indoors or outdoors.
Light is the primary energy source for plants, driving photosynthesis, where light energy is converted into chemical energy to produce glucose and oxygen. This process is essential for growth, and the plant uses this energy for development.
| Germination | Light Cycle | 18–24 hours of light | |
| Purpose | Light is less critical; however, a consistent light source helps seedlings orient upward. | ||
| Seedling | Light Cycle | 18–24 hours of light | |
| Purpose | Extended light promotes rapid root development and healthy leaf growth. | ||
| Vegetation | Light Cycle | 18/6 (18 hours of light and 6 darkness) | |
| Purpose | Extended light boosts photosynthesis, encouraging strong vegetative growth. | ||
| ALTERNATIVES | 20/4 or 24/0, can be used, but may increase stress on the plants. | ||
| Flowering | Photoperiod Strains | ||
| Light Cycle | 12/12 (2 hours of light and 12 hours of uninterrupted darkness) | ||
| Purpose | Simulates late summer/fall conditions, triggering hormonal changes that induce flowering. | ||
| Precautions | Any interruption during the dark cycle can cause plants to revert to vegetative growth or become hermaphroditic. | ||
| Autoflower Strains | |||
| Light Cycle | 18/6, 20/4 or 24/0 | ||
| Purpose | Autoflowering strains flower based on age, not light cycle, and benefit from longer light exposure for better yields. | ||
| Causes | Light leaks in indoor grow | Small light leaks during the dark cycle can disrupt flowering. |
| Environmental factors in outdoor grows | Streetlights or security lights can extend the perceived daylight hours, disturbing the plants’ internal clock. | |
| Human error | Mistimed light cycle adjustments or manually turning lights on during dark periods. | |
| Effects | Revegetation | Plants return to vegetative growth, delaying flowering and reducing yields. |
| Hermaphroditism | Plants may produce both male and female | |
| Stunted growth | Stress caused by irregular light cycles can result in smaller plants and lower yields. |
| 1. | Adjustments | During flowering | Some growers use an 11/13 cycle to enhance resin production for specific strains. |
| Pre-harvest | Reducing light to 10/14 or complete darkness for 24–48 hours before harvest can help boost cannabinoid and terpene levels. | ||
| Revegetation for cloning | After harvesting, switching back to 18/6 helps regenerate plants for cloning purposes. | ||
| 2. | Precautions | Use timers to ensure consistency and avoid manual disruptions. | |
| Key points | Consistency | Use reliable timers for precise control over the light schedule. | ||
| Light intensity (PPFD) | – Seedlings | 200–400 PPFD | ||
| – Vegetative | 400–600 PPFD | |||
| – Flowering | 600–900 PPFD | |||
| Light Spectrum | Full-spectrum LED lights mimic natural sunlight, making them ideal for all growth stages. | |||
| Key points | Choosing the right strain | Photoperiod strains require specific seasonal conditions, whereas autoflowering strains are more adaptable to varying light cycles. |
| Managing light pollution | Use barriers or select secluded planting areas to avoid interference from artificial lighting. | |
| Supplemental lighting | Extend light hours during the vegetative stage using outdoor grow lights. |
The FAQ recap for this section is included in the following chapter, where the two topics are addressed together as part of a single subject.
