The seed-to-harvest cycle of photoperiod-sensitive plants, such as cannabis, is regulated by the amount of light and darkness they receive daily. This biological response, known as photoperiodism, allows the plant to coordinate its development with the changing seasons. By manipulating light duration indoors, growers precisely control the plant’s growth phase. Different stages require distinct light schedules to encourage specific outcomes, moving from building a robust structure to producing flowers.
Initial Stages: Germination and Seedling Development
The life cycle begins with germination, followed by the seedling stage, which establishes a healthy root system and the first true leaves. During this initial phase, the young plant requires lower intensity light to prevent damage to its delicate tissues. While light is not required for the seed to break dormancy, it is needed immediately after sprouting to initiate photosynthesis and growth.
For the first few weeks, growers expose seedlings to long periods of light, often using schedules like 18 hours of light followed by 6 hours of darkness (18/6) or continuous light (24/0). These extended light periods encourage rapid initial development and biomass accumulation. This quickly establishes a strong foundation before the plant enters the next phase of growth.
The Vegetative Phase Light Schedule
Once the plant establishes several sets of true leaves, it enters the vegetative phase, dedicated to maximizing size, leaf structure, and stem strength. During this stage, the plant requires a long-day light schedule to suppress hormonal signals that trigger flowering. The standard indoor light cycle for this phase is 18 hours of light and 6 hours of uninterrupted darkness (18/6).
The extended light duration mimics the long days of summer, signaling that conditions are favorable for growth. This schedule maximizes photosynthesis time, allowing the plant to rapidly increase its size and develop a dense canopy. As long as the period of darkness remains short (less than 12 hours), the plant remains in the vegetative state, focusing on building structural support for future flowers.
The 12/12 Light Cycle: Triggering the Flowering Phase
The stage of the seed-to-harvest cycle that uses a 12 hours of light and 12 hours of darkness (12/12) schedule is the Flowering Phase. This schedule is a deliberate change made by the grower to simulate the shortening days of late summer and autumn, signaling the plant to begin reproduction. The plant is a “short-day” plant, initiating flowering when the nights become longer than the 12-hour threshold.
The 12 hours of complete, uninterrupted darkness triggers the hormonal shift that initiates flower production. During the dark period, a photosensitive protein called phytochrome senses the prolonged absence of light and degrades into an inactive form. When this inactive form reaches a high enough concentration—after about 12 continuous hours of darkness—it activates the hormonal cascade, causing the plant to transition from vegetative to reproductive growth.
This transition phase is often accompanied by a rapid increase in vertical growth, known as the “flowering stretch,” as the plant develops its reproductive structures. The 12/12 cycle must be maintained strictly throughout the entire flowering period, which typically lasts 8 to 12 weeks depending on the plant’s genetics. Interrupting the 12 hours of darkness, even briefly, can stress the plant and cause it to revert to the vegetative state or develop undesirable traits.
Recognizing Readiness and Post-Harvest Procedures
The 12/12 light cycle continues until the plant reaches peak maturity, judged by visual cues on the forming flowers. The most reliable indicator is the appearance of the trichomes, the tiny, resinous glands covering the buds and surrounding leaves. These structures contain the plant’s active compounds, such as cannabinoids and terpenes.
Growers use a magnifying tool to observe the trichome heads, which progress from clear to cloudy (milky white) and finally to amber. Harvesting when the majority of trichomes are cloudy and a small percentage are amber indicates the optimal point for maximum potency. Another common indicator is the color of the pistils, the hair-like structures on the buds that start white and turn brown or orange as the plant matures.
Once harvested, the plant material undergoes essential post-harvest procedures to ensure a high-quality final product. The first step is drying, which slowly removes excess moisture from the flowers, reducing mold risk and preparing them for storage or curing. This is typically done in a controlled environment with temperatures between 60–70°F and a relative humidity of 50–60%.
Following drying, curing begins, where the dried flowers are sealed in airtight containers and “burped” periodically to exchange air and release moisture. Curing is a controlled aging process that allows the breakdown of residual chlorophyll and enhances the terpene profile. This results in a smoother, more flavorful, and aromatic final product, refining the quality before consumption.