Indoor cultivation of Cannabis requires providing the precise amount of light energy necessary to drive photosynthesis and optimize plant development. Light is the most important factor determining the final yield and overall quality of the harvest. Growers must manage two aspects of light delivery: the intensity (strength) and the duration (number of hours the light is active). Controlling these variables allows the grower to mimic natural seasonal changes, directing the plant’s growth from foliage to flower production.
Understanding Light Intensity Metrics
Serious indoor growers use specific scientific metrics to precisely measure and manage light intensity, moving beyond simpler measurements like lumens. The most common metric is Photosynthetic Photon Flux Density (PPFD), which measures the number of usable light photons hitting a square meter of canopy per second. These photons fall within the Photosynthetically Active Radiation (PAR) range, which plants use for growth. PPFD is expressed in micromoles per square meter per second (\(\mu\text{mol}/\text{m}^2/\text{s}\)) and serves as an instantaneous measure of light strength at the plant’s surface.
While PPFD measures momentary intensity, the Daily Light Integral (DLI) provides a more complete picture of the plant’s total light intake. DLI quantifies the total amount of light received over a full 24-hour period, measured in moles of light per square meter per day (\(\text{mol}/\text{m}^2/\text{day}\)). DLI combines light intensity (PPFD) and the duration of the light cycle. This metric is considered more informative than PPFD alone because it represents the cumulative energy the plant uses for growth each day. For example, a plant receiving light at a lower PPFD for 18 hours may receive the same DLI as a plant with a higher PPFD for only 12 hours, illustrating the relationship between intensity and duration.
Photoperiod Requirements by Growth Phase
The growth cycle of photoperiod-sensitive cannabis strains is directly controlled by the duration of light and darkness, a process called photoperiodism. This mechanism signals to the plant when to remain in the vegetative phase and when to transition into flowering. The plant requires a long period of continuous light to focus its energy on growing roots, stems, and leaves.
During the vegetative phase, growers typically maintain a light cycle of 18 hours of light followed by 6 hours of darkness (18/6). Some growers may use a 20/4 or 24/0 cycle to maximize growth rate, though the 18/6 schedule allows the plant a necessary dark period for metabolic processes. The plant will continue to grow indefinitely under these long-day conditions.
To trigger the flowering phase, the light schedule must be abruptly changed to 12 hours of light and 12 hours of uninterrupted darkness (12/12). This extended darkness mimics the shorter days of late summer, signaling the plant to produce flowers. Maintaining a strict, uninterrupted dark period is essential during this phase. Even brief light exposure can confuse the plant, potentially causing it to revert to vegetative growth or develop hermaphroditic traits.
Selecting the Right Indoor Lighting Fixtures
The choice of lighting fixture is the practical mechanism for delivering the required light intensity and spectrum to the plants. Modern cultivation primarily relies on three technologies, each with distinct advantages and drawbacks.
Light Emitting Diode (LED)
Light Emitting Diode (LED) fixtures are the most energy-efficient option, converting a greater percentage of electricity into usable light. LEDs produce minimal heat, which significantly reduces the need for extensive ventilation and cooling systems, though the initial cost for high-quality units is typically the highest.
High-Pressure Sodium (HPS)
High-Pressure Sodium (HPS) lights were the industry standard for many years, offering high intensity and a lower initial purchase price. However, HPS fixtures are power-hungry, have a shorter lifespan, and generate a substantial amount of heat, which complicates temperature management. They are often favored for flowering due to their spectrum but are less flexible than modern LED systems.
Ceramic Metal Halide (CMH)
Ceramic Metal Halide (CMH) fixtures provide a full-spectrum light considered superior for overall plant health and quality compared to the limited spectrum of HPS. CMH lights fall between LED and HPS in terms of energy efficiency and heat output, making them a balanced, middle-ground option. While their initial cost is higher than HPS, they offer a better lifespan and a more suitable spectrum for both vegetative and flowering stages.
Optimal Light Intensity for Each Life Stage
The light intensity must be carefully adjusted throughout the plant’s life cycle to match its metabolic capacity for photosynthesis. During the seedling and cloning stage, plants are fragile and require a gentle light level to prevent stress. The recommended PPFD for this phase is low, ranging from 100 to 300 \(\mu\text{mol}/\text{m}^2/\text{s}\), corresponding to a DLI of 6 to 12 \(\text{mol}/\text{m}^2/\text{day}\).
As the plant establishes itself and enters the early vegetative phase, the light intensity should be gradually increased to promote robust growth. Growers should aim for a PPFD between 300 and 600 \(\mu\text{mol}/\text{m}^2/\text{s}\). This intensity, combined with the 18-hour light cycle, results in a DLI of approximately 20 to 35 \(\text{mol}/\text{m}^2/\text{day}\).
Once the plant develops a dense canopy and transitions to the flowering phase, its light demands peak. During the main flowering phase, the ideal PPFD range is between 600 and 900 \(\mu\text{mol}/\text{m}^2/\text{s}\). Since the light cycle is now 12 hours, this higher intensity is necessary to achieve the target DLI for maximum flower development (35 to 50 \(\text{mol}/\text{m}^2/\text{day}\)). Intensity adjustment is often accomplished by raising or lowering the fixture distance from the canopy.