Cannabis, like all green plants, uses light energy to power photosynthesis, converting carbon dioxide and water into sugars for growth. The amount of light required is complex and changes drastically throughout the plant’s life cycle. Requirements differ significantly between indoor and outdoor environments. Understanding these changing needs is essential for optimizing health, growth rate, and final yield.
Light Cycles and Duration (The Photoperiod)
The duration of light and darkness, known as the photoperiod, is the primary control mechanism for most cannabis varieties. These photoperiod-sensitive strains use the changing length of the dark period to signal the transition from vegetative growth to flowering. During the initial vegetative stage, the plant focuses on developing a robust structure of leaves, stems, and roots.
To maintain this vigorous vegetative growth indoors, growers typically provide 18 hours of light followed by 6 hours of darkness. This extended light period mimics the long days of late spring and early summer, signaling the plant to grow larger. To initiate the flowering stage, the light cycle must be strictly reduced to 12 hours of light and 12 hours of uninterrupted darkness. This longer dark period simulates the shorter days of late summer and autumn, triggering the hormonal shift that leads to flower production.
Autoflowering varieties are a genetic exception, as they do not rely on a change in the photoperiod to transition. These strains begin flowering based on their age, usually around three to four weeks after germination, regardless of the light schedule. While they benefit from extended light hours, they do not require the strict 12/12 cycle to produce buds.
Determining Light Intensity (PPFD and DLI)
Light intensity, or strength, is crucial for determining the plant’s growth potential. This strength is measured using two specific metrics: Photosynthetic Photon Flux Density (PPFD) and Daily Light Integral (DLI). PPFD measures the instantaneous number of photosynthetically active photons hitting a square meter of canopy per second. DLI is a more comprehensive metric that integrates the PPFD value over the course of a 24-hour period to show the total usable light energy delivered daily.
Light intensity requirements increase dramatically as the plant matures, corresponding to its ability to process more energy. Young seedlings and clones require the lowest intensity, thriving with a PPFD of 100 to 300 \(\mu\text{mol}/\text{m}^2/\text{s}\), which translates to a DLI of approximately 9 to 12 \(\text{mol}/\text{m}^2/\text{d}\).
During the vegetative phase, the plant can handle and use significantly more light, with optimal intensity ranging from 400 to 600 \(\mu\text{mol}/\text{m}^2/\text{s}\). This pushes the DLI into the 20 to 35 \(\text{mol}/\text{m}^2/\text{d}\) range. Peak flowering requires the highest light, as the plant channels maximum energy into developing dense buds. The ideal PPFD target is between 700 and 1,000 \(\mu\text{mol}/\text{m}^2/\text{s}\) (for non-CO2 supplemented grows), achieving a DLI of 35 to 50 \(\text{mol}/\text{m}^2/\text{d}\) under a 12-hour cycle.
Practical Light Management by Environment
Managing light intensity is a dynamic process that depends on whether the plant is grown indoors with artificial fixtures or outdoors under the sun. For indoor cultivation, management involves adjusting the distance between the fixture and the top of the plant canopy. As the plant grows taller, the light must be raised or dimmed to maintain the target PPFD and prevent light stress. Many modern LED fixtures include dimming features, allowing growers to precisely tune the intensity without changing the physical distance.
Outdoor Management
Outdoors, the sun provides intensity far greater than any artificial light source. The primary management concern is ensuring the plant receives enough hours of direct sunlight. Ideally, outdoor plants should receive 10 to 12 hours of direct sun exposure daily to maximize their growth potential. A minimum of six to eight hours of unobstructed direct sunlight is required for a healthy plant and respectable harvest.
The geographical latitude of the grow site also plays a role, as it affects the sun’s angle and the total available hours of daylight. Growers in northern latitudes must be particularly mindful of planting times. This ensures the photoperiod variety has adequate time in the vegetative stage before the naturally decreasing daylight hours trigger flowering.
Recognizing and Addressing Light Stress
Plants will exhibit clear visual symptoms when they are receiving either too little or too much light, providing a necessary warning that adjustments are needed.
When a plant receives insufficient light, it enters a state of under-exposure, characterized by stretching or etiolation as the plant desperately reaches for the light source. Leaves will often appear pale or sickly yellow due to a lack of chlorophyll production, and the stems will be thin and weak, unable to support the plant’s structure. The corrective action for under-exposure is to increase the light intensity by lowering the fixture or increasing the dimmer setting.
Conversely, too much light leads to high-light stress or “light burn,” typically seen in the leaves closest to the light source. Symptoms of this over-exposure include the leaves curling upward at the edges in a “taco” shape as the plant tries to shade itself. The most telling sign is bleaching, where the leaf tips and margins turn yellow or white, though the veins often remain green, distinguishing it from a nutrient deficiency. To fix light burn, the intensity must be reduced immediately, either by raising the light fixture further from the canopy or by dimming the light to a lower PPFD setting.