Light is the most influential factor in indoor cannabis cultivation, serving as the energy source that drives growth, development, and the quality of the final product. Photosynthesis converts light energy into the chemical energy a plant uses to build its structure and produce valuable compounds. Unlike outdoor growing, indoor cultivation requires the grower to carefully control the light’s quality, intensity, and duration. Mastering this control is necessary for successfully cultivating healthy cannabis plants with high yields.
Essential Spectral Requirements
The light spectrum used for photosynthesis is called Photosynthetically Active Radiation (PAR), covering the visible light range of 400 to 700 nanometers (nm). The intensity of this usable light is measured by Photosynthetic Photon Flux Density (PPFD), expressed in micromoles per square meter per second (\(\mu mol/m^2/s\)). PPFD quantifies the number of photons striking the canopy every second, measuring the energy available for growth.
Different wavelengths within the PAR range trigger specific biological responses in cannabis plants. Blue light (400–500 nm) is important during the vegetative stage. This spectrum encourages compact, bushy growth, promoting the development of strong stems and thick leaves.
The red light spectrum (600–700 nm) is crucial for stimulating flowering and boosting biomass and yield. Red light efficiently drives photosynthesis and plays a direct role in producing buds and resin. A higher proportion of red light signals the end of the growing season, prompting the shift from vegetative growth to flower production.
Secondary wavelengths also contribute to plant health and potency. Green light, which plants reflect, can penetrate deeper into the canopy than red or blue light, allowing lower leaves to maintain some photosynthetic activity. Ultraviolet (UV) light, particularly UVA (315–400 nm), does not contribute to photosynthesis. Instead, it acts as a stressor that encourages the plant to produce protective compounds like cannabinoids and terpenes, potentially boosting potency.
Far-red light (700–750 nm) influences development by affecting photoreceptors that regulate plant structure. This spectrum can induce a “shade avoidance” response, causing the plant to stretch taller and develop larger leaves to maximize light capture. Strategic use of far-red light can also accelerate the transition into the flowering stage.
Comparing Lighting Technologies
Selecting the right hardware involves balancing upfront cost, energy efficiency, and heat management. Light Emitting Diode (LED) fixtures are the most energy-efficient choice, converting approximately 50% of electricity into usable light, compared to about 30% for older technologies. LEDs produce very little radiant heat, which significantly reduces the need for cooling and ventilation systems in the grow space.
High-quality LED systems offer a full and often tunable spectrum, allowing growers to adjust the ratio of blue and red light to match the plant’s specific growth stage. Although the initial investment for an LED fixture is typically the highest, their long lifespan (50,000 to 70,000 hours) and lower energy bills make them the most cost-effective option over several years.
High-Pressure Sodium (HPS) lights are a traditional choice known for their high intensity and lower initial purchase price. HPS bulbs emit a spectrum weighted toward red and orange wavelengths, which is excellent for flowering but less ideal for vegetative growth. HPS fixtures produce significant heat, necessitating substantial cooling infrastructure and increasing operational cost.
Ceramic Metal Halide (CMH) or Light Emitting Ceramic (LEC) fixtures offer a broad, high-quality spectrum that closely mimics natural sunlight, including beneficial UV wavelengths. CMH lights are more energy-efficient than HPS and produce a balanced light suitable for both growth stages. They generate moderate heat, falling between HPS and LED, and their bulbs last longer than HPS.
Fluorescent lights, such as T5 high-output bulbs or Compact Fluorescent Lights (CFLs), have very low intensity and minimal heat output. These lights are not powerful enough for heavy flowering and are reserved for the earliest stages of growth. Fluorescent lights are excellent for starting seeds, rooting clones, and maintaining small mother plants due to their low cost and ability to be placed close to the canopy without causing heat damage.
Managing Photoperiod and Intensity
Light application requires careful management of both the daily duration (photoperiod) and the intensity (measured in PPFD). Cannabis is a photoperiod-dependent plant, meaning the length of the dark period dictates its transition to flowering. During the vegetative stage, plants need a long light cycle, typically 18 hours of light followed by 6 hours of darkness, or 24 hours of continuous light.
The flowering stage is triggered by shortening the light cycle to 12 hours of uninterrupted light and 12 hours of complete darkness. This change mimics the shortening days of autumn, signaling the plant to produce flowers. Maintaining a strict, uninterrupted dark period is necessary, as even brief light exposure can confuse the plant and revert it to vegetative growth.
Light intensity must be carefully calibrated to the plant’s stage of development to maximize growth without causing damage. Seedlings and clones require gentle intensity, thriving with a PPFD between 100 and 300 \(\mu mol/m^2/s\). As the plant matures in the vegetative stage, intensity should increase to 400 to 600 \(\mu mol/m^2/s\) to support rapid biomass accumulation.
The highest intensity is required during the flowering stage, with an optimal PPFD target of 700 to 900 \(\mu mol/m^2/s\) for growers not supplementing carbon dioxide. Fixture placement is the primary method for managing intensity, since the PPFD value drops significantly as the light source moves farther from the canopy. Growers must regularly monitor the distance between the light and the plant tips, adjusting the fixture height to maintain the target PPFD. This prevents light burn or heat stress, which presents as yellowing or bleaching of the top leaves.