Autoflowering cannabis strains are genetically unique because their flowering phase is triggered by age, not a change in the light schedule. Incorporating genetics from Cannabis ruderalis, these plants have a fixed and typically short life cycle. Since autoflowers cannot be kept in a vegetative state indefinitely to increase size, maximizing the light they receive daily is the most important factor for maximizing final yield. The duration and intensity of light directly translate into the plant’s capacity to produce energy, making light management the primary focus for high productivity.
Understanding Autoflower Light Requirements
The biology of an autoflower plant is fundamentally different from a photoperiod strain. Photoperiod strains rely on a reduction in daily light hours, typically a transition to 12 hours of darkness, to begin flowering. Autoflowers are day-neutral, meaning they transition automatically, regardless of the light-dark cycle, usually three to four weeks after germination. This fixed, short timeline limits the window for vegetative growth, shifting the goal to providing maximum light energy throughout the entire lifespan. The more light an autoflower receives daily, the more energy it produces for growth and bud production, allowing for longer daily light exposures than photoperiod varieties.
Recommended Daily Light Schedules
The duration of light is a primary decision point for autoflower growers, as it directly impacts the Daily Light Integral (DLI) and final harvest size. The most common approach is the 18 hours on and 6 hours off (18/6) schedule, considered the optimal balance. This cycle provides extensive light for robust photosynthesis while offering a six-hour dark period for essential metabolic processes, such as nutrient transport and recovery. The 18/6 schedule also helps manage energy costs and heat generation compared to longer cycles.
A more intensive option is the 20 hours on and 4 hours off (20/4) schedule, which significantly increases the total light energy delivered daily. This schedule often results in higher yields because the plants have four more hours to photosynthesize. While the short four-hour darkness period is debated, it is generally thought to allow for adequate rest and recovery processes. Using a 20/4 schedule demands careful environmental control, as longer light operation increases ambient temperature and humidity.
Some growers run their lights for a full 24 hours a day (24/0) throughout the entire life cycle. The rationale is that constant light equals maximum energy production and the fastest possible growth. However, this schedule is controversial, as many believe the plant requires darkness to complete crucial non-photosynthetic functions, such as moving sugars and starches. The 24/0 cycle also results in the highest electricity consumption and the most difficult conditions to manage regarding heat and humidity.
Optimizing Light Intensity for Growth Stages
Beyond the duration of light, intensity is the most important factor for maximizing yield. Intensity is measured using Photosynthetic Photon Flux Density (PPFD), which quantifies the number of photosynthetically active photons hitting a square meter per second. PPFD is a more accurate measurement than watts because it focuses on the light energy plants use for growth. The goal is to provide a specific PPFD range that matches the plant’s increasing energy needs throughout its development.
Seedling Stage
During the seedling stage, the young plant is delicate and requires a gentle introduction to light to avoid stress. A low PPFD of approximately 100 to 300 µmol/m²/s is ideal for this phase. This intensity encourages healthy root development without overwhelming the small leaves, preventing photo-bleaching and helping the seedling establish a strong foundation.
Vegetative Phase
As the autoflower enters the vegetative phase, its energy demands increase substantially to support the rapid growth of leaves and branches. The PPFD should be gradually increased to a medium intensity range of 400 to 600 µmol/m²/s. This increased intensity fuels chlorophyll production and allows the plant to develop a dense canopy capable of supporting heavy flower production.
Flowering Stage
The flowering stage is where the plant’s energy consumption peaks as it focuses on developing dense, resinous buds. During this time, the light intensity should be maximized to a PPFD of 600 to 900 µmol/m²/s. This high intensity provides the necessary energy to drive flower development and increase potency. The Daily Light Integral (DLI), the total amount of photosynthetically active radiation received over 24 hours, is highly relevant as it combines high intensity and long duration.
Recognizing and Correcting Light Stress
Providing too much or too little light intensity leads to stress, negatively affecting plant health and yield. Recognizing the visual symptoms of light stress is necessary to diagnose and correct lighting issues. Symptoms of excessive light, or light burn, typically appear on the parts of the plant closest to the light source, primarily the top canopy.
Light burn often manifests as a yellowing or bleaching of the leaves, concentrated at the top canopy, which can resemble a nutrient deficiency. Leaves may also exhibit a characteristic upward curling, known as “tacoing,” as the plant attempts to protect itself from the intensity. In the flowering stage, extreme light stress can cause buds to turn pale white or bleached, indicating damage to cannabinoids and terpenes. Correcting this requires immediately raising the light fixture or dimming the light’s output to reduce the PPFD hitting the canopy.
Conversely, light deficiency results in symptoms related to the plant trying to stretch closer to the light source. The plant appears pale and weak, with elongated stems and large spaces between the nodes, a condition known as stretching. This morphology is an attempt to reach higher intensity light, resulting in weak branches that struggle to support the weight of mature flowers. If light deprivation is the issue, the action is to lower the light fixture to increase the PPFD or increase the total daily duration of the light cycle.