The question of when to switch to a bloom light for autoflowers often stems from confusion between light cycle and light spectrum. Autoflowering plants flower based on an internal genetic timer, independent of the hours of light or darkness they receive. Autoflowers transition automatically, unlike photoperiod strains which require a reduction in light hours to trigger flowering. Therefore, the “switch” is not about changing the timer but about immediately adjusting the light spectrum and intensity to support bud production once the plant begins its natural transition.
The Critical Difference of Autoflowering Genetics
Autoflowering cannabis strains carry the genetics of Cannabis ruderalis, a subspecies native to Central Asia and Eastern Europe. These plants evolved in regions with short summers, making a photoperiod trigger unreliable. This genetic heritage makes them “day-neutral,” meaning their shift from vegetative growth to flowering is driven by age rather than a change in the light-dark cycle.
This inherent difference means the grower is not responsible for initiating the bloom phase by changing the light schedule. While photoperiod plants require 12 hours of uninterrupted darkness to begin flowering, autoflowers bypass this requirement. They rely on their internal clock to start floral initiation, typically three to five weeks after sprouting, allowing the grower to maintain a long light duration throughout the entire life cycle.
Identifying the Natural Transition to Bloom
The plant’s natural transition provides the visual cue for adjusting the light spectrum. The first sign of flowering is the appearance of pre-flowers—tiny, white pistils that emerge at the nodes where the fan leaves meet the main stem. This pre-flowering stage usually begins between the third and fifth week after the seedling emerges.
This initial stage is often accompanied by a rapid vertical growth spurt known as the “flowering stretch.” The plant focuses energy on developing taller branches and elongating its internodes to support the developing buds. Once the vertical stretch slows down or halts, the plant is fully committed to floral production and is ready for a spectrum change.
The precise timing for this transition is strain-dependent, making visual inspection the most accurate method. Some fast-maturing varieties may show pre-flowers as early as two weeks, while others might take closer to six weeks. Observing these white pistils and the onset of the stretch phase indicates the plant has hormonally shifted, signaling the grower to make lighting adjustments.
How and When to Adjust the Light Spectrum
The “switch” to bloom light should occur concurrently with, or immediately after, the first appearance of pre-flowers. This adjustment involves changing the spectral output, or color, to better support the plant’s new biological function. During the vegetative stage, plants benefit from a light spectrum higher in blue wavelengths, corresponding to a higher Kelvin temperature (e.g., 6500K).
For the bloom phase, the focus shifts to increasing the red and far-red light components (around 600–700 nm and 730 nm). Red light is highly efficient for photosynthesis and promotes bud development, density, and flower mass. A common color temperature recommendation for flowering is a warmer, red-heavy spectrum, typically around 2700K to 3000K.
For growers using modern LED fixtures, this spectral change is managed by activating the “bloom” or “red booster” channels, which increase the ratio of red light relative to blue. If the fixture is not adjustable, the goal is to switch to a light source that naturally emits a warmer, red-dominant spectrum. Providing this red-heavy light helps the plant redirect energy from vertical growth to reproductive development, maximizing the eventual yield.
Optimizing Light Intensity and Duration for Maximum Bloom
While the spectrum changes at the start of flowering, the plant’s light duration remains long for autoflowers. Most growers maintain a long photoperiod, such as 18 hours of light (18/6) or 20 hours of light (20/4), from seed to harvest. This extended duration maximizes the plant’s daily light intake, directly contributing to higher yields.
The light intensity must also be increased during bloom to support the higher energy demands of flower production. Intensity is measured using Photosynthetic Photon Flux Density (PPFD), which quantifies the active light reaching the canopy. During the vegetative phase, an autoflower typically requires 400–600 \(\mu\)mol/m²/s.
Once flowering is established, the PPFD should be increased to a range of 600–900 \(\mu\)mol/m²/s to drive robust bud growth. The grower must also consider the Daily Light Integral (DLI), which is the total amount of light received over a 24-hour period. Because autoflowers are kept under longer light cycles, a DLI of around 40 mol/m²/day is a common target during peak bloom.