The seedling phase is the delicate early stage of a cannabis plant’s life, marked by rapid but vulnerable development. This stage begins once the first true leaves emerge and lasts until the plant has developed three to four sets of nodes, signaling it is ready for the vegetative growth light schedule. Providing a gentle yet consistent light source is paramount during this window, as the seedling focuses on establishing a robust root structure to support its future size. Incorrect lighting now can permanently stunt growth, making precise control over intensity, spectrum, and duration a foundation for a successful harvest.
Setting the Right Light Intensity and Distance
Light requirements for cannabis seedlings are measured using Photosynthetic Photon Flux Density (PPFD). This metric quantifies the number of photosynthetically active photons that hit a square meter every second, expressed in micromoles per square meter per second (\(\mu \text{mol/m}^2/\text{s}\)). Seedlings thrive in a relatively low PPFD range, typically between 100 and 400 \(\mu \text{mol/m}^2/\text{s}\).
While experienced growers may target the upper end of this range (300 to 400 \(\mu \text{mol/m}^2/\text{s}\)) for faster growth, a safer starting point for delicate seedlings is often 200 \(\mu \text{mol/m}^2/\text{s}\). Light intensity is primarily controlled by the distance between the fixture and the plant canopy. The inverse square law dictates that light intensity drops dramatically as the distance from the source increases.
For low-power sources like T5 fluorescent tubes or low-wattage LED panels, a distance of 24 to 36 inches is often suggested to achieve the appropriate low intensity. Placing the light too close can overwhelm the plant before its root system is fully established. It is always better to start with the light higher and gradually lower it over the course of the seedling stage as the plant gains strength.
Excessive light intensity at this stage is detrimental, forcing the young plant to expend energy on defense mechanisms rather than growth. Treating seedlings like mature vegetative plants results in light stress that can bleach the tender leaves or stunt the overall development.
Choosing the Best Light Spectrum
The light spectrum, or color, plays a significant role in guiding the seedling’s physical development. Spectrum is often described using a color temperature scale measured in Kelvin (K). During the seedling phase, the preferred light color is a cool, bluish-white light, which corresponds to a color temperature range of 5000K to 6500K.
The blue wavelengths (approximately 400–500 nm) within this cooler spectrum are readily absorbed by the plant’s chlorophyll pigments. This absorption encourages a compact, sturdy growth habit, prompting the seedling to develop thick stems and short internodal spacing. This response prepares the plant for the heavier foliage and future flower weight.
Growers should avoid using lights with a warmer, red-heavy spectrum, typically below 3500K, during this early stage. While red light is highly efficient for photosynthesis, it can cause seedlings to stretch excessively. This phenomenon, known as etiolation, results in tall, spindly plants that lack the structural integrity for optimal growth.
The Optimal Daily Light Schedule
Once the correct intensity and spectrum are set, the next consideration is the duration of light exposure, or the photoperiod. Cannabis seedlings are typically given a long period of light each day to maximize the energy available for growth. The two most common schedules used during the seedling stage are 18 hours of light followed by 6 hours of darkness (18/6) or 24 hours of continuous light (24/0).
The 18/6 schedule is widely used because the six-hour dark period allows the plant to perform metabolic processes that are thought to improve overall health and root development. Additionally, the short dark cycle can save on electricity costs and reduce heat output in the grow space.
The 24/0 schedule maximizes the total light energy the seedling receives, which can result in a faster growth rate due to constant photosynthesis. While cannabis does not strictly require a dark period, many growers prefer the 18/6 schedule to provide a metabolic rest period and to acclimate the young plant to a diurnal rhythm. Since seedlings are not yet sensitive to the photoperiod, the choice between 18/6 and 24/0 is largely a matter of grower preference and environmental control.
Troubleshooting Common Lighting Mistakes
Observing the seedling’s physical response is the most reliable way to determine if the lighting setup is correct.
One common sign of insufficient light is “stretching,” where the stem becomes tall and thin with large gaps between the nodes. This is the plant’s desperate attempt to grow upward and closer to the light source. The immediate correction is to either lower the light closer to the canopy or increase its intensity.
Conversely, symptoms of too much light—known as light stress or light burn—appear on the leaves closest to the light source. These signs include the leaves curling upward along the edges, resembling a taco shell, or the tips of the leaves turning yellow or bleached white. Bleaching occurs because the intense light has destroyed the chlorophyll in the leaf tissue. The solution is to immediately raise the light fixture away from the plant canopy to reduce the PPFD and allow the seedling to recover.