A grow light mimics the solar spectrum necessary for photosynthesis, allowing plants to grow indoors. The required duration depends on the plant’s species, its growth stage, and its natural life cycle. Timing this light exposure, known as the photoperiod, is a fundamental tool used by indoor growers to control vegetative growth and the initiation of flowering.
Understanding the Necessity of a Dark Period
While light provides the energy for a plant’s primary function—photosynthesis—a period of consistent darkness is equally important for metabolic and signaling functions. During the light cycle, a plant converts carbon dioxide and water into oxygen and glucose, which is primarily stored as starch in the leaves. The dark period is when the plant actively transports these stored sugars from the leaves to other parts, such as the roots, stems, and new growth sites, fueling structural development.
This nightly transportation and growth process is a form of energy rationing, which is why continuous light can sometimes lead to reduced growth efficiency and plant stress. Additionally, darkness is the mechanism by which many species measure the change in seasons, a process called photoperiodism. The uninterrupted length of the dark cycle, not the light cycle, serves as the trigger for hormonal changes that signal the plant to transition from the vegetative stage to the reproductive, or flowering, stage. Interrupting this dark period, even briefly, can confuse the plant’s internal clock and prevent it from flowering or fruiting.
Standard Light Schedules by Plant Maturity
For most photoperiod-sensitive plants, the light schedule is divided into two distinct phases that correspond to the plant’s natural life cycle. The vegetative phase is dedicated to rapid growth of leaves, stems, and roots. To encourage this, the standard light schedule is typically 18 hours of light followed by 6 hours of darkness, commonly referred to as 18/6. This extended light duration maximizes the time the plant spends producing energy while still providing the necessary dark period for sugar transport.
When a grower decides the plant has reached a sufficient size, the photoperiod is intentionally altered to trigger the reproductive stage. This transition is initiated by switching to a 12-hour light and 12-hour dark schedule, known as 12/12. This equal split mimics the shorter days and longer nights of late summer and early autumn, which is the natural signal for many species to begin flowering and producing seed. The 12 hours of total, uninterrupted darkness is a strict requirement for the plant to release the flowering hormones that regulate the switch to reproduction. Consistency is paramount in this stage, and any accidental light exposure during the dark period can reverse the hormonal signal, causing the plant to revert to the vegetative state or suffer from stress.
Maintaining the 12/12 schedule throughout the entire flowering and fruiting period ensures the plant dedicates its energy to bud or fruit development until harvest. The controlled shift to 12/12 is the primary method indoor gardeners use to manage the timing of their harvest.
Exceptions for Seedlings and Non-Flowering Plants
Newly germinated seedlings require a high amount of light duration to establish strong, stocky growth and prevent etiolation, which occurs when the seedling stretches rapidly, resulting in a thin, weak stem. To prevent this, seedlings benefit from a slightly longer photoperiod than mature vegetative plants, generally between 16 and 18 hours of light per day. This extended light duration provides the energy needed for rapid cell division and root development, creating a sturdy foundation for the rest of the plant’s life.
Certain plants are categorized as day-neutral, meaning their flowering cycles are not primarily dictated by the length of the dark period. These include many common leafy greens and herbs, such as lettuce, spinach, and basil, which are grown solely for their foliage. Since there is no reproductive stage to trigger, these plants can often thrive under a continuous 16 to 18-hour light cycle. In some setups, certain day-neutral plants can even tolerate 24 hours of light without suffering the stress that would affect photoperiod-sensitive species.