How Many Light Hours for the Indoor Flowering Stage?

Reaching the flowering stage of indoor gardening is an exciting milestone. To ensure a successful transition into this productive phase, providing the correct light signals is necessary. Light is not just food for plants; it is also the primary environmental cue that dictates their developmental stages. For many species grown indoors, manipulating the duration of light and darkness is the trigger that shifts them from growing leaves and stems to producing flowers.

The Standard 12/12 Flowering Light Cycle

The most widely accepted lighting schedule to initiate flowering in photoperiod-sensitive plants is the 12/12 cycle. This means providing 12 hours of continuous light followed by 12 hours of continuous darkness each day. This timing simulates the autumnal equinox, when day and night are of equal length. For many plants, this seasonal shift signals that winter is approaching, and they must enter their reproductive phase.

The change in day length triggers internal hormonal shifts, redirecting the plant’s energy from vegetative growth toward producing flowers and buds. This redirection is the goal of manipulating the light cycle. Implementing this schedule is best managed with an outlet timer. Manually turning lights on and off is prone to error, while a timer automates the process, ensuring the light cycle is precise.

Once the switch to a 12/12 schedule is made, the plant often undergoes a period of rapid stretching before its vertical growth slows and energy is committed to developing buds. The duration of the flowering stage itself can vary depending on the specific plant, ranging from six to over fourteen weeks. Throughout this time, maintaining the strict 12/12 cycle is the foundation for a productive flowering phase.

The Critical Role of Uninterrupted Darkness

While the 12/12 schedule is discussed in terms of its light hours, the true trigger for flowering lies in the 12-hour period of complete, uninterrupted darkness. For photoperiod plants, it is the consistent, extended duration of darkness that signals the plant to change its developmental course from vegetative growth to flowering. This response is governed by light-sensitive pigments called phytochromes.

During the day, light converts the pigment to its active form. During darkness, this active form slowly reverts to the inactive form. The length of the dark period determines how much of the phytochrome is converted back. When the period of darkness is long enough, the concentration of the inactive form reaches a tipping point. This chemical change signals the production of a flowering hormone that instructs the growing tips to start forming buds instead of leaves.

Any interruption of the dark period can disrupt this process. Even a brief exposure to light can be enough to convert the phytochrome pigment back to its daytime form, effectively resetting the plant’s internal clock. This can confuse the plant, causing it to halt the flowering process or prevent it from starting altogether. Light leaks from another room or indicator lights on equipment can have negative consequences.

Alternative Flowering Light Schedules

While the 12/12 cycle is the standard, some growers experiment with alternative schedules to save energy or accelerate the harvest. These methods are considered more advanced and are often employed for particular plant varieties. It is important to understand the standard before attempting these variations.

One alternative is the 11/13 schedule, which provides 11 hours of light and 13 hours of darkness. The extended dark period can encourage certain strains to flower faster, potentially shortening the total flowering time. The added hour of darkness each day also results in a small energy saving over the entire flowering cycle.

A 10/14 schedule can be used to push this concept even further. With a 14-hour dark period, the signal to flower is stronger, which can be useful for stubborn varieties. However, the reduced light hours mean the plant has less time for photosynthesis, which could lead to smaller yields if the light intensity is not sufficient to compensate.

A more complex technique is the Gas Lantern Routine, which breaks the dark period into two smaller segments. A typical example is 12 hours of light, 5.5 hours of darkness, a single hour of light, and then another 5.5 hours of darkness. The brief one-hour light interruption is intended to break the plant’s respiration cycle, theoretically saving energy while still tricking the plant into flowering.

Lighting Needs for Different Plant Types

The discussion of light cycles for flowering primarily revolves around photoperiod-sensitive plants. These are species that rely on the length of day and night to dictate their life stages. Common examples grown indoors include cannabis, poinsettias, and many “short-day” vegetables. For these plants, adhering to a 12/12 light schedule is the method to induce and maintain flowering.

The world of indoor gardening includes a major exception to this rule: autoflowering plants. Autoflowers do not depend on light schedules to begin flowering; instead, their transition is determined by age. These plants carry genetics from subspecies that evolved in regions with very short summers, where waiting for changing light cues was not a reliable survival strategy.

Because their flowering is pre-determined by age, autoflowers do not require a 12/12 light cycle. Growers can provide them with a consistent light schedule throughout their entire life, such as 18 hours of light and 6 hours of darkness (18/6) or 20 hours of light and 4 hours of darkness (20/4). Providing more than 12 hours of light per day allows autoflowers to absorb more energy, which can fuel more photosynthesis and lead to larger yields.

Common Lighting Mistakes to Avoid

Even with a solid understanding of light cycles, growers can encounter issues that hinder a successful flowering stage. Avoiding a few common mistakes can be the difference between a bountiful harvest and a frustrating experience. These errors often relate to the consistency of the light schedule and the physical environment.

• Allowing light leaks during the dark period. As detailed earlier, interruptions from stray light can confuse photoperiod plants, delaying or preventing flowering. Growers must make their grow space completely light-proof by checking for light seeping under doors or from equipment LEDs.
• Using an inconsistent schedule. Relying on manually switching the lights on and off can cause stress that negatively impacts development. Using a reliable timer is a necessary component of a controlled indoor garden.
• Switching to the 12/12 schedule at the wrong time. Inducing flowering when a plant is too small will result in a diminished yield. Conversely, waiting too long can lead to plants outgrowing their space, as many will double in size after the switch.
• Accidentally reversing the process. Re-vegetation can occur if a flowering plant is exposed to a vegetative light cycle (more than 12 hours of light) due to a timer failure. The plant will try to revert to vegetative growth, resulting in strange growth and a loss of harvest quality.