The heightened level of control over all environmental factors is arguably the biggest advantage of indoor horticulture. In an indoor garden, the horticulturist has ultimate control over the atmospheric conditions, watering schedule, nutrition, and lighting. In short, the horticulturist has control over all the inputs for the chemical reaction commonly known as photosynthesis.

By manipulating any of the inputs for photosynthesis, growers automatically influence the growth and development of plants. The heightened control allows a grower to provide his or her plants with conditions that maximize photosynthesis and would not normally be found in nature. This can equate to faster growth rates and larger yields.

Our cumulative knowledge of indoor horticulture has established a set of standards for growing indoors. Put another way, there are specific ranges in temperature, humidity, CO2 levels, nutrition, and lighting that are considered optimal. And for good reason, as these standards are tried and true.

However, indoor horticulturists’ quests for efficiency and effectiveness have them continuously experimenting with alternative environmental conditions. It is the heightened control offered only by indoor horticulture that allows for experimentation with environmental conditions that would rarely, or never, exist in nature.

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One of the factors that can be manipulated beyond those found in nature is lighting. In nature, light cycles are determined by the rising and setting sun. No two days have the exact same duration of light and dark hours, as they change slightly every day. In a light-tight indoor garden, a grower can create any sort of light cycle he or she desires. Although erratic light cycles will most likely stress the plants and cause undesirable effects, there are some indoor horticulturists who achieve success with alternate light cycles.

The Standard in Light Cycles for Indoor Gardens

Before discussing alternative light cycles for indoor gardens, it is important to mention the standard in lighting cycles for artificial horticultural lighting. Most indoor growers employ a 24/0, 20/4, or 18/6 light/dark cycle during vegetative growth. These cycles dictate the lights on or daytime durations are 24, 20, or 18 hours and the lights off or night time durations are zero, four, or six hours, respectively.

Though the optimal lights on and lights off duration for vegetative growth seems to differ depending on who you ask (or the particular species of plant), it seems that the vast majority of indoor horticulturists use a 24/0, a 20/4, or an 18/6 light cycle for vegetative growth.

The standard for the flowering or blooming light cycle in an indoor garden is a 12/12 light cycle. This means the lights are on for 12 hours and off for 12 hours in every 24-hour light cycle. This standard cycle for flowering or blooming is largely agreed upon by indoor growers and is employed for both effectiveness and convenience.

The standard light cycles used by indoor growers are based solely on a 24-hour day, which makes sense because that is the cycle which plants have evolved in for millions of years. However, some alternative light cycles manipulate this 24-hour cycle to trick the plants into thinking there is more than one light cycle in a 24-hour period. Some alternative light cycles used by indoor growers actually replace the 24-hour cycle with an 18-hour cycle.

Gas Lantern Routine

The gas lantern routine is an alternative light cycle technique that reduces energy consumption but allows the plants to continue to grow in a vegetative stage. The gas lantern routine follows a 24-hour clock in the following way: 12 hours lights on, followed by 5.5 hours of lights off, one hour of lights on, and then 5.5 hours of lights off.

The gas lantern routine is similar to the standard flowering 12/12 lighting cycle with an interruption of one hour of light directly in the middle of the lights-off period. This interruption inhibits florigen production within the plant. Florigen is thought to be the flowering hormone responsible for triggering flowering production in plants. The logic behind the gas lantern routine is to interrupt the dark cycle with enough light to trick the plants into remaining in vegetative growth.

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For most photosensitive plants, florigen is only produced once the plant has experienced 12 hours of continuous darkness. In other words, it is the amount of darkness, not light, that triggers a plant’s flowering response.

The largest advantage of the gas lantern routine is the savings in electrical consumption. The gas lantern routine reduces the amount of electrical consumption per 24 hours, while maintaining a light cycle that will not trigger flowering. When used correctly, the gas lantern routine will reduce electrical usage by an average of seven hours per day (when compared with standard vegetative light cycles). This adds up to close to 200 hours per vegetative cycle.

After achieving the desired plant height with the gas lantern routine, a grower can simply switch to the 12/12 standard light cycle for flowering. However, many growers using the gas lantern technique employ a 10/14 flowering light cycle or taper the flowering light duration. A 10/14 or a tapered light cycle during the flowering stage will continue to reduce the overall energy consumption compared with operating a standard light cycle. A tapered light duration in flowering more closely mimics the light duration during the autumn season (which is sort of ironic considering the whole point of the gas lantern routine is to use a light cycle that is unavailable in nature).

Regardless of the irony, many growers who use the gas lantern routine prefer to taper the light duration during the flowering stage. In order to taper the light cycle in the flowering stage, a grower should start with a 12/12 light cycle for the first two weeks of flowering. At week three, the grower should reduce the light hours to 11, creating an 11/13 light cycle for weeks three and four. At the beginning of week five, the grower should reduce the light cycle another hour to create a 10/14 cycle for weeks five and six.

Finally, for the last two weeks of flowering, the light cycle should be 9/15 or nine hours on and 15 hours off. All in all, both the gas lantern routine for vegetative growth and the tapering light duration for flowering offer the benefit of reduced production costs. The counter argument to this is less radiant energy (light energy) equates to reduced yields. At the end of the day, it takes a grower willing to experiment with his or her crop to determine if the gas lantern routine produces an adequate yield for the cost of production.

18-Hour Days

Another alternative light cycle employed by some indoor growers is the 18-hour daylight cycle. The theory behind the 18-hour day is a plant can only process a certain amount of light per 24 hours and as the light cycle continues, a plant’s efficiency of absorbing light decreases.

Put another way, it is theorized that during a normal 24-hour light cycle, plants will achieve the fastest growth rates during the first 50-60 per cent of the light hours. The growth rates will then diminish rapidly and the last 20-30 per cent of the light hours only produce minimal growth.

By reducing the length of the day from 24 hours to 18 hours, a grower can create an environment where the plants receive enough light hours to achieve peak growth and still enough radiant energy (light energy) to produce comparable yields.

In order to experiment with an 18-hour daylight cycle, a grower will need a programmable digital timer or an analog 18-hour timer. The vegetative cycle would have a light cycle of 14/4 or 14 hours lights on and four hours lights off. During the flowering stage, the plants would need at least 12 hours of darkness so a 6/12 light cycle (six hours on/12 hours off) would be used.

The benefits of operating an 18-hour day in an indoor garden include reduced electrical costs and shorter vegetative and blooming duration. A reduction in the duration of time to harvest equates to more harvests per year. One disadvantage of operating an 18-hour day is the lights will come on and turn off at different times each day. This can be inconvenient to a grower’s feeding and watering schedules. Lastly, it should be noted that although some indoor growers swear by an 18-hour day, most professional growers still operate light cycles on a 24-hour clock.

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Indoor horticulture is all about control. Having absolute power over the environmental conditions is what makes indoor gardening stand apart from outdoor gardening. With the control offered only in an indoor environment, cultivators have been able to experiment with and fine-tune all the factors that contribute to a healthy, productive garden.

Though the standard light duration used by most professional growers exists for a reason, there are many cultivators willing to experiment with photosynthesis’s most important input: radiant energy (light energy). For photosensitive plants, it is the duration of the dark cycle that initiates florigen production and, in turn, the plant’s reproductive processes. By manipulating the duration of the dark period with the gas lantern routine, growers can significantly reduce electrical costs during the vegetative stage of growth in their gardens.

As if manipulating a 24-hour clock wasn’t enough, the use of digital timers allows a grower to get even more inventive and experiment with just about any day/night length he or she desires. Experimentation is the process that collectively shapes the future of indoor horticulture. Who knows… it just may be your experimentation that leads to the next standard in light duration for horticultural lighting.