Photoperiod Manipulation in Marijuana Grow Room for Higher Yields

Photoperiod strains switch from the growth phase to the flowering stage when they get fewer hours of light per day. In nature, this means that they will start to flower in the late summer.

Autoflowering strains are not dependent on the length of daylight. They start to flower automatically when they’re a few weeks old (usually 3-4 weeks). This length of daylight vs. age is the main difference between photoperiod and autoflowering strains in a nutshell.

Excluding auto-flowering strains, most plants in vegetative growth are kept in 18-24 hours of light per day and only when the photoperiod is changed to provide longer dark periods will the cannabis plant flower.

Photoperiod strains are easy to manipulate, which is both a positive and negative trait. If we assume the manipulation is intended, photoperiod strains can grow taller and produce bigger yields than their autoflowering counterparts.

It is possible to keep a photoperiod plant in a vegetative state until you are sure it has the strength and structure to support efficient bud production.

From seed to harvest, growth will take longer than with an autoflowering strain, which makes timing essential for outdoor growers. If you live in a climate that is less than perfect for growing cannabis, the time at which you choose to grow will be vital to ensure it doesn’t get killed off by an early frost or similar environmental factor.

The Science


Cannabis plants are described as long evening or brief day plants because they need a long period of darkness to cause the plant’s hormones to change from vegetative growth to flowering.

These light receptors are color pigments in the fallen leaves called Phytochrome Red (PR) and Phytochrome Far Red (PFR). These pigments acquire their labels from the types of light they take in.

PR absorbs red light in between 660 and 760 nm and PFR takes in red light between 760 and 800 nm. These two pigments chemically react to the light and trigger the plant to flower or not.

In cannabis plants, the typical presence of PFR turns off the flowering signal. The degree of PFR is what you can manipulate by adjusting the photoperiod and PFR is quickly created when plants are left open to light which contains far-red wavelengths.

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When there is light, the PFR and PR keep a balance. When the sunlight decreases, or the lighting schedule changes, the darkness slowly switches over from PFR to PR. As a result of this, PR levels gradually enhance and the PFR gradually lessen throughout the dark duration.

When the light returns, or if a small amount of much red light interrupts the dark period, the PR immediately changes back to PFR. If the plants lack light for long enough, the PFR will reduce beyond the tipping point.

This reduced level of PFR indicates to the plants that autumn is approaching, and the marijuana begins flowering. Basically, the existence of PFR, due to long hours of light and short hours of darkness, keeps the plants in the vegetative phase.

If the plant experiences sufficient hours of darkness, many of the PFR revert to PR; and the reduced degree of PFR indicates the plant hormones to begin flowering.

Photoperiod Manipulation for Maximum Yields

The natural transition of light to dark is very subtle, brightening or dimming slowly as the sun rises or sets, which may add unnecessary trauma to the plants. To remedy the issue, some grow light manufacturers have created special lights that fade in and out much like the sun. This not only helps reduce trauma but also helps increase cannabinoid and terpene production while helping cut back on electrical bills. Though the “smart light” technology is young, it may prove to be the next best thing in cannabis photoperiod manipulation.

Many cannabis grow lights are designed to mimic the type of light they would naturally receive outdoor during the various times of the season which can include varying wavelengths of light for the different phases of the grow. However, for the plants to produce flowers, the light cycle must be carefully timed, as well. While seedlings fare well under constant light, vegetative plants should be kept under a light cycle consisting of no less than 18 hours of light. While some growers will leave their vegetative plants under constant light, it is believed that the healthiest and most bountiful plants receive at least a few hours of darkness within a 24 hour period. Though this happens naturally as the earth begins to tilt away from the sun, indoor growers must time it themselves. The easiest way to do this is with simple timers that plug in between the light source and the power source. These timers can be set to turn on and off at the same time each day to help reduce the chance of grower error.

Many cannabis growers are beginning to find ways to utilize this method to get higher yields. A popular one is a method that involves 21 hours, 36 minutes day length with a 12-hour dark period, carried out with the aid of a 24-hour digital timer. This switch is done after about 10 weeks and leads to an almost 80% increase in lighting with a commensurate 40% increase in the flowering period. It is carried out at select times over 2 weeks while still maintaining the existing air, grow space and nutrient conditions, increasing the plant’s metabolism.


If you really are the weed enthusiast you claim to be, growing your own organic herb is the next step on the road to marijuana heaven. Manipulating growth will not influence potency; it will, rather, enable one to keep the plants’ energy focused on the ultimate goal of yielding flowers. For the home-grower, it means getting as much as possible out of those few seedlings growing in the garden.

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