A lot of growers are leaving money on the table due to a failure of imagination.

The idea that cannabis should always veg at eighteen hours of light and flower at twelve is wrong.

Cannabis plants do not require total darkness to flower and trivial light leaks will not cause plants to “herm”

For those of us who are prepared to replace cannabis mythology with plant science there are opportunities to improve yields by better understanding our plants photoperiod requirements.

Photoperiodism

A couple of brilliant botanists named Allard and Garner theorized that plants could measure daylength.

To test this hypothesis, they built light tight boxes, and placed them over field grown MM tobacco plants each afternoon, to simulate short winter days. The plants flowered, proving their theory correct.

This technique is referred to as light-dep and is still in use a hundred years later.

Short Day Plants

In the 1920’s Allard and Garner conducted a series of experiments to further characterize this phenomenon and coined the term Photoperiodism.

They determined that plants could be characterized as Short Day (SD) plants which flower during short days, Long Day (LD) plants which flower during long days, or Day Neutral (DN) plants which can flower regardless of day length.

Although tobacco is typically a DN plant, the MM mutant is a SD variant which enables continual vegetative growth during the long days of summer.

Most commercial cannabis cultivars are SD plants. This feature evolved to enable cannabis plants to flower and produce seed before the onset of winter in temperate climates.

Light cycle manipulation, and mechanical blackout systems enable growers to initiate flowering of SD cannabis plants at the optimal time for canopy development.

Night Interruption

Early photoperiod trials often produced surprising results.

The photoperiodic flowering response is determined by the night length (skotoperiod) rather than the day length (photoperiod). Use of lights to artificially lengthen the photoperiod will keep a short-day plant in vegetative growth.

A brief exposure to light during the middle of the skotoperiod has the same effect. This is referred to as Night Interruption (NI) lighting and is an efficient way to manipulate the photoperiod without upsetting your neighbors with continual light pollution.

Experiments with different colored lights reveal that red-light produces the strongest photoperiod response.

A cannabis plant which is exposed to a 12-hour (flowering) photoperiod will not flower if there is a 10-minute exposure to red light of a moderate intensity near the middle of the skotoperiod.

In other short-day plants, if the red-light exposure is followed by exposure to far-red light of a similar intensity and duration, the plant will flower as if the red-light interruption did not occur.

This cycle of red and far-red light exposure can be repeated many times over a dark period and the plant will respond to whichever light stimulus it was exposed to last.

This is referred to as a toggle effect and led to the theorized existence of a pigment called phytochrome which changes form when exposed to specific qualities (colors) and intensities of light.

Many decades later, phytochrome was isolated.

Phytochrome

Phytochromes exist in two interconvertible states depending on the quality of light they absorb.

Phytochrome red is the red-light absorbing state. When this pigment is saturated, it converts to Phytochrome far-red.

These states of phytochrome are constantly interconverting to mirror the light environment in a process known as photo equilibrium.

During the photoperiod, there is typically far more red than far-red light and phytochrome far-red is the dominant state. During the dark period the Phytochrome far-red slowly converts back to Phytochrome red.

This daily pattern of interconversion creates a rhythm which enables photoperiod perception.

Some horticulture LEDs now enable spectrum tuning so that growers can develop light recipes and provide different light spectra during various developmental phases.

A blue shifted spectrum will promote compact vegetative growth, while a red shifted spectrum promotes generative (flowering) development. Far-red light is typically used as an end of day treatment to quickly convert phytochrome at the onset of the skotoperiod.

Light Quantity

A plants photosynthetic potential is determined by the cumulative amount of light received each day.

The metric by which light quantity for horticulture production is measured is known as the Daily Light Integral (DLI).

This measure is a factor of the light intensity multiplied by the photoperiod duration and is expressed in mol. DLI targets are determined for various crops based on the point at which additional light no longer has a linear effect on yield.

For many crops, this target represents the point at which additional supplemental light becomes un-economical.

The target DLI for tomatoes, for example, is generally set at 25 to 30 mol. Cannabis is a light-loving crop with a proposed DLI target of at least 50 mol.

It is important to understand that plants will perform based on their limiting factors, and additional light will not improve performance unless other cultural conditions are met.

Inductive Photoperiod

The photoperiod required for a specific cultivar to flower is known as the critical inductive photoperiod.

Although some tropical landrace varieties may require 12 hours or more of darkness to flower, most commercial cultivars will flower with 10-10.5 hours of darkness.

Once a cannabis cultivar has been exposed to its critical photoperiod for four- or five-days, hormones and other signaling compounds form a cascade effect which commits the plant to flower formation.

Recent research shows that different flowering photoperiods have a big impact on yield and potency and that this response varied widely among the cultivars that were trialed.

• Cannatonic, a CBD rich cultivar more than doubled its yield when flowered under a 14/10 light cycle as compared to the control at 12/12. The CBD percentage of the flower was the same for both treatments.

• Hindu Kush, a THC cultivar saw an increase of biomass, matched by a decline in potency under a continual 14/10 light treatment as compared to the control.

• Northern Lights performed best when a 14/10 light schedule was applied for the first half of flower, then a 12/12 photoperiod for the second half. This treatment resulted in a 50% gain in yield with no reduction in potency.

It is important to trial any changes to cultivation inputs prior to applying them to commercial crops. The critical photoperiod can be measured using a small grow chamber or tent where the photoperiod is slowly reduced until flowering is observed.

Factors including irrigation dry-back, and climate cues enable growers to steer crop development by stimulating hormone signaling systems.

Light Leak Myth

This idea that cannabis is super sensitive to the presence of low levels of light during the skotoperiod is important to debunk.

Growers who have been force flowering cannabis outdoors for decades using blackout tarps understand that low levels of light have no effect on skotoperiod perception. Blackout tarps with holes big enough for babies to crawl through can provide adequate darkness.

This observation has recently been proven by Dr. Bruce Bugbee who demonstrated that none of the cultivars studied could perceive light below about 10 nanomoles.

This is about 5X as much illumination as full moonlight and is enough light to read a large print book.

If the genetics that you are using are prone to hermaphroditism, any type of stress can cause this expression, and those cultivars should be retired from production. Don’t blame the little red light on your dehumidifier though.

Blackout curtains have big effects on the cultivation climate and can make humidity control more difficult during the dark period.

Understanding that cannabis can tolerate incidental light during the skotoperiod empowers growers to close blackout curtains for the minimum amount of time required to induce flowering.

Autoflowers

Photosensitive Cultivars

When autoflowering and SD plants are hybridized, the offspring typically have an intermediate photoperiod trait.

These plants often flower at 15- or 16-hour photoperiods and share other characteristics including a propensity to begin flowering spontaneously when rootbound.

Other cultivars that may not have autoflower pedigrees can also exhibit photosensitive flowering.

Photosensitive cultivars can flower on the longest days at subtropical latitudes. This allows growers in these regions to skip expensive blackout systems and use minimal NI lighting to vegetate plants until they are the right size to flower.

In northern climates, photosensitive plants can produce quality flower before the close of the short seasonal window.

Conclusion

Cannabis production is moving out of the basement and into the sunshine.

This transformation will require growers to develop a better understanding of the botany of this incredible plant.

The techniques that served us well during decades of prohibition are holding us back in this brave new world.

Don’t let a lack of imagination limit your potential.