Why Cannabis Photoperiod Considerations Matter

Commercial cannabis growers should quantify the specific inductive photoperiods required to initiate flowering in the cultivars being grown at their facilities.
Photo by Stewart Maxwell

Most commercial cannabis cultivars are obligate short-day plants. This means that they have evolved the ability to synchronize their physiological flowering response to seasonal changes in day-length to optimize reproductive success.

Photoperiodic flowering responses in these cultivars are regulated by the length of the skotoperiod or dark period. When the skotoperiod exceeds a critical threshold (about 10 hours for most), flowering is initiated. The photoperiod duration required to initiate flowering for a specific cultivar is referred to as the Inductive Photoperiod (IP).

Cannabis growers typically influence flower initiation through use of supplemental lighting and blackout curtains to control photoperiod. The photoperiod is usually set at 18/6 (light hours/dark hours) for vegetative growth and moved to 12/12 for flower initiation when plants reach an appropriate developmental stage. While this approach remains valid for indoor cultivation, for commercial greenhouse operators, a more dynamic approach to photoperiod control can increase profits and efficiencies.

The Chemistry of Light Perception

Photoperiod perception in plants is achieved through interactions among pigments (phytochromes and cryptochromes) and portions of the plant’s genome that are associated with flower regulation. These pigments are light-labile, and exist in interconvertible states, as influenced by light quality and dark reversion. This chemical interconversion is continuous and mimics the changing light environment in a process known as photoequilibrium.

Put simply, plants can perceive the length of the skotoperiod by the relative state of various pigments that are present following dark reversion. This information is transduced by metabolic pathways involving hormone signaling, as mediated by the plant’s genome. This perception and response create a feedback loop that results in flower initiation. This interaction promotes a cascade effect wherein an external incident (change in photoperiod) causes a series of developments that, once established, are resistant to reversion.

Cannabis Daily Light Integral Potential

A plant’s photosynthetic potential is determined by the total amount of light available in the environment during a given photoperiod. The metric by which light sufficiency for horticulture production is measured is known as the Daily Light Integral (DLI). This measure is a factor of the light intensity (PPFD) 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 of 40 to 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 (CO2 levels, in particular) are met.

Increase DLI by Increasing Photoperiod

For greenhouse crops grown under supplemental lighting conditions, the easiest way to increase the DLI is to increase the photoperiod. Most modern cultivars will happily flower under 13.5 hours of light. An effective lighting strategy to achieve this goal is to shift the photoperiod to a 12/12 IP for six to eight days to ensure a fulsome flowering response. The photoperiod can then be adjusted to a 13.5/10.5 IP for four or five weeks, and then back to a 12/12 IP for the final ripening stage. This strategy will result in an increased DLI during the critical weeks of floral expansion and the attendant linear increase in yield.

Genetics Dictate Photoperiod Requirements

While most cannabis plants are short-day plants, the specific photoperiod requirements vary among cultivars. All commercial cannabis growers should quantify the specific inductive photoperiods required to initiate flowering in the cultivars being grown at their facilities. This metric can be easily determined by exposing target cultivars to a gradually descending photoperiod within a grow chamber or tent. This data point can significantly affect crop production strategies and yields.

The use of day neutral (DN) cultivars, or autoflowers, can increase DLI in supplemental lighting conditions. The ability of these cultivars to flower under 20 hour or greater photoperiods increases their ability to synthesize cannabinoids during some seasonal conditions, particularly under low-level supplemental lighting. As this type of germplasm is further developed, it may eliminate the requirement for many producers to install and maintain expensive blackout systems.

Photoperiod and Climate Control

In addition to DLI considerations, an enhanced understanding of cannabis photoperiodism can benefit greenhouse growers from a climate perspective. Use of supplemental lighting, and blackout curtains have profound effects on greenhouse climates. Cannabis plants do not require absolute darkness to flower, and light levels below 20 to 30 lumens have no effect on skotoperiod perception. This factor, in combination with an understanding of the critical skotoperiod for the cultivars in production, allows the use of more aggressive blackout gap strategies to moderate temperatures and humidity.

Cannabis is an emergent crop with important economic implications. Producers do not have a century of trial and error to rely on for guidance as we do for other crops. An understanding of the botany underlying plant response provides opportunities for the evolution of greenhouse cultural systems currently in a nascent state.