Cannabis Photoperiodism

Written By: Stewart Maxwell | 

Indoor cultivators have traditionally taken a binary approach to photoperiod manipulation for cannabis production. The photoperiod is usually set at 18/6 for vegetative growth and moved to 12/12 for flower initiation when plants reach an appropriate developmental stage. This is a valid orthodoxy in many situations.

As cannabis production moves from basements and warehouses, to fields and greenhouses, a more nuanced approach to photoperiod manipulation is useful in improving plant performance. To this end, a brief review of the biomechanics of cannabis photoperiodism is in order.

Most commercial cannabis cultivars are obligate short-day plants. 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. This is referred to as the inductive photoperiod.

Photoperiod perception in plants is achieved through a series of complex and elegant interactions among various light signaling systems (phyB, CRY2, etc.), and portions of the plant’s genome that are associated with flower promotion (CONSTANS, Ehd1, etc.). These signals are transduced by light-labile flavoproteins, or flavin chromophores, which exist in interconvertible states, as influenced by light quality and dark reversion. Put simply, plants can perceive the length of the skotoperiod by the relative state of light-labile phytochemicals that are present following dark reversion. This perception and response creates a feedback loop that results in flower initiation. This interaction is akin to a cascade effect wherein an external incident (change in photoperiod) causes a series of developments that, once established, are resistant to reversion.

These phenomena are easily demonstrated in cannabis. If a cannabis plant is moved from a non-inductive (NIP) to an inductive (IP) photoperiod for 2 days and then returned to a NIP, the plant will continue to vegetate. If the duration that the plant is exposed to an IP is increased to 5 or 6 days, it will begin to flower in the NIP before slowly re-vegging. This reversion to vegetative growth can take several weeks, and is associated with atypical phyllotaxy, single blade leaf sets, and other morphological phenomena. The result is less than aesthetically pleasing.

An enhanced understanding of the processes underlying photoperiodism in cannabis should lead cultivators to consider photoperiod strategies that will improve the yield and quality of their product.

Most cannabis greenhouses in Canada are Venlo greenhouses that have been hastily converted from olericulture production. As such, supplemental lighting levels are often suboptimal for cannabis flower cultivation during low light months. 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 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.

For greenhouse crops grown under suboptimal lighting conditions, the easiest way to increase the DLI is to increase the photoperiod. Most modern cultivars will happily flower under 14 hours of light. An effective lighting strategy to achieve this goal is to shift the photoperiod incrementally from an 18/6 NIP to a 12/12 IP over a period of 10 days or so. This will ensure a fulsome flowering response. The photoperiod can then be adjusted to a 14/10 IP for 4 or 5 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.

Another way for greenhouse cultivators to increase the DLI their crops receive is by using day-neutral cultivars (autoflowers). These cultivars initiate flowering independent of photoperiod based on developmental maturity and root expansion factors. Autoflowers will flower profusely under 20 hours of light. There are some complicating factors in deploying autoflowers for commercial production. These cultivars must be propagated from seed. Quality feminized seed must be sourced in sufficient quantities. Cultural practices need to be adjusted to accommodate a propensity for root-disruption flowering. As with any cannabis plant grown from seed, a minimum of 1 plant per thousand should be expected to express intersex traits. Pollination events can result if intersex plants are not quickly culled. Despite these limitations, autoflowers have a bright future in commercial cannabis production.

In addition to photoperiod manipulation, a clear understanding of cannabis photoperiodism can benefit greenhouse growers from a climate control perspective. Cannabis plants do not require absolute darkness to flower. Light levels below 20 to 30 lumens have no experiential 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.

For outdoor cannabis cultivators in northern latitudes, photoperiod manipulation is essential to the efficient production of quality flower. Most cultivars will not initiate flowering until mid-August in Canada. This results in crops that mature in suboptimal lighting conditions and ripen during peak seasonal pathogen pressures. These crops are usually harvested early, cannabinoid and terpene production is low, and the product is generally underwhelming. The logistical difficulties of processing an entire season’s product  in a short amount of time cannot be overstated.

Enlightened cultivators will not engage in this paradigm. It is more efficient and rewarding to flower outdoor crops during peak summer months. Outdoor harvests should begin in July and finish in early October. To accomplish this goal, a multitude of strategies can be employed.

Photoperiod-sensitive cultivars will remain an important component of field production. Many of these will initiate in July, for a September harvest. The best of these cultivars exhibit elevated resistance to fungal pathogens, as they were selected for in northern outdoor environments. Due in part to their compressed flowering periods, most do not express the terpene complexities that sophisticated consumers should expect, and are most suitable for extract purposes. Photoperiod-sensitive cultivars can be propagated clonally, but care must be taken when holding mother plants, as these have a propensity for root-bound flowering. These cultivars will often flower prematurely if planted out too early in the season.

Light deprivation methodologies are an important component in the outdoor cultivation of smokable flower. Light deprivation does not necessarily entail major infrastructure or access to power. Low-tech manual blackout systems have been employed in California for many years with great success. With minor modification, these techniques have proven valid  in BC, and can produce top-shelf smokable flower. If power and infrastructure investments are available, these systems can be automated at scale.

Day-neutral cultivars should also be considered for field production. The day-neutral flowering trait has been successfully ingressed into drug varietals over many generations, and the quality of some of the resultant varietals is on par with photoperiod obligate plants. Autoflowers have come a very long way since Lowryder.

An additional strategy that can be deployed for earlier ripening of field crops is to plant out cultivars that have already been induced to flower. This involves initiating small plants within a light deprivation structure before moving them to the field. At 49 North, this usually involves initiation at the end of July for transplant outdoors mid-August. Cultural practices should be modified to limit root disturbance, and densities adjusted to accommodate smaller plants. This is an effective way to flower a second crop, following early autoflower or dep crops, with harvest occurring by the end of September.

Photoperiod manipulation is one of many tools available for cultivators seeking to realize the potential of their crops.

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