Photoperiodism: Principles and Applications in Cannabis Cultivation

By: Due Diligence Horticulture

December 11, 2024

Photoperiod, or day-length, is an important cue used to regulate seasonal activities by many organisms, including birds[1], insects[2], and even some microbes that only live for a few hours[3]. In plants, photoperiod is crucial for timing key developmental processes such as flowering, tuber formation, and bud dormancy. Plants measure and respond to daylength to synchronize their reproductive cycles with favorable environmental conditions, ensuring greater survival and reproductive success. This review explores the principles of photoperiodism, focusing on how plants perceive and respond to photoperiod. These principles are then applied to cannabis cultivation, where photoperiod is a key regulator in determining yield, morphology, and chemical quality. We will examine traditional practices like the 12/12 light cycle and explore novel approaches that leverage extended photoperiod to improve cultivation outcomes.

Photoperiodic Responses of Flowering Plants

Plants are categorized into three main groups based on their photoperiodic flowering response: short-day, long-day, and day-neutral. Short-day plants flower if day length is less than some critical photoperiod. Long-day plants flower if day length is greater than some critical photoperiod. The effects of light pollution and night break lighting are a topic for another review. Day-neutral plants flower independently of photoperiod.

Short-Day (SD) Plants: 

These plants require long nights to flower. 

• Poinsettia: A festive and photoperiodic plant, poinsettias require short days to flower and develop the iconic red bracts characteristic of the holiday season (Figure 1)[4].
• Chrysanthemum: Commonly used in horticulture, chrysanthemums provide much needed color as the days shorten in late summer and fall.
• Soybean: An important agricultural crop that flowers when daylength decreases. Due to extensive breeding efforts, a wide range of cultivars are available that can flower even under ‘long-day’ conditions.
• Cannabis: Classically considered a short-day plant, the flowering physiology of cannabis is complex, but this complexity may offer opportunities to increase yield and improve efficiency. More on Cannabis later.

Long-Day (LD) Plants:

These plants require short nights to flower. 

• Spinach: Bolts and flowers rapidly under long-day conditions. This response is also highly influenced by high temperature, making it more well suited to the cooler climate and shorter days of spring and fall.
• Wheat: Many varieties are long-day plants that flower as days lengthen in spring and early summer. Continuous light accelerates seed set in controlled environments.
• Petunia: A common bedding plant, petunia is classically considered a long day plant that requires at least 13 hours of light to flower (Figure 2).

Day-Neutral (DN) Plants: 

Flowering of day-neutral plants is usually triggered by plant maturity, light intensity or temperature. 

• Amaryllis: Another festive flower, amaryllis flowering is more regulated by temperature and a period of dormancy than by photoperiodism (Figure 3). 
• Cucumber: Flowering is influenced more by temperature and growth conditions than by photoperiod. However, photoperiod influences sexual expression of flowers, with long days promoting more male flowers over female flowers.
• Rose: Most varieties are day-neutral, with high temperature and light intensity being more important than photoperiod for driving the flowering response.
• Cannabis: Some cultivars of cannabis, derived from the ruderalis subspecies, can flower regardless of photoperiod. These cultivars, known as ‘autoflowers,’ have the potential to be a valuable tool for growers who do not have good control over photoperiod or want to push plants under continuous light to increase yield. More on autoflowers later.

Facultative vs. Obligate Plants:

The classification of short-day, long-day, and day-neutral plants is not always straightforward. Plants can be further categorized as either facultative or obligate based on their photoperiodic requirement. Obligate plants have a strict photoperiod requirement—short day or long day—to initiate flowering. If these conditions are not met, obligate plants will not flower. In contrast, facultative plants respond more flexibly – while they have a preferred photoperiod for optimal flowering, they can initiate flowering irrespective of photoperiod, albeit less efficiently or later[5].

Perception of Photoperiod

Phytochrome and cryptochrome are key photoreceptors that regulate photoperiodic flowering in plants. 

Phytochromes (primarily PhyA and PhyB) are sensitive to red and far-red light and exist in two interconvertible forms: the inactive form (Pr), which absorbs red light (around 660 nm), and the active form (Pfr), which absorbs far-red light (around 730 nm). When phytochrome absorbs red light, it is converted from Pr to Pfr, the biologically active form that promotes or inhibits flowering depending on the plant species. In long-day plants, Pfr often promotes flowering, while in short-day plants, Pfr can inhibit it. During the night, Pfr slowly reverts back to Pr, effectively resetting the system. Temperature also plays a role in this process, as warmer temperatures accelerate the reversion of Pfr to Pr, thus influencing flowering responses by altering the balance between the active and inactive forms (Figure 4).

Cryptochromes (Cry1 and Cry2) respond to blue light and are also involved in the regulation of flowering, particularly by interacting with the circadian clock to measure photoperiod. 

Both phytochromes and cryptochromes influence the expression of genes related to the synthesis of florigen, a signaling molecule that promotes flowering. The interaction between the plant’s internal circadian clock and external light cues regulates the timing of florigen production. In many plants, the CONSTANS (CO) gene acts as a key mediator in this process. In long-day plants, CO accumulates under long-day conditions, promoting the expression of FLOWERING LOCUS T (FT), which leads to flowering. Conversely, in short-day plants like cannabis, flowering is induced when CO levels accumulate under short-day conditions[6].

Classic 12/12 Schedule and Application in Cannabis Cultivation

A 12-hour photoperiod, often referred to as the 12/12 schedule, is a widely adopted practice for inducing flowering in SD plants, including cannabis. It is effective in both large-scale commercial cultivation and small home grow operations because of its simplicity, broad applicability, and efficacy initiating flowering of SD plants. It is worth noting, however, that the natural photoperiod, except at the equator, is greater than 12 hours during the growing season (Figure 5). This means that most plants have evolved to flower under photoperiods longer than 12 hours and suggests that natural flower responses are influenced by multiple factors and relative changes over time rather than a fixed photoperiod.

In the context of commercial cannabis cultivation, photoperiod is key for optimizing flowering time and maximizing yields. Cannabis is classically considered a short-day plant, so, by definition, it requires long nights to initiate flowering. However, recent research in controlled environments argues that flower initiation is independent of photoperiod. Solitary flowers develop on ‘vegetative’ plants, regardless of photoperiod (Figure 6). The shift from ‘vegetative’ to ‘reproductive’ growth has been shown to happen at the seventh node[7]. Instead, short days promote intense branching of the inflorescence[8]. While this may seem like an academic distinction (and it is), it again highlights the complex physiology of flowering, and the role photoperiod plays. From a practical perspective, cannabis is a short-day plant, since nobody is harvesting solitary flowers.

Genetic Variability in Critical Photoperiod

The critical photoperiod at which cannabis initiates flowering varies widely among cultivars and even among plants within a cultivar propagated from seed. Figure 7 highlights this variability within seed propagated plants. These were two plants of the high-CBD cultivar ‘remedy’ propagated from seed and grown in the field. This photo was taken in early September 2019, and despite being from the same seed lot, one plant was approaching full flower while its neighbor was almost entirely vegetative. 

Variability in seed aside (seed production has come a long way and these issues have since been worked out by reputable companies through breeding), the critical photoperiod of a cultivar is related to genetic origin. Varieties that originated from lower latitudes with longer growing seasons tend to have a shorter critical photoperiod, while varieties originating from higher latitudes tend to have a longer critical photoperiod and therefore may be successfully flowered under longer photoperiods. 

Zhang et al. (2021) evaluated a wide variety of high-CBD and fiber cultivars and demonstrated wide variability in critical photoperiod among cultivars, with some nearly all cultivars having a critical photoperiod of 14 hours or more (Figure 8)[9]. This is incredibly valuable for controlled environment cannabis cultivation, as it demonstrates the possibility to cultivate at longer photoperiods, increasing the total amount of light delivered to the crop and potentially increasing yield without necessarily increasing light intensity.

Increasing Yield with Extended Photoperiods

While the 12/12 schedule is effective, reliable, and operationally convenient, recent research suggests that longer photoperiods have the potential to increase yield and quality in some cultivars. For example, Ahrens et al. (2023) demonstrated that extending the photoperiod beyond the traditional 12/12 cycle may increase yield in some cannabis cultivars. However, significant variability exists among cultivars regarding their response to extended photoperiods, emphasizing the need for tailored approaches based on genetic differences. A key limitation of the study is that it did not take plants all the way to harvest; the study was concluded after 21 days of treatment, which limits the ability to fully assess long-term yield and cannabinoid content outcomes[10].

In a follow-up study[11], the response of two cannabis cultivars to photoperiods of 12 or 13 hours throughout harvest was evaluated (Figure 9). Flowering was slightly delayed in one cultivar, but not the other, and both cultivars showed a substantial increase in yield at a 13-hour photoperiod compared to 12-hours. However, the difference in daily light integral (DLI) between the treatments makes it challenging to separate the specific effects of photoperiod from the overall impact of increased light exposure. The substantial yield increase observed in the 13-hour treatment could be attributed to greater photon capture, as the plants under this treatment were larger, allowing them to intercept more light, in addition to receiving more total light due to the extended photoperiod.

One potential strategy to increase cannabis yield without negatively impacting development or reverting to vegetative growth is to increase the photoperiod from 12 to 24 hours during the final week of flowering. This approach takes advantage of the fact that plants are less sensitive to photoperiod changes towards the end of their flowering cycle, allowing for increased light exposure without interrupting the flowering process. Assuming an 8-week flowering cycle, with the first 7 weeks at 12 hours of light per day and the final week at 24 hours, the total light delivered over the entire cycle would increase by 12.5% compared to 8 weeks at a 12-hour photoperiod. Assuming this increase in light directly translates to increased yield (a relatively safe assumption), growers could potentially increase yield by 12.5% without increasing light intensity, but further research is needed to evaluate potential changes in cannabinoid or terpene concentrations. Nevertheless, it is an intriguing idea.

Autoflowering Varieties

Autoflowering cannabis varieties, derived from the ruderalis sub-species, provide an alternative to traditional photoperiod-sensitive strains. These plants flower (develop dense inflorescences) independently of daylength, making them a versatile choice for growers with power constraints or those cultivating in environments where natural light patterns are less predictable. Autoflowering plants have a shorter life cycle (Figure 10), often completing growth in less than 10 weeks, making them attractive for rapid production cycles.

Recent research has uncovered important genetic information about autoflowering cannabis plants. For example, Dowling et al. (2024) identified genes linked to flowering, including a gene similar to FLOWERING LOCUS T (FT), which plays a major role in determining when plants start to flower[12]. The identification of these genes facilitates a deeper understanding of how autoflowering plants flower regardless of the day length, unlike traditional cannabis varieties. In autoflowering varieties, these genes regulate flowering time based on the plant’s age and internal signals rather than photoperiod. The FT gene and related circadian clock genes coordinate the development, promoting flowering after a certain growth period, not photoperiod. Understanding the genetic basis of autoflowering traits enables growers to leverage these varieties for consistent flowering regardless of daylength, allowing for more flexible cultivation schedules and potentially reducing resource costs, making them particularly beneficial in environments with unpredictable light conditions

Autoflowering plants offer advantages for commercial cultivation, such as higher potential yields without the need for increased light intensity or installing additional lights. However, there are some challenges with autoflowering varieties in large-scale production. Unlike photoperiodic cannabis, which can be maintained in a vegetative state and cloned through cuttings, autoflowering plants must be grown from seed every time. This makes it harder to maintain consistency in crop quality and yield. Despite these challenges, ongoing research may help overcome these limitations and make autoflowering varieties more practical for large-scale cannabis production.

Conclusion

Photoperiod is a key regulator of flowering in many plants, including cannabis. The complexity of photoperiodic flowering responses is mediated by phytochrome and cryptochrome, which interact with internal signals to regulate flowering.  Species respond to photoperiodism in various ways, with short-day, long-day, and day-neutral classifications highlighting the diversity of flowering mechanisms. The photoperiodic flowering response of cannabis is complex, but it can be practically defined as a short-day plant. The classic 12/12 light schedule is effective, but recent research suggests that longer photoperiods have the potential to increase yield in some cultivars. Autoflowering varieties offer an alternative that bypasses photoperiod requirements altogether, providing additional flexibility for growers. Future research should focus on refining photoperiod control strategies to optimize yield, quality, and economics in cannabis cultivation.

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