8 Factors Affecting the THC Potency of Your Cannabis Flowers

One of the holy grails of cannabis cultivation is achieving consistently high concentrations of THC. A recent trend in both the recreational and medical markets is the increase in the threshold for what defines a high concentration.

In the medical sector, anything over 18% THC was considered on the higher end of the potency spectrum. However, recent advancements in breeding and the continuous pursuit for even higher testing cultivars that are seen in the adult-use market have meant this number now pales compared to emerging prized cultivars that are testing closer to 25% THC.


Whether or not the pursuit of potency will reach a ceiling, or if the market’s obsession with THC concentration is a suitable marker for quality, are discussions for another day. Instead, this article looks at some of the factors that can affect cannabinoid synthesis and the subsequent potency of flowers as measured by % THCA (the precursor acid form of THC) and %THC of cannabis flowers.

The focus of this article is the cultivation processes that can be managed in the grow room, although it is by no means an exhaustive list. We won’t focus too much on post-harvest processes related to packaging or processing methods like freeze-drying and extraction that could also impact the potency of flowers. By the end of this post, you should be familiar with several biological, environmental, and manufacturing determinants of potency. Use this list to optimize your current processes, identify potential risks, and guide you in what to look out for if the potency of your cultivars starts changing. Learn more at BigLeafConsultants.com.

A Quick Note on Secondary Metabolite Function and Plants

Cannabinoids like THC and CBD and compounds like terpenes and flavonoids are all types of secondary metabolites. Their accumulation in cannabis occurs during the flowering stage via various processes that change throughout a plant’s life cycle. This suggests that their levels alter depending on their various functions within the plant (Feder et al, 2021).

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One of the most popular theories about THC is that it protects cannabis from UV radiation (Lydon, 1987, Eichhorn Bilodeau et al, 2019). It has also been that suggested THC prevents a loss of moisture and protects plants from high temperatures and heat stress (Gülck and Møller, 2020).

Another theory is that its production as sticky resin in glandular trichomes prevents insect herbivory (Gülck and Møller, 2020).

Finally, some work has shown THC can induce cell death in leaf cells as part of the plant defense system against pathogens (Morimoto et al., 2007).

In spite of some promising observations, we still don’t know exactly why cannabis produces THC, but all theories suggest it is broadly involved in plant defense as levels change as a direct response to stress.

Factor #1: Genetics

As the most obvious and well-known factor, we had to put it first. The profile of your cultivar when it comes to secondary metabolite production is inherently pre-determined by its genes. In cannabis, the total potential THC levels are determined by the levels of THCA synthesis within the plant which are under genetic control (De Meijer 2014).

It is important to remember that the information provided by breeders and clone nurseries on THC potency should be used as a guideline for the maximum potency of a particular cultivar and should not be viewed as a guarantee.

Some of the better genetics distributors will also be transparent when it comes to information regarding the specifics of the cultivation methods used to achieve THC levels. This not only helps to manage your expectations when growing new cultivars for the first time but also reaffirms that the potency of a genetic is tied to a number of other contributing factors.

Although genetics set the scene, the overall performance of a cultivar is also heavily influenced by its environment. Therefore, the growing conditions and a host of other details also play a part in determining potency.

Factor #2: Weeks in Flower/Harvest Time

The time plants spend in flower is also another well-known determinant of flower potency. When researching cultivars, it is common to find information relating to their flowering time. This can be used as an approximate estimate of the expected harvest window. But remember that this is also intrinsically tied to a whole host of other factors.

Generally speaking, the potency of cannabis flowers rises as flowering progress. However, what is less known is that many studies have identified that cannabinoid concentrations in some cultivars actually reach a point where they decrease with increasing yield. This phenomenon is known as the cannabinoid dilution effect. It means that waiting to harvest your crop until the optimum harvest window, as recommended by genetics distributors and other in-field observations such as trichome and pistil color, may not actually correspond to peak potency.

In reality, cultivars may reach their peak potency before ripeness meaning the decision to harvest could become a compromise between THC concentration and final yields. This highlights the importance of in-process testing as the most reliable marker for peak potency.

Weekly testing from mid-flower allows cultivators to generate and plot data on a curve to see how potency is changing between weeks. This facilitates a more informed decision with regard to the ideal harvest window for specific cultivars.

Chopping plants too late can also decrease potency, since missing a harvest window can result in a degradation of THCA into CBN which can be caused by prolonged exposure to light (Ferrer 2020).

Factor #3: Plant Architecture/Location of Flowers on the Plant

Canopy management techniques including topping and defoliation have been used by cultivators for years. These methods are reported to increase yields and improve the density of flowers by improving light penetration to parts of the plant below the canopy.

Studies have recently begun confirming what growers have long suspected: that the cannabinoid profile varies spatially within the plant.

Initially, it seemed that the concentrations of cannabinoids including THC followed a pattern of “vertical stratification,” and increased with height and were the greatest in apical flowers found at the top of the plant (Namdar et al 2018, Bernstein et al 2019). Flowers on the bottom parts of plants were found in one study to have half the THC content of flowers of those on the top (Bernstein et al 2019). Another study showed light intensity was up to 60% less in lower branches compared to the plant apex leading to the popular conclusion that light penetration affects THC content (Namdar et al 2018).

These observations seemed to confirm the common theory that flowers higher up in the canopy and exposed to the most light were higher in THC due to the proposed role of THC as a UV protectant or plant sunscreen (Lydon 1987). However, this seemingly probable notion has been challenged by more recent data from a group of researchers in Israel.

Trials manipulating the plant architecture of plants using different degrees of pruning and defoliation have found these techniques can indeed standardize the concentration of THC by improving light penetration to lower parts of the plant. But, quite surprisingly, they also found that cannabinoid concentrations were not always the highest in apical flowers.

In five out of the eight architecture manipulations (including a control), the concentrations of THC were greater in locations lower than the apical meristem (Danzinger and Bernstein 2021b). This confirmed the findings from another similar experiment from the same group on different high THC cannabis genotypes, where they also found cultivating smaller-sized plants improved the overall uniformity of their chemical profile (Danzinger and Bernstein 2021a).

It’s important to remember that cannabinoid concentrations in flowers are affected by various environmental (exogenous) and developmental (endogenous) effects. Environmental effects include microclimates related to factors including different light intensities at different parts of the plants and also different air circulation, temperatures and humidity.

Developmental effects include changes in the chemical maturation of flowers on the same plant that occur due to their slightly different individual development and how this contributes to their cannabinoid profile. The exact mechanisms that cause developmental changes to occur, whil obeserved, remain unclear.

In short, determining how and why canopy management techniques alter THC concentration is complicated and cannot simply be put down to the effects of light intensity. To simplify we have chosen to capture this contributing factor under the plant architecture heading as a way of highlighting what research has observed but is yet to convincingly explain.

Gotta love that science rabbit hole!

Factor #4: Hemaphrodism and Pollination

There is a reason why “sinsemilla” or “sensimilla” Cannabis (from the Spanish word meaning “without seeds”) has been cemented in innumerable songs and refers to top-quality bud. The diversion of energy from cannabinoid synthesis to seed production that occurs when a cannabis plant is pollinated does indeed decrease the cannabinoid content of its flowers.

One trial found fertilization decreased the total cannabinoid content of a high THC variety by 75% with significant decreases in the THCA and THC concentrations specifically (Feder et al 2021). Interestingly, changes were greater in the high THC chemovar compared to the high CBD chemovar.

The fertilization trial also tested the viability of both donor pollen from male plants and pollen from male flowers from induced hermaphrodites. Both were effective in pollination (Feder et al 2021).

Hermaphroditism refers to a cannabis plant that displays both female and male reproductive organs and is a source of concern since it can lead to self-pollination and the pollination of other nearby plants. It can be thought of as a plant’s last-ditch attempt to fulfil its evolutionary demand to procreate which becomes more desperate when it feels under threat. Although it can occur spontaneously, it is more commonly associated with various environmental stressors, including an interruption of the photoperiod, temperature extremes, and nutrient deficiencies (Punja and Holmes 2020).

It is therefore important to regularly scout your crop for male flowers if your plants are subjected to unforeseen stress. This should be part of a preventative procedure that aims to eradicate male flowers before they open and you risk pollinating your entire crop.

It is also worthwhile keeping a careful eye on plants that are germinated from seed, including feminized seeds, as there is always a probability some of these could either be genetic males (depending on the quality of the manufacturing process) or could spontaneously hermaphrodite.

Factor #5: Hop Latent Viroid (HpLVd)

Identified in Europe in the late 1980s, this pathogen was found to affect the production of secondary metabolites including acids, resin, and essential oils that were critical for beer brewing. It also impacted both the yield and overall quality of infected plants. Recently, HpLVd has become a well-known pathogen affecting cannabis and has been identified as the causal agent of symptoms of a disease colloquially known as dudding. These symptoms include reduced vigor, stunted growth, reduced rooting, abnormal branching, brittle stems, reduced flower mass and overall yields, malformed and discolored leaves, and importantly for this article, reduced trichome and cannabinoid production.

Pinnacle Analytics, a cannabis and hemp testing laboratory in Oregon, claimed they have sampled infected material that showed a concentration of cannabinoids of between 6% and 50% in infected plants compared to healthy plants.

The primary means of spread of HpLVd is mechanical means through infected tools and equipment. Therefore, prevention of the pathogen relies heavily on strict hygiene and sanitization procedures and sourcing new genetic stock from tissue culture since this is currently the best method to guarantee pathogen-free plants. Identifying HpLVd is challenging and the identification of the viroid is through genetic testing using RT-PCR assays. Currently, this is not a service easily accessed in Europe.

Factor #6: Drought Stress

Due to their sessile (i.e. immobile), nature, plants can rapidly respond to alterations in their environment and regulate their behavior to withstand changes in environmental conditions.

In some circumstances, when a particular stress is controlled and carefully monitored, it can be used as part of management practice to elicit changes in the secondary metabolite profile of plants (Gorelick and Bernstein 2014, Gorelick and Bernstein 2017). This has been successfully demonstrated in various medicinal and aromatic plants (Kleinwächter and Selmar 2015).

Research on controlled drought-stress in cannabis has shown that a single application lasting 11 days during week seven of flower increased the THCA concentration in tested plants by 12% (Caplan et al 2017). Rather surprisingly, it did this without negatively impacting yield.

Defining the timing, intensity, and duration of drought stress are essential as part of primary research into the potential benefits of the technique which can differ between cultivars. Applying drought during the vegetative period can negatively impact your final yields and it should instead be applied during mid-late flower (Caplan et al 2017).

A word of warning: inducing stress is a risky and advanced technique for more seasoned cultivators. It should always be attempted on a smaller test plot of plants before being used on an entire room.

Factor #7: Nutrients

Breakthroughs in nutrient research on cannabis are beginning to shed light on how different concentrations of macro, secondary, and micronutrients affect THC potency. These insights have improved our understanding of how nutritional management is related not only to final yield but also to phytocannabinoid production.

The cannabinoid dilution effect that has described the general trend that increasing yields leads to decreasing potency is linked to the effects of increased nutrient availability. Remember that secondary metabolites like THC are thought to be produced by plants in response to stress as part of their defense mechanisms. Therefore, it follows that a reduced nutrient availability can trigger this response.

Many researchers point out this is cultivar-specific since the optimum ranges recommended for specific nutrients by studies sometimes differ. The effect of macronutrient availability on the phytocannabinoid profile could be cultivar specific since the nutrient profile and requirements of different genotypes is unique.

Alongside differences in the specific growing conditions used in experiments, this explains why the recommended nitrogen levels for both phytocannabinoid and flower yields are different between papers.

Factor #8: Testing Procedure

Last but not least, we are ending our list with the very prickly problem of cannabis potency testing protocols. Although testing protocols don’t affect the potency of your cannabis flowers per se, the different approaches to sampling preparation and potency testing from methods including gas chromatography (GC) and high-performance liquid chromatography (HPLC) can result in differences in the reported potency of the same product (Dotzert, 2019).

Currently, there is no single analytical method to asses potency, and this lack of standardization of testing procedures in the industry means there is significant variability between protocols (Jikomes and Zoorob 2018). This has caused critics to question the reliability of lab reports they believe are increasingly compromised, especially in a climate where consumers and distributors place %THC as the biggest marker of quality. This is becoming more concerning amidst the growing controversy that questions the legitimacy of testing protocols given the emerging practice of lab shopping, i.e. when facilities look for labs whose procedures give higher potency results allowing them to sell a product at higher prices. Combined with this are the corrupt practices that are reportedly used by laboratories to manipulate results to suit client requests for high numbers, so-called potency inflation. (Jikomes and Zoorob 2018).

To complicate matters further, the problem of standardization doesn’t only apply to testing protocols but also to the lack of homogeneity in samples that are sent to laboratories. Even if your facility’s testing procedures are internally validated, don’t be surprised if the third-party testing results of your flower are even slightly different.

What about the Role of Light Intensity and Light Spectrum?

Although plants have been shown to respond to light stress by producing and accumulating secondary metabolites (Thirumurugan et al 2018), the effects of wavelength and intensity on the production of secondary metabolites including THC in cannabis are limited and not well understood. Much of the research investigating how the different qualities of light (intensity, photoperiod, and spectrum) determine THC synthesis and the final concentration in flowers is contradictory.

For example, the well-known plant sunscreen theory for the role of THC in cannabis has not been confirmed by more recent trials that exposed plants to varying doses of UV radiation. These found no differences in the THC levels of tested cultivars (Rodriguez-Morrisson et al 2021b, Llewellyn et al 2021). Another trial also found UV radiation increased cannabinoid levels other than THC (Maganini et al 2018).

Some of the research out there measures THC potency in leaves instead of flowers. A majority do not test intensities over 500μmol·m−2·s−1 so are not representative of the parameters most commercial facilities employ (Desaulniers Brousseau et al 2021).

One of the difficulties in evaluating the influence of light quality is the lack of standardization that exists between experiments and the light intensity (PPFD) they test, the specific wavelengths of light they use (spectrum), and the timing of application. This can prevent meaningful comparisons between experimental results which are also impacted by the different cultivars used.

Light spectrum research is coming along nicely thanks to the new era of LED technology which has granted researchers unparalleled control over wavelengths of light.

As the industry continues to develop and more Cannabis specific research is done, there is no doubt that more factors affecting the THC potency of flowers will become apparent. These include root-zone oxygen availability, elicitors, CO2 concentration, temperature, plant density, and irrigation strategies.

The effect of interlinked factors in influencing THC in a way that is greater than the sum of their parts – could also be revealed. Especially if theories like the entourage effect are experimentally demonstrated, this could be particularly exciting. Observations of the dilution effect are also revealing that some factors such as nutrition and time in flower, need to be adjusted to reflect production goals.

The era of controlled environment agriculture coupled with an advancement in breeding and biotechnology has allowed researchers and cultivators to continuously push the limits of possibilities with regard to optimizing cultivation for specific plant compounds. Future frontiers include developing specific cultivars capable of harboring a pre-planned group of secondary metabolites (Romero et al 2020). This research is the focus of a partnership between the World Horticenter and Wageningen University in their project “Cultivation for Compounds.” It has a special focus on how cultivation techniques can be used to steer plant growth into producing compounds that could be of potential therapeutic interest.

Discussing potency, like countless other topics in the cannabis cultivation space, is a complex task that requires navigating murky waters. The current controversies surrounding the reliability of laboratory testing are a prime example of this.

Consistency is essential for producing a product that is in high demand in a recreational market but also for patients who are dependent on a particular type of cannabis-derived therapy. One of the biggest limitations in generalizing the results of Cannabis-specific research remains the role of cultivar-dependent behavior.

The factors listed in this article are part of a grower’s investigative toolkit and are a good starting point for those looking to investigate any changes in potency that might occur from batch to batch.