Nowadays, after the legalization of medical and recreational use in many countries, Cannabis cultivation is a rapidly growing industry, with many farmers and growers looking to increase their yields in order to meet the growing demand for the plant. However, previous restrictions led to a void in scientific research, leaving scientists and growers with an open question: What is the difference between the maximum possible flower yield compared to current yields? And how can we close the yield gap? 

First and foremost, as it was explained in previous newsletters, big steps need to be done to provide a detailed knowledge base about existing cannabis strains, at the molecular level, to understand how the genotype influences the chemotype. Yet, production methods seem to influence yield and cannabinoid content too, showing that more research is needed to understand how growth conditions affect Cannabis. Recently, a new meta-analysis was published, which gathered data from previous research on cannabis cultivation to identify key factors that affect yield (1). Results showed that the variety plays a major role, but production conditions and methods can also highly affect yield and cannabinoid content (1).  In this newsletter, we’ll show different environmental factors to take into consideration when growing cannabis for medicinal/recreational purposes, such as temperature, light intensity, light spectrum, light type, growing medium  and fertilization. 

When looking at temperature, C. sativa seems to have the highest photosynthesis rate around 25-30°C (1,2). However, the authors of the meta-analyses argue that increasing temperature might trigger a protective reaction in the plant, known as stress response, resulting in increased production of THC  (1). Indeed, higher THC levels have been previously achieved by exposing  Cannabis to abscisic acid, a plant stress hormone (3). Also, when grown in stress conditions, such as small pots and high light intensity,  cannabis plants produce higher levels of THC and CBD, showing the accumulation of these compounds might be part of the plant’s stress response (1,3). 

When grown in industrial conditions, the use of ideal lighting is crucial to increase cannabis yields, as it can optimize costs and the space required for cultivation. While cannabis plants grown in high light intensity usually have higher  THC/CBD levels, the yields (per watt) obtained from the same plants seems to decrease (1). Not only light intensity, but also the length of the flowering period plays a crucial role in cannabis cultivation, in which plants that go through a longer flowering period and a shorter vegetative period seem to produce higher yields and increased THC levels per square meter. This is attributed to the plant spending more energy on growing  buds , rather than  stem or leaves  (1,4). Moreover, additional factors that influence cannabis yield are the type of light used and the light spectrum that cannabis responds to. High pressure sodium (HPS) lights produce higher yields than metal halide (MH) lights, which have a lower luminous efficiency (1). Whereas for the light spectrum, like all other plants, cannabis is responsive to the spectral composition of their source of light, but still today the best light spectrum to achieve optimum yields has yet to be found  (1). Modern lighting sources, like LED, come with many advantages, including high energy efficiency and longer life, but especially, with the possibility to customize their wavelengths. This is important in research settings, to test the effects of different wavelengths on plant growth and development (1). Positioning of light is also known to make a difference. In traditional horticulture, lighting is usually placed on the ceiling, which reduces light efficiency on the sub-canopy while resulting in highly light-saturated top-canopy (1,5). Since LEDs emit little heat compared to HPS lights, these lights could also be placed at a lower level, exposing the subcanopy to stronger lights, thus increasing the proportion of light used for photosynthesis and consequently cannabis bud yields (1,5).

Another important factor to take into consideration when growing cannabis in industrial settings is the impact of the growing substrate. While soil is the traditional and most commonly used growing medium for cannabis cultivation, it is also the most susceptible to soil-borne diseases, pests, and nutrient imbalances, which can affect growing conditions and cannabis yield (6). To reduce these issues, new soilless cultivation methods were developed,  allowing faster growth and higher yields than traditional soil-based growing systems. An example are hydroponic systems, where plants are grown in water with added nutrients. In hydroponic cultivation, different substrates can be used to grow the plant, and all of them come with advantages and disadvantages. Coco-coir, for example, a residue of the coconut industry, provides good water-holding capacity, air porosity, and root aeration, but it can also have a high salt content, which, if not properly washed or buffered, can impact plant growth (6). Rockwool, made from melted volcanic rock, is another commonly used growing medium in hydroponic systems. It provides similar properties to Coco-coir, , with the advantage of also being sterile (6). However, it is not environmentally friendly, it is subjected to pH changes and, since it always stays moist, algae growth is not uncommon, which can harbor bacteria or viruses, and attract fungus gnats (6). Finally, peat-moss can also be a great option thanks to its high porosity, but since it is acidic, hydrophobic and low in nutrients, additives need to be incorporated to make it suitable for growing plants (6,7). Overall, despite peat-based media seeming to give highest plant yields, the choice of substrate will depend on factors such as local regulations, cost, and the grower’s experience and preferences (6,8). Another soilless cultivation method that, due to a greatly reduced environmental footprint, recently gained great success, is aquaponics, which adds fish to the hydroponic method (9). In this closed-loop system, fish effluents function as natural fertilizer, which drastically reduces  fertilizer and pesticides use (10). According to another study,  the hydroponic method with peat as a substrate seems to produce higher yields compared to the aquaponic system (9). Yet, higher levels of THC and CBD were found in strains grown with aquaponics compared to hydroponics, probably because the plants are in a slight stress state, suggesting that environmentally friendly methods still need further improvement for the high-nutrient demands of C. sativa (9).

Agriculturally speaking, cannabis is known to be a crop that produces a high yield with minimal cultivation efforts (from that, the name “weed”, an invasive plant). However, research has shown that the nutritional needs of cannabis, particularly nitrogen fertilization, require careful consideration (10). Fertilizer type seems to affect cannabis yields and THC content (1). A research study demonstrated that the optimal fertilizer rate to produce high-yielding, cannabinoid-rich plants, was slightly less than 400 mg nitrogen per liter during the vegetative growth stage (12). Knowledge around cannabis fertilizer is mainly based on growers experience, while peer-reviewed research is currently lacking. In general, slow release fertilizer seems to have a positive effect in increasing cannabis yield and cannabinoid content (1). Future studies should examine the effect of fertilizer application at different growth stages, varieties and growing substrates. Another promising approach is the use of plant growth-promoting rhizobacteria, which have the potential to increase crop yields by inducing a systemic resistance in plants and reducing losses caused by plant pathogens (1). The use of these microbes is of particular interest in the cannabis industry, as the flowers of the plant are highly susceptible to infection and strict legal regulations require low levels of pesticide residue on the flowers, but further research is needed to determine its effectiveness in the cannabis industry (1).
Finally, indoor cultivation has different advantages compared to outdoor cultivation. Controlled conditions seem to give higher yields and to produce more homogeneous batches, assuring quality, but also reducing the risk of potentially harmful pests (13). Nonetheless, a controlled environment requires high energy consumption, making indoor cultivation less sustainable. Future research should focus on how to make indoor cultivation more sustainable, by making it more efficient in terms of energy/resources use and pest control. At the same time, efforts should be made on making greenhouse and outdoor cultivation more efficient.

With this week’s newsletter, we dived into the state of knowledge of cannabis growth conditions for medical/recreational use: how different temperature, lighting methods, substrate and fertilizer can affect yield, but also cannabinoid content. Due to the different applications these plants have, when growing cannabis it is important to consider the cultivation purpose. For example, when cannabis (also known as hemp) is grown for textiles, plants need low THC content, and strong plant fibers. Therefore, hemp has different fertilization needs than cannabis grown for medical/recreational purposes (14).This suggests that cannabis cultivation should be customized by the growers, based on the product that they need to grow. It is also important to note that limited information is available in peer-reviewed scientific literature, therefore cannabis producers rely on cultivation methods derived from experience and anecdotal information (11). Also, it is common to obtain different results between industrial settings and lab settings, indicating that efforts should be made to make research settings completely reproducible in industrial settings. More research around cannabis growing practices will be crucial for the future of this industry, which needs to become more efficient in terms of costs, but also in terms of sustainability.