The UK is the world’s largest producer of legal cannabis, the growth of the market being in response to a swelling market for cannabis products. The emergence of approved and regulated drugs to treat conditions such as epilepsy, along with a gradual relaxing of legislation around medicinal use – including in the UK in November 2018 – has contributed to a growing demand for high quality, carefully grown cannabis plants. According to the publication Health Europa, the global medical cannabis market was worth $13.4bn (£10bn) in 2018 and is projected to grow to $148bn by 2026.
Vast greenhouses are expanding throughout the UK, from Dumfries and Galloway, to Lincolnshire, Wiltshire and Norfolk. One of the most important factors in the cultivation of cannabis is humidity control. Humidity impacts almost every aspect of a grow operation – including photosynthesis and transpiration, vapour pressure deficit (VPD) and nutrient uptake, pest and pathogen pressure, room temperature and leaf temperature, and ultimately the yield and quality of the product grown. Accurate and consistent humidity control will reap great rewards in the increasingly competitive arena of indoor horticulture.
Transpiration is an important physiological process that plants go through to take in nutrients and maintain turgor pressure. Transpiration results in plants releasing water into the air, which has a significant environmental impact on both temperature and relative humidity. Transpiration directly impacts VPD, or the pressure difference between the intracellular membrane of a plant’s leaf and the air of a grow room. VPD is the physical force drawing water vapor up the stem from the roots and out of the leaf. A VPD that is too low will inhibit transpiration, decreasing the rate at which water and nutrients flow through a plant, which ultimately will slow growth.
Relative humidity and the plant life cycle
Genotype + Environment = Phenotype.
Plants need several basic inputs in order to grow: light, water, nutrients, oxygen, root zone temperature, microbes, temperature, humidity, airflow and carbon dioxide. Plant systems are interconnected in such a way that growers must understand the impact of one input on potentially several others.
Some plants grow better in different relative humidity ranges during different stages of growth, so growers must control relative humidity specific to each stage of the plant life cycle. Doing so can influence plant vitality, product quality, genetics, and overall yield.
In general, the ideal humidity for cannabis plants in seedling form is 70%, dropping to 50%-55% while in flowering and vegetative form, and reducing furthermore for the drying purpose to around 40-50%. As stated, these figures could differ depending on the genetics of the cannabis plant, but with any drying process it is important not to dry too quickly so a gradual drying process is required.
Conversely, if relative humidity levels are too high, the transpiration process will slow down and there may be many other negative impacts on plant health and secondary metabolite production.
In all stages of growth, cannabis plants need a constant intake of water, but the amount of water they need will fluctuate based on a variety of factors. Because humidity will directly impact how much water the plants take in, controlling the humidity gives growers increased control over nutrient uptake and therefore control over plant growth and plant health.
Humidity also has a direct effect on disease and pest infestation issues such as powdery mildew and botrytis. Mould can be problematic for cannabis production so it is vital the correct humidity is adhered to as mould can spread quickly through the crop and, once present, can lead to the whole crop being rejected as strict standards need to be adhered to in order to ensure the consistency and quality of the products that medical practitioners prescribe to their patients.
Controlling Relative Humidity
Effective and efficient humidity control begins with selecting the correct level and combination of environmental control systems like heating, ventilation, air conditioning and humidity control specific to each unique operation. The correct equipment can be used to maintain temperature, manage humidity levels and minimise threat of disease at each stage of the plant growth cycle.
Right-sizing cultivation and drying systems means balancing capital costs with risk tolerance to maximise profit and optimise long-term operating expenses, all while ensuring the system has the capacity required to manage the growing environment and meet production goals.
Even small changes in humidity setpoints can have a dramatic impact on both plant growth and equipment selection. Humidity is one of the most critical factors in cannabis cultivation, yet many growers lack a complete understanding of controlling and manipulating humidity levels to maximize plant health and yield, minimise risk of disease, and increase revenue.