From Trial & Error to Truth: Researching the Impact of Supplemental Lighting on Cannabis Production

Cannabis cultivation is booming and as more growers enter the market, the ability to produce consistent, high-quality yields has become business-critical across the industry. Growers now have an imperative to explore the best methods to produce more potent, higher yielding harvests; harnessing the latest in science and technology to enhance productivity and profitability, without ignoring the global imperative to employ more sustainable farming practices.

With greater leeway now afforded to research, plant scientists are seeking to develop a greater understanding of cannabis cultivation, checking the received knowledge gleaned from previous generations of trial-and-error and establishing new best practice, backed by science as well as experience. As a leader in horticultural lighting, GE Current, a Daintree company works with some of the world’s pre-eminent horticulture research institutions to ensure that its lighting technology is rooted in science and a deep understanding of plant biology.

Growers want the ability to choose between completely enclosed grow rooms, where all aspects of the growing environment can be dialled in, or greenhouse environments which allow them to take advantage of available natural sunlight. In either case, the use of sole source or supplemental lighting in cannabis cultivation remains a crucial part of the equation. In order to help growers establish the most effective set-up for them, Current is working with the Hernández Lab, part of the Department of Horticultural Science at North Carolina State University (NCSU), to support the research of Prof. Ricardo Hernández and his PhD student Cristian Collado, into the impact of supplemental lighting on cannabis at every stage of growth.

Using Current’s Arize^® Element L1000 LED top lighting, the team has explored, in a greenhouse setting, the impact of different quantities of light on the growth of a Cannabis sativa variety commonly cultivated for its high levels of CBD. By maintaining consistent levels of all other environmental factors such as temperature, fertigation, CO₂, etc., the team has isolated the impact of different levels of light throughout the propagation, vegetative, and reproductive phases of growth, allowing them to establish baseline guidance that will support growers throughout the industry – from nurseries supplying young plants, to producers of flowers - both for dispensaries and the growing CBD consumables market.

According to Prof. Ricardo Hernández, head of the Hernández Lab and lead researcher Cristian Collado working on supplemental lighting for greenhouse production of Cannabis sativa through LED light supplementation, “Our starting hypothesis, that higher levels of light intensity throughout the photoperiod would lead to higher yields, was based on anecdotal reports from growers. However, growers must weigh inputs such as energy costs, equipment, fertilisers, water, etc. with the outputs – or income generated from rooted clones or viable flowers that are sold. Our aim was to study how light supplementation during the vegetative and flowering stages affects plant growth and flower production and quality. However, from a practical perspective, we were also interested to learn if our research would highlight an optimal daily light integral before returns started to diminish.”

Given the scale of the research, the findings for each of the growth phases will be shared separately, starting with the vegetative phase, followed by the flowering phase, and finally the propagation phase results.

Shining a light on vegetative growth

Although shorter than the flowering stage at just 21 days, the vegetative growth stage underpins every part of the cannabis market, either laying strong foundations for flower production or creating well-developed mother plants from which cuttings for new plug plants can be taken.

For nurseries whose primary goal is to produce cannabis clones for sale to growers, maximising the number of branches and stimulating the nodal points between which cuttings can be taken, is critical.

This figure shows the total number of branches and their division into primary and secondary branches. Primary branches are on the main stem and secondary branches are on the primary branches. Scientists counted all the branches equal to or larger than 5 cm (~2in).

Using a standard photoperiod of 18 hours, the Hernández lab team established four separate lighting treatments using the Arize Element L1000 top lighting to deliver a supplemental light intensity of 150, 300, 500 and 700 μmol m⁻² s⁻¹). The plants under the 150 μmol m⁻² s⁻¹ treatment received approx. 50% of the daily light integral (DLI) from the sun, whereas 80% of the DLI from the 700 μmol m⁻² s⁻¹ treatment was provided by the Arize LED lighting.

Sixty rooted cuttings of the Cannabis sativa strain were grown under each lighting treatment, with all other environmental factors maintained across the entire study. At the start of the study, the plants measured between 17 and 18cm (6.5-7.0in).

At the end of the 21-day growth stage (at first harvest), the plants had grown to a height of approximately 51 cm across the study. However, with the increased light intensity came far higher rates of secondary branching, with little increase in the number of primary branches, creating a direct, linear correlation between light intensity (and cumulative DLI across the vegetative stage) and the number of secondary branches that could be used to clone the plant successfully.

What does this mean for growers?

Cannabis is known to be a resilient plant but the Hernández Lab research into the impact of supplemental light intensity on the vegetative growth stage highlights its ability to effectively convert high levels of PPFD (photosynthetic photon flux density) into useful plant growth. The steep, linear relationship between the number of secondary branches and the quantity of light delivered by each treatment suggests that even the highest levels of light intensity (and therefore the highest associated energy costs) would be rewarded by a 300%+ increase in the number of secondary branches (potential cuttings) with respect to the lowest light intensity tested.

Assuming that this increase in secondary branching would also lead to a similar increase in the number of viable clones produced from each mother plant, cannabis nurseries could dramatically increase the productivity of every plant within their existing operation, simply by upgrading their supplemental lighting to a more intensive set-up.

Additional effects of the higher light intensity levels on plant morphology during the initial 21 days of growth (such as the development of thicker leaves to maximise photosynthesis) could also have a beneficial impact on the quality and quantity of flowers produced during the reproductive stage. This will be explored further in upcoming trials to determine the impact of supplemental light intensity on the quality and quantity of flowers produced under lighting treatments that elicit different branching responses.

With the cost of energy and other factors such as climate varying across geographies, growers must weigh the potential increase in income against their own predicted energy bills associated with more intensive lighting installations. However, new advancements in renewable energy production and using the latest, energy-efficient LED lighting technology can help mitigate high energy costs and lower the carbon footprint of a greenhouse operation, creating a cleaner path to profit.