Use High Intensity Light, Not Ultraviolet, For Maximum Cannabis Yield, Study Suggests

If you want to extract the maximum possible value from your indoor cannabis grow, popular belief tells you to use high intensity lighting to increase yield and ultraviolet (UV) light exposure to stimulate the production of cannabinoids.

But according to a new study published in Frontiers in Plant Science, this conventional wisdom may not hold water.

While cannabis yields did increase proportionately with light intensity in this experiment, the researchers determined that UV radiation had no significant effect on either cannabis yield or total cannabinoid content.

Higher light intensity equals higher yield

The study was conducted in a commercial cannabis production facility in Ontario, Canada, where the researchers set up a series of blacked-out cannabis grow plots with different lighting set-ups. Each experiment began with 21-day-old cannabis plants from a high-THC strain, which had been grown under LED lighting at an average canopy-level photosynthetic photon flux density (PFFD) of around 600 micromole per second and square meter (μmol m⁻² s⁻¹).

To study the effect of increasing light intensity, the researchers increased the strength and/or number of LED light bars to deliver average canopy-level PFFDs of 600, 800, or 1000 μmol m⁻² s⁻¹.

In comparing a range of biomass metrics – including the fresh weight of leaves, stems, and harvested inflorescences – the researchers found that all relevant parameters were between 1.3 to 1.5 times higher in the plants from the highest PFFD plots compared to the plants from the lowest PFFD plots. The only exception was inflorescence dry weight, which was actually 1.6 times greater in the plants with the highest PFFD exposure.

High-performance liquid chromatography with UV spectroscopy (HPLC-UV) analysis at a third-party testing laboratory revealed no significant differences in detectable cannabinoid concentrations in the floral tissues of plants grown under different lighting intensities. Visual inspection also confirmed that all of the plants’ inflorescences matured at a similar rate regardless of light intensity.

Ultraviolet ineffective at stimulating cannabinoid levels

To evaluate the effect of exposure to ultraviolet A (UVA) and ultraviolet B (UVB) radiation, the researchers compared three plots with LED lighting systems set to deliver PFFDs of 600 μmol m⁻² s⁻¹. One plot was kept as a control, one had a UVA light installed to deliver an additional UV photon flux density (PFD) of 50 μmol m⁻² s⁻¹, and the third plot had a UVA + UVB lighting system set to deliver a PFD of 3 μmol m⁻² s⁻¹.

The researchers found no evidence of a UV treatment effect on any biomass metrics. There were also no statistically significant effects seen in inflorescence cannabinoid concentrations. While sugar leaves (small leaves associated with inflorescences) did display 30% higher THC concentrations in the plants that received UVA+UVB treatment, total THC levels were still unaffected in foliar tissue.

“While we did not quantify trichomes, we noted that there appeared to be higher trichome density on the sugar leaves in the UVA + UVB vs. the UVA and control treatments, particularly in areas proximate to the petioles,” the researchers wrote.

“This observed increase in trichome density may explain the ≈30% higher foliar THC content [...] However, since foliar cannabinoid content was much lower than inflorescence tissues due to lower trichome density, these tissues are of relatively low value in commercial indoor cannabis production and are often discarded.”

Is high intensity lighting economically viable?

The findings of this study suggest that while UV radiation may not increase THC content in any way that is of commercial interest, higher light intensity can near-linearly increase inflorescence dry yield. According to other recent research, this linear increase may even continue up to 1800 μmol m⁻² s⁻¹.

Now for the elephant in the room: price. Higher light intensity naturally necessitates higher electricity costs, and so the added value precipitated by having higher yields needs to be enough to mitigate these increased energy costs.

According to the study authors, based on the current wholesale price for dried inflorescence ($4 CAD/gram), the added electricity cost for increasing lighting intensity “comprises only ~1% of the total price” and so may be financially viable. Of course, individual circumstances, such as the need to buy additional lighting infrastructure, may affect this calculation.

Optimizing indoor cannabis grow lighting

While some regions can make extensive use of outdoor farming, weather and local climate can force others to strongly rely on artificial lighting systems in order to produce a healthy crop.

“If you’re at a more southern latitude, and you have the option of growing in a field with a lot of sunlight, that's going to be a good option,” David Hawley, head of research and development at the horticultural lighting company Fluence, told Analytical Cannabis in 2019.“[But] if you’re a little bit more northern, up into Canada or in the Netherlands where it’s dark, you’re going to be supplementing quite a lot of light.”

More experimental studies, such as the one recently published in Frontiers in Plant Science, will help cultivators to determine the optimal conditions for cannabis and hemp farming in the future. Outside of light spectrum, there are two more factors that cannabis growers may wish to consider when setting up a new grow operation: facility layout and cost and efficiency.

For example, with vertical farming operations increasing in popularity, growers may consider a multi-level lighting system that enables plants to be grown in a tiered layout. In terms of costs, high-intensity discharge (HID) lights tend to be cheaper than LEDs. But it is also important to consider the total running costs of such lighting systems, as Brendan Delaney, director of cultivation at Solar Cannabis Co., previously told Analytical Cannabis.

“Plain and simple, LEDs are extremely energy efficient,” Delaney said. “When talking about efficiencies, it’s not just about the light itself but more in terms of environmental variables.”

“LEDs throw off considerably less heat than other lights, such as HPS [high-pressure sodium]. This has a dramatic impact on cutting cooling costs, making it more efficient all the way around.”