5 tips on how to light your hemp crop for maximum yields, quality and potency

P.L. Light Systems

Light Plan, Lighting Strategies, Medicinal crops | October 2, 2020

Lighting for commercial hemp production – 5 tips on how to light your hemp crop for maximum yields, quality and potency 

We know that light is essential for plant growth, and for a plant such as hemp, which is adapted to high light levels, and where yields and quality are heavily influenced by light intensity – having supplemental or a high-quality sole source lighting systems is critical to maintain plant health, and maximize cannabinoids content  Factors such as photoperiod, light intensity and quality all play important roles in controlling yield, flowering, CBD concentrations and even helping decide the sex of the plant.  

Greenhouse and indoor hemp production allows for a consistent environment that can help to maximize yields, CBD, terpene content and other cannabinoids, while protecting against wind, drought and other environmental related threats.  Hemp (Cannabis Sativa L.) differs from marijuana crops in that it must have less than 0.3% THC to qualify as hemp.  Growers who choose to grow hemp indoors, are often growing high value hemp for CBD production. Some growers also use greenhouse and indoor growing for production of clones and seeds, because it helps limit exposure from outside pollen and provides an ideal environment for strong healthy plants.   

Hemp production has distinct growing stages: Propagation, vegetative and flowering; and each has important photoperiod and light intensity requirements.  You need 12 hours of darkness to initiate flowering on hemp plants (other than auto-flowering varieties), and a minimum of 18 hours of light for vegetative growth stages to produce strong plants.  As plants move from propagation to vegetative to flowering stages, while the photoperiod decreases, light intensity needs to be increased to allow for maximum yields and high Cannabinoids content. We typically recommend growers aim for 30 moles of light per day once in flowering cycles.  If you are unfamiliar with light measurements and lighting strategies, we have outlined 5 tips to on how to light your hemp crop for maximum yields and quality. 

Person measuring DLI under HPS lights in hemp cultivation

Tip 1: Know your DLI and how to measure your light intensity 

There is a strong correlation between light intensity and yields, so directing as much PAR light as possible onto the crop, is essential for all growers. The way we measure light intensity over the course of a day, is called the Daily Light Integral or DLI. DLI refers to the number of photosynthetically active photons in the PAR (photosynthetically active radiation) range accumulated in a square meter over the course of a day.  DLI will vary depending on your geographygreenhouse glazing materials and time of year – it is always highest in the summer and lowest in the winter. 

Not achieving your targeted DLI can mean low yields, low quality, and longer crop cycles. For example, in the flowering stage of development, it is recommended to target a minimum DLI of 30 mol/m²/d¹ for proper development and to achieve a high-quality product and yields.  In autumn, winter and early spring, outdoor light levels fall well below 30 moles per day, so supplemental light becomes essential to maintain your desired DLI. To appropriately measure light levels within your growing facility, it is recommended to use a light sensor and not rely on the human eye to detect light levels.  

Tip 2: Work with a reputable lighting company with dependable high-quality products that stand up  to the harsh environments of greenhouse/indoor growing 

Do your research into the companies you want to work with and ensure that they have a background in horticulture and can stand by their products.  There have been many new lighting companies on the market in the last few years, selling products that, although inexpensive, are not designed for plant production or the harsh environments of growing, and often fail.  Companies like P.L. Light Systems have been designing and manufacturing horticultural lighting products for 40 years, so we understand the challenges of growing, and is why so many growers depend on our products because they know they are built to last.  

Another thing to look for, is to ensure products are CSA or UL certified, which means they have been deemed safe for use.  If you are opting for LED luminaires, check if they are DLC certified, which can help you receive rebates from your local state or provincial energy providers.  DLC certification also validates any product performance claims, including light output and energy-efficiency. 

LED trial area with hemp crop underneathTip 3: Set up a trial area to figure out what lighting works best for your application 

Deciding whether to go HID or LED can be a big decision and will depend on many factors, such as your environment, goals, strains and budget.  Both technologies have their advantages and disadvantages, so the decision should be made carefully.  Setting up a trial R&D area for LED technology, is recommended since different varieties and strains can respond differently to the same wavelengths.  Another great option is to install a hybrid lighting system with both HID and LED luminaires, so you can take advantage of both the energy savings and spectral efficiency of the LEDs as well as the radiant heat and uniformity of the HID luminaires. 

Tip 4. Follow the light plan 

Any reputable lighting company will design you a light plan which calculates the best possible coordination of luminaires in terms of their distance and orientation from one another, as well as the crop.  

The way light is distributed over the crop is equally important as the light intensity, because you need a uniform light distribution for even photosynthesis to ensure uniform growth and development.  Uneven light will lead to plants drying out at different times, and you can have some plants developing faster and others slower 

It’s important to remember all luminaires are different in their layouts.  One manufacturer’s light plan may differ from another, not just with the pattern of luminaire layout but also the number of luminaires it will take to reach the targeted light level and uniformity.  

P.L. Light Systems advises growers to consider a light plan that shows a guaranteed uniformity and light level.   

Hoop house with hemp crops and NXT-LP luminaires

Tip 5: Determine your ROI  

Yield is the most important measurement of profitability. When growing in a controlled environment, understanding yield metrics is important to make sure you are getting the most out of your lighting – and to make sure you are being profitable.  Using supplemental lighting can allow for an increase in plant density, while still obtaining greater quality and yields. Crop growth, uniformity and productivity are influenced by the homogeneity of light distribution and amount of light delivered to the entire foliar canopy.  

The most common way to measure a crop like hemp, is in grams/kilograms per square foot.  Measuring yield allows the grower to understand how well the space is being utilized, if grow conditions need to be adjusted, or if some varieties/strains produce better than others.  Growers should not be using metrics like Grams per Light since it is not the most telling of ways to determine if your space is efficient, since this measurement only tells us the yield under a single lightOther considerations, when it comes to profitability, should also be price per unit, energy use efficiency, maintenance costs, and infrastructure costs (if doing a retrofit, LEDs can require extra track, wiring, etc.).