How do you achieve the highest yields with your lighting system? As we know, increasing light intensity up to a certain species-specific point results in a corresponding increase in yield. When light levels are not optimized for production or the type of crop you are growing, you have slower flowering, poor root development and plant structure, and reduced yields.
Four factors should be evaluated when considering supplemental light for your crop: light quantity (light intensity), light duration, light quality (spectrum), and distribution (light uniformity). Optimizing all these factors for your specific crop will help you gain increased productivity, plant quality, and yields. However, if you want to maximize profits, you need to focus on what will deliver the highest yield on high-revenue items in the shortest time possible. One way to do this is to increase light intensity, as light intensity is directly linked to yield.
How to measure Light
The measure of light intensity is photosynthetic photon flux density (PPFD), which is an instantaneous (spot) measurement of the amount of PAR light distributed to the crop canopy. PPFD is measured in micromoles per square meter per second (μmol/m2/s). This measurement is important because you need to know how much light is distributed at the crop level from the sun and/or luminaires to ensure the plants receive the correct amount of light to hit your target DLI (daily light integral). DLI is another way we calculate light intensity and refers to the cumulative amount of PAR light received in a 1 square meter (10.8 sq. ft.) area each day. DLI has become an important tool for managing the light environment to optimize crop growth. Most crops have optimum DLI ranges. For example, bedding plants fall between 20-25+ moles per day and vine crops need around 25-30+ moles per day.
Light Intensity and Yields
While the light spectrum is important for initiating plant responses such as germination, leaf unfolding, and flowering, light intensity is a far better indicator and tool for achieving high yields because it contributes to increased plant biomass production (up to a point). Plant growth is also driven by other environmental factors such as CO2, water, nutrients, humidity, and genetics. Achieving high yields and quality in your crops should be based on these environmental factors. You should first know the appropriate DLI of your crop so all other areas can be managed to maximize yield potential.
The light intensity rule of thumb is that a 1% increase in light equals a 1% percent increase in yield. In a study by Wageningen University, researchers discovered this to be true of all horticultural crops they evaluated including fruiting (vine) crops and flowers. This literature review found that a 1% increase in light from supplemental lighting resulted on average a 0.5-1% increase in harvestable products. They also found that aspects of the environment play a large role in determining how light intensity will impact yields. For instance, the effect of increased light intensity is most pronounced under low light levels, at higher CO2 concentrations, and at higher temperatures.
Is there a point where a plant can have too much light? All plants have a point where light becomes excessive and will damage the plant, referred to as the light saturation point. Therefore, it is important to know what your crop’s optimum DLI is. Crops such as leafy greens, strawberries, and some ornamentals will not benefit from high light intensities and have daily light integrals at the low to medium intensity level. For example, lettuce will grow best at 17 moles per day, orchids should be kept between 4-10 moles per day, and strawberries between 18-25 moles per day. Increasing the light intensity over a day to lower DLI crops can be consequential to yields and can increase disease incidence. However, there are many high-value crops like cannabis, tomatoes, cucumber, and some ornamentals where high light intensities are needed to improve yields, quality, and crop cycles – these are crops that need daily light integrals around 25-30+ moles of light per day.
One crop that can take a high amount of light intensity is cannabis. While exact light intensity requirements will vary by cultivar, it has been shown that cannabis yields will linearly rise with increasing PPFDs up to 1400umol/m2/s. Yield and fruit quality are also positively related to high light intensities in fruiting crops like tomatoes. Higher DLIs with supplemental light can also impact bedding plant plugs by increasing quality and decreasing the time to flower, with DLIs up to 26 moles per day. So, if you are struggling with yields, crop times, or quality, it could be related to the quantity of light you are giving your plants. Again, always ensure other areas like CO2, temperature, and nutrients are optimized to your desired light levels, so energy is not wasted.
A question we often hear is does the light spectrum matter when it comes to yields?
While spectrum is important in many areas of plant morphology, it is shown to have little effect on increasing yields. The PPFD will have a greater impact on yields than the spectrum of light. Greenseal Cannabis compared cannabis yields and morphology under HPS luminaires to multiple LEDs. The results showed that the finishing stage was approximately 5 days shorter, with higher dried flower yields on average under LED grow lights. This is most likely because the LEDs were mounted much closer to the crop canopy—directing more photons to the plants. Conversely, HPS lights, due to their radiant heat output, need to be mounted further above the plant canopy. However, one of the big takeaways from the study indicates that light intensity was the primary driver of yield in broad-spectrum lighting applications and that differing light spectrums between the various manufacturers’ LEDs made little difference when it came to increasing yields.
Author James Eaves also stressed in his paper that $/PPFD should be the key metric growers look at, rather than $/W since the incremental yield outweighed the incremental electricity costs. Of course, this statement was made in 2018 when the Canadian cannabis market was at its peak and the price of cannabis remained high. If the price of cannabis drops and/or the price of electricity is high, then that equation might yield a different result. The study thus concluded that yields increased up to at least 1500 μmols/m2/s, which is almost twice the intensity often used by cannabis growers. This implies that tuning the spectrum of lights is not an important factor for yields. However, it is possible that spectral tuning could still impact the chemical profile of the plant and increase yield value that way.
Are LEDs better for increasing yields compared to HPS luminaires?
Many studies, including the one by Eaves, indicate that photons from LEDs are more efficient for photosynthesis and increasing yields. Still, many of these studies were performed on cannabis. More research needs to be performed on other lower-value crops like cut flowers and fruiting vegetables.
In some cases, the high costs associated with LEDs, including increased price per unit, labor, and infrastructure, may not outweigh the benefits of slightly increased crop cycles or yields. This is why it is important to ensure that your ROI (return on investment) makes sense for your application.
Many growers we work with in both cannabis and traditional horticulture still decide to grow with HPS lighting because they can achieve high yields with a lower upfront cost, which makes more sense for them in terms of profitability. If you decide that LEDs make the most sense for your application, it is recommended to set up a trial area to see how your crops perform under LEDs, especially if you are used to growing with HPS lighting.
For growers who want to transition to LEDs for increased efficiencies, a hybrid HPS-LED system can be a great solution, especially in colder climates where HPS luminaires help offset winter heating costs. The heat from the HPS lamps also helps heat the plants while maintaining transpiration rates, allowing growers to keep their existing infrastructure and initial costs fairly low compared to a full LED installation.
The overall takeaway from this is that light intensity is a far better indicator of yield than light source or spectrum. As a manufacturer of both LED and HID technologies, we know that either can be used to achieve high intensities and therefore high yields. Most important is that your ROI makes sense and you can optimize other areas of your grow like CO2 and temperature to ensure a balanced growing environment.