Understanding how carbon dioxide and vapor pressure deficit impact cannabis growth can save resources and lead to healthier, more productive crops
As the price of cannabis decreases and the cost of doing business increases, growers are actively looking for ways to optimize their use of energy, water and other resources, while maintaining the same level of productivity or even increasing output. At the same time, regulators and utility companies are concerned about the cannabis industry’s impact on regional power supplies, local ecosystems and watersheds, and waste stream management.
For those entities outside the sphere of cannabis cultivation and horticultural science, the solution seems obvious: regulate or incentivize more energy-efficient technologies, such as LED lighting, variable refrigerant flow HVAC systems, or other ready-for-market alternatives that have been successfully implemented on commercial and residential projects. Unfortunately, it’s not that simple, because changing an environmental input can have a profound effect on how the plant responds to other inputs, ultimately affecting how the grower manages crop productivity and profitability.
But how do energy-efficient technologies and strategies impact plant productivity? Are these technologies the right solution for maximizing efficiency and reducing impacts on utilities and the environment? Unfortunately, these technologies do not live in a vacuum.
What is VPD?
Vapor pressure deficit (VPD) is the difference between the amount of water in the air and the maximum amount of water the air can hold for a given temperature (saturation). It is usually measured in kilopascals (kPa) or bars. A low VPD is indicative of high moisture content in the air and a high VPD is indicative of low moisture content.
The surface of leaves is commonly assumed to be saturated with water. Plant stomatal opening is directly related to VPD. If VPD is too high, stomata will close to conserve water. If it’s too low, stomata may be fully open, but evapotranspiration will be slow and nutrient uptake will be impacted. Both conditions can cause wilting, leaf tip burn and other crop maladies. When VPD is managed correctly, plants will transpire freely, move nutrients readily to cells and maximize CO2 uptake.
Plant Responses to Environment
As a C3 plant (see sidebar), cannabis is particularly well-suited for temperate climates, which are characterized by relatively small swings in daily and seasonal temperatures and abundant rainfall.
Regardless of type, all green plants are autotrophs, meaning they use the chemical reactions of photosynthesis to self-generate the energy required to grow and reproduce. To support photosynthesis, all plants require the same basic environmental inputs: light, water, nutrients and CO2.
For flowering plants, such as cannabis, a change in one or more environmental factors can shift it from a vegetative state (leaf production) to a reproductive state (flower and fruit production). During its vegetative state, cannabis likes long, sunny days to maximize photosynthesis and chlorophyll production, resulting in big, beautiful green leaves. As the sun wanes after the summer solstice (June 21), the plant recognizes that resources will become more scarce, triggering the plant to move toward a reproductive state.
By the time days and nights reach a 12-hour split, the cannabis plant is in full flowering mode. Greenhouse growers extend their growing season by using a combination of light-dep curtains and supplemental lighting to manipulate the photoperiod (day length) and evoke the flowering response, regardless of time of year. Indoor growers do the same thing by switching from an 18-hour photoperiod to a 12-hour photoperiod when they want to induce flowering.
Although light is essential for plants to photosynthesize and sets a timer for reproducing plants, it is only one of many environmental factors that plants require to grow. Air temperature affects the metabolic rate of plants and their ability to synthesize CO2 and nutrients into primary compounds (think chlorophyll) and secondary compounds (think terpenes). The relative humidity at a given air temperature impacts the opening of leaf stomata, which are responsible for the exchange of gases, specifically CO2 and water vapor. The larger the stomatal opening, the more CO2 can come into the leaf and the more water vapor can evaporate from it (via evapotranspiration).
Because the combination of air temperature and relative humidity (commonly referred to as vapor pressure deficit or VPD) affects the rate of evaporation from the leaf, it also affects the rate at which water is taken up by the roots and delivered through the plant. Therefore, if the stomatal opening is large, the plant will need more water to keep up with the rate of evaporation. If the roots do not receive enough water, the plant will respond by wilting. Because plants also use evapotranspiration to transport nutrients from the water to plant cells, if water deficiencies persist, the plant will begin to exhibit nutrient deficiencies, such as leaf tip burn (calcium deficiency). Plants can even lose their ability to ward off pests and pathogens due to a depressed immune system.
Alternately, when stomatal openings are smaller (due to too either too much or too little moisture in the air), evapotranspiration rates will be lower and plants will require less water. But if too much water is supplied to the plant under this condition, the roots can become starved for oxygen, causing them to exhibit behavioral signs similar to under-watering.
CO2 enrichment is often used by indoor and greenhouse growers to increase the rate of photosynthesis and plant growth. Typically, growers target CO2 levels between 1,200 and 1,500 parts per million in the production room. Although there have not been academic studies specifically researching the optimal CO2 level for cannabis cultivation, research into other similar horticultural crops reveals that these target levels are likely too high based on growers’ targets for other environmental factors, specifically air temperature and light intensity.
In general, the rate of CO2 uptake has a positive relationship with air temperature and light intensity, such that plants uptake CO2 at a higher rate when both air temperature and light intensity are increased. But there is an upper limit, at which no matter how high the temperature or light level, the plant simply cannot use more CO2. For most plants, this upper limit comes around 1,200 to 1,500 parts per million. Additionally, when CO2 levels are elevated, the plant uses more resources to support leaf development, which is great during the veg stage, but could reduce yields if continued during flowering.
LEDs to the Rescue?
A lot of attention has been given to the energy-saving potential for replacing high-intensity discharge (HID) lamps with LEDs. Utility companies are especially interested in this technology, as they witness steep spikes in electricity demand across their service areas where cannabis cultivation facilities are permitted.
Growers are also looking for solutions to reduce operating costs, with energy use commonly the largest expense after labor. Because LED lamps are more efficient at converting electricity into light, they produce less waste heat, thereby reducing the HVAC cooling requirements. It is this cascade effect of simultaneously reducing the electricity needs for both lighting and HVAC that causes many people to consider LEDs the “holy grail” of energy-efficiency opportunities.
But plants respond differently to different environmental conditions. Most people recognize that LEDs generally put out lower light intensities than HID lamps, even with full-spectrum ratings, and adjustments are made to lighting layouts and floor plans to compensate for the plants’ response to different light inputs. Rarely, however, are adjustments made to the room or other systems to also compensate for the reduction in waste heat.
Plant leaves are essentially black bodies, similar to our hands, in that they capture radiant energy from the surfaces around them. Radiant energy comes in the form of light and heat. Any spectrums of radiant energy outside the photosynthetically active radiation (PAR) spectrum — about 400 to 700 nanometers — is considered “waste heat.” This radiant heat affects leaf temperature, metabolic activity in the cells and the rate of evapotranspiration.
If you started growing in a facility outfitted with HID lamps, you have likely installed HVAC equipment and developed crop management strategies to accommodate this “high waste heat” condition, including finding the right balance of air temperature, relative humidity, irrigation rate and frequency, and fertilizer mix. But under a “low waste heat” condition, plants respond differently to these variables. Because they are being irradiated by less heat, the leaves will be cooler and evapotranspiration rates will be less, effectively reducing the plant’s thirst for water. And because there is less heat and moisture generated in the room, there will be a smaller requirement for cooling and dehumidification. Additionally, the combination of low light intensity under the same air temperature setpoint will reduce the rate at which plants fix CO2. Therefore, lower CO2 levels and less enrichment is required.
But that also raises other questions. If plants need less water and have a slower rate of metabolism under LED lights, does that also mean they need less nutrients, more concentrated nutrients, or a different balance of nutrients? Can your existing HVAC equipment handle the lower cooling and dehumidification loads? Can other environmental inputs be modified to achieve the same growth rate and produce the same phytochemical profile you’ve become accustomed to under an HID light environment?
One strategy to combat the change in metabolic activity is to increase the air temperature. If the plant isn’t going to receive radiant heat from the lamps, they can still receive convective heat from the air. It doesn’t take much to facilitate heat transfer from the air to the leaf — just 1-3 degrees Fahrenheit higher air temperature than leaf temperature.
By increasing the air temperature, you can further reduce the HVAC cooling requirements, though you may still need to dehumidify at a relatively high rate because plants transpire harder under higher temperatures. One potential drawback, if you use the same HVAC equipment under this new light condition, is that it could be too large for the needs. This could cause it to cycle on and off more frequently, increasing the risk of frozen cooling coils and shifting the equipment’s balance from heat to moisture, a condition most HVAC units aren’t adept to handle.
Ultimately, every environmental setpoint you target — light level, VPD, CO2, etc. — affects how the plant responds to other environmental factors in the production space. By understanding these plant-environment interactions, growers can make better decisions about how to manage their crop from seed (or clone) to flower, including what equipment to use, what environmental levels to target and when, and ultimately how to maximize yield and quality by managing resource inputs and minimizing waste.
If you are thinking about switching to LED lamps, be patient and recognize that everything you’ve known about growing a crop under HID lighting could change dramatically. But that doesn’t mean it’s not worth it. If you can discover the new balance between environmental factors, crop management strategies and technologies, LED lamps could be one solution to a profitable and more sustainable future.
Nadia Sabeh is the president and founder of Dr. Greenhouse (www.doctorgreenhouse.com), an engineering consulting firm focused on designing HVAC systems for indoor farms. She is a licensed mechanical engineer in California, received her Ph.D. in agricultural and biosystems engineering from the University of Arizona’s Controlled Environment Agriculture Center, and has nearly 20 years of experience helping clients maximize crop productivity by translating the plants’ needs into the design and operation of the facility.