abadonian | Adobe Stock

New principles of greenhouse crop management have emerged from Dutch horticulture industry experts and scientists from Wageningen University, a Netherlands institution recognized for its agricultural science program. Hundreds of Dutch growers have been trained in greenhouse climate control practices called Growing by Plant Empowerment (GPE), fundamentals of which have been outlined in a 2018 book, “Plant Empowerment” and in related online tools at Letsgrow.com.

The methods presented in the book diverge from our understanding of greenhouse climate control. They remind us that plants are physical objects subject to the laws of thermodynamics in addition to being biological organisms. Rather than focus climate control on static air temperature and humidity or vapor pressure deficit (VPD) targets, GPE controls the growth and flowering processes based on three balances: the energy balance, the water balance and the assimilates balance. Assimilates are the sugars made during photosynthesis that are used for growth. The three balances are managed simultaneously and kept in equilibrium by the tiny pores in leaves called stomata that allow water vapor to be evaporated and carbon dioxide to be absorbed. In effect, GPE is about managing the stomata rather than the environment per se, keeping the pores in open position to maximize photosynthesis and evaporation.

Though developed for greenhouse vegetable and flower growers in drier northern climates not found in much of the U.S., some controlled environment ag experts agree that GPE may be well-suited for U.S. cannabis production. Cannabis responds very well to greenhouse tomato temperature and nutrient protocols and can be illuminated similar to roses grown for cut flowers, which have a thick canopy of petals, according to an article in Greenhouse Management magazine (CBT’s sister publication). Routinely, news stories highlight large-scale greenhouses being built or converted for cannabis in North America. A 2019 CBT study found that 43% of cultivators who participated in the research plan to build greenhouses in the next two years. Yet many growers have privately told me they struggle with product uniformity and quality control when comparing their greenhouse operations to their indoor grows.

GPE emphasizes plant health, fruit or flower quality and resistance to stress and disease. This results in predictable yields despite the dynamic nature of greenhouse environments. Energy savings may also be possible but are considered a bonus.

GPE in a Nutshell

Greenhouse control is based on supporting the three plant balances rather than conditioning the air to certain setpoints. Instead of a fixed temperature regime, setpoints are adjusted according to predicted daily light integral (DLI) to create a more constant ratio of temperature to radiation in order to balance growth. The emphasis is placed on growing at warmer temperatures. During sunny periods, rather than increasing ventilation or mechanical cooling, temperature and humidity are allowed to increase in the greenhouse to keep stomata open for CO2 absorption, maximizing photosynthesis. Under high light and enriched CO2, most plants’ optimum temperature is 86 degrees Fahrenheit, according to the white paper “Next Generation Growing: Plant empowerment and plant balances.”

The “Plant Empowerment” book’s authors use the term evaporation to encompass both water transpiring from stomata and evaporating from micropores in the leaves, and the GPE methods seek to avoid interruptions in this flow of water vapor from the plant. Heating pipes or warm air currents are used in the absence of light at night. To keep up with water demand, irrigation is triggered based on all energy flows (light, heat, convection, evaporation) rather than just light. This emphasis on evaporation is to keep water and mobile nutrients such as calcium flowing to the plants’ growing points.

The authors also state that without night ventilation, plant cells can be damaged by root pressure (water turgor) building up, creating sites for possible fungal infection. Diseases seldom occur due to poor climate conditions exclusively, they report, but rather because the sub-optimal conditions combined with disturbances in the plant balances lead to lower resiliency. For example, disease is prevented by avoiding condensation that occurs when leaf temperature drops below dew point. This is accomplished with thermal screens to block radiation of heat energy from the plant to a colder object, such as the greenhouse roof. This method is based on the law of conservation of energy, which states that energy can’t be created or destroyed. It can only be converted into another form of energy.

Assimilates Balance

Maintaining a balance of photosynthetic assimilates in the plant is paramount in the GPE methodology. Photosynthesis increases along with light radiation up to a point called the light saturation point. Cannabis has a light saturation point of 1500 µmol/m2/s, according to multiple reports, but can be as high as 2000. (Even a greenhouse in summer sunlight is unlikely to reach this point, so light stress should not be a concern.) Under these high-light conditions, photosynthesis can be enhanced by increasing humidity to keep stomata open for CO2 absorption. According to GPE, rather than cooling the greenhouse, temperature should be allowed to rise to speed the biochemical reactions of photosynthesis, as long as water stress can be avoided.

But this is only one half of the balance. Now that the plant is maximizing creation of assimilates, the goal is to use these sugars immediately for growth, flowering or root production, rather than convert them to starch for storage. Accumulation of starch in the leaves can slow photosynthesis by regulating enzymes. Some sugars might also be converted to cellulose for heavier stems and leaves, which provide no value.

In essence, GPE calls for the use of light to make sugar and heat to forge that sugar into new material. On the other hand, growers don’t want this high-temperature regimen during dim conditions, as the plant may need more assimilates for “everyday maintenance” of the leaves and cellular apparatus than the plant can make under low light, resulting in cessation of growth and lack of resilience.

Given the dynamic nature of greenhouse environments, how do growers balance the production and usage of assimilates? Cultivators have traditionally observed their crops and lowered light or nitrogen to stop excessive vegetative growth, and lowered temperature for excessive flowering. These reactive measures are a poor way to steer a crop toward a successful yield. GPE provides proactive methods that render more predictable, uniform yields by maintaining a more constant ratio of temperature to radiation. First, environmental control systems with photosynthetically active radiation, or PAR, sensors can track and estimate DLI, and growers can program temperatures to then increase with increasing predicted DLI. Additionally, night temperatures can be adjusted to dial in the proper 24-hour temperature once that day’s DLI has been locked in. Determining the target temperature follows the formula: Target Temperature = 18 + (2 x DLI/10) with 18 being the base temperature in Celsius for a dark day. A more typical DLI of 40 for cannabis would have a temperature of 26 degrees Celsius (78.8 degrees Fahrenheit). The GPE authors give examples of when to adjust this formula for challenging conditions. For example, in very hot climates or seasons, the baseline temperature of 18 degrees Celsius could be shifted upward to 20 degrees Celsius to account for the difficulty of cooling the greenhouse environment and to lessen fluctuations when cooling cycles on and off. (Note: This same formula might prove useful for determining DLI targets as you lower temperatures near harvest.)

It is also important to consider the plant load, which, in the case of cannabis, is defined as the number of plants per square meter and the number of flowers per plant. Can the assimilates the plants are producing actually sustain the number of flowers you hope to yield? Though no recommendations have been devised for cannabis, GPE principles suggest that it is best to use low plant loads with high temperature regimes to maximize quality and produce predictable, uniform yields. Higher plant loads would require lower temperatures, negating the benefits of the higher temperatures described previously. If both plant load (flower canopy) and temperatures are high, growth and flower yield may be diminished due to competition for assimilates for maintenance. 

As far as equipment, assimilates production also can be improved by increasing light or light interception, using more lights, intracanopy lighting, light-diffusing roof and wall panels, or light-diffusing shade screens inside the structure. Diffusing light improves penetration into the lower foliage, according to a 2015 research paper published by Frontiers in Plant Science. This prevents lower leaves from turning from sources of photosynthetic assimilates to “sinks” (organs that use them up).

Energy Balance

The “Plant Empowerment” authors also discuss “energy balance,” which refers to the four different types of energy flows: light, heat, convection by air currents and evaporation. These four energy flows can be measured, and their values must add up to zero, according to the law of conservation of energy. Plants can’t make their own light (that we can observe easily) or heat, so these can only be energy inputs toward the plant.

A key insight of GPE is that water evaporation through micropores in the leaf and through stomata themselves occurs at night and should be encouraged. The authors cite data that a full-grown tomato crop evaporates 25 g/m² of water overnight, and American Society of Plant Biologists research shows transpiration via stomata can be up to 30% of daytime rates, though the function of this water loss is still unknown. As much as this nocturnal evaporation challenges conventional wisdom—and as troubling as this sounds for humidity control concerns—that evaporative flow is bringing water and nutrients, particularly calcium, to the tips of the plant. It also relieves root pressure that results in cell damage to the edges of young leaves and guttation droplets—a recipe for possible fungal infection. But it requires energy to perform. Evaporation of water requires 2.3 megajoules (MJ)/kilo of energy, according to data from the Engineering ToolBox. Without light, plants need energy from other sources to evaporate. Convection currents occur from heat rising off heating pipes or tubes or from heated air moving through the greenhouse. If the temperature of this convective air flow is warmer than the plants, that energy can be absorbed and used for evaporation.

Heat emission is when one body radiates infrared radiation to a cooler body until they are both in equilibrium. Plants emit heat toward a cooler greenhouse roof or light deprivation curtain at night, losing energy that could otherwise be used to sustain evaporative flow. Furthermore, heat emission can cause the plant to cool, possibly below the dew point of the greenhouse air, allowing condensation to form on the leaf surfaces. Preventing heat emission involves closing energy curtains to create a barrier between the roof and the plants or the surrounding light deprivation cloth. The book authors cite a study indicating the decisive factor for botrytis infection of greenhouse gerbera daisies was heat emission and lack of movement under the light-deprivation screen, rather than high relative humidity. Could this be the case in flowering cannabis?

Loss by heat emission also can be notable during early morning and evening in a greenhouse when the roof temperature is cool. Growers should open energy curtains long after sunrise and close them long before sunset. Just exactly when to open/close them would require measurements that can’t be made from the typical aspirated sensor box hanging in a greenhouse. The authors recommend a sensor called a pyrgeometer for outdoor weather stations and a net radiation sensor inside, which both measure heat emission. For tall crops, they recommend thermographic cameras that would “heat map” and quantify leaf surface temperatures along the length of the plant. Tops of the plant are often cooler due to heat emission and more likely to stop evaporating and form condensation, something to consider with tall strains. Detailed control advice for curtains and heating is provided in the “Plant Empowerment” book.

Water Balance

Water uptake must be balanced against evaporation to prevent drought stress and linked not only to sunlight but to other energy flows. These flows of light, heat, convection and evaporation can be accumulated in an energy sum to refine irrigation triggering, particularly by accounting for the night environment. Additionally, irrigation can be triggered by gravimetrics, or weight scales. For rockwool production, each day is divided into four periods with different objectives:

  • Period 1 to refill the rockwool block or slab from overnight decrease
  • Period 2 to maintain the volumetric water content and electrical conductivity (EC), dependent on the stage of growth
  • Period 3 to maintain water content and control EC rise especially in bright afternoon light
  • Period 4 to allow the water content to drop to night target level

Learning More

“Plant Empowerment” and Letsgrow.com have detailed, real-world greenhouse climate graphs, thermographic diagrams of greenhouses to describe energy flows, design advice, plain-language summaries and chapters for getting started in small steps. Free online interactive tools show how changes in indoor or outdoor conditions impact the balances. Experts endorse the work, including a foreword by Gene Giacomelli of the University of Arizona’s Controlled Environment Agriculture Center. I am particularly intrigued by the capability of growing crops at high temperatures, as many cannabis fungal diseases are not infectious above 82 degrees Fahrenheit.

Many GPE concepts are geared toward massive, naturally ventilated greenhouses in The Netherlands, making translating to smaller, mechanically cooled greenhouses challenging and a potential shortcoming, though the authors state the concepts could be used for nearly any greenhouse and even for indoor vertical farms. Whether the principles work in more humid climates will also need to be investigated.

Robert Eddy is director of Ag Projects for Core Cannabis in East Lansing, Mich.