Learn about controlled environment agriculture (CEA) and why we grow in sterilized growth rooms. Understand vapor pressure deficit (VPD), temperature differential (DIF), Carbon Dioxide (CO2), and HEPA air filtration. We fine tune all of these parameters to bring you the best cannabis!
At Golden Apple Cannabis Co. we cultivate our plants in completely sealed sterile rooms. This style of growing is also called controlled environment agriculture (CEA). The aim of CEA is to provide consistency, protection, and to maintain optimal growing conditions throughout the development of the crop. Production takes place within enclosed growing structure or room.
A main benefit of CEA growing is that one is able to control the environment. This includes the temperature of the grow environment, the relative humidity (RH), the Carbon Dioxide (CO2) levels, ethylene levels, air flow, pathogen counts, mold spore counts, dust, insects, etc.
Essentially, when we grow indoors in an environmentally controlled sealed room (closed environment agriculture) we are able to control each of these important variables to provide the best, and cleanest pharmaceutical grade cannabis possible.
Air Temperature and Humidity:
Humidity and temperature has a direct effect on a very important aspect of plant growth – plant transpiration! Healthy transpiration (the right amount), leads to healthy plants. Transpiration can be controlled through the right combination of temperature and relative humidity (RH).
As most people already know, plants obtain their draw water by drawing it in through their roots. The water is then distributed through the plant via the stems and eventually to the leaves and flowers. On the underside of plant leaves there are microscopic “breathing” pores called stomata. The plant uses these stomata to absorb CO2 which is required for photosynthesis, and also uses them to release the oxygen which is created as a waste product.
Stomata have the ability to be open or close (or partially open or close). The goal is to have the stomata closed during the dark period, but open during the light or day period of the plant. The more open stomata, the more transpiration that occurs which ultimately leads to more nutrient uptake (of certain elements), photosynthesis, and healthy growth.
The goal for a cultivator is to get as open stomata as possible, or ideal stomatal conductance, which in turn allows for ideal rate of transpiration which allows for ideal uptake and translocation of Calcium (Ca) and Boron (Bo) because they need to be ‘sucked’ up into roots with water, unlike other elements.
Vapor Pressure Deficit (VPD)
Vapor Pressure Deficit (VPD) is basically the drying ability of the air.
Moisture in the air causes a sort of pressure which bears down on the plants. Although this pressure is somewhat different to the normal concept of pressure.
High air temperatures and low humidity are a combination that will cause moisture in or on something to dry off more quickly than in cooler and higher humidity conditions. A more scientific definition of VPD is that VPD is the difference (deficit) between the amount of moisture in the air and the amount of moisture the air can hold when saturated.
In layman’s terms, VPD is a single number that takes into account both temperature and humidity.
VPD values run in the opposite way to RH vales, so when RH is high and temperature is low, the VPD value is low. The higher the VPD value, the greater the potential the air has for sucking moisture out of the plant. VPD provides a more accurate picture of how plants feel their environment in relation to temperature and humidity which gives us growers a better platform for environmental control.
In a nutshell, VPD is used to quantify what the plant’s ‘feel’ and the effects it has upon stomatal conductance and chemical leaf chemical responses.
Here is a link to an excel based VPD calculator that one can use to find VPD with the above known values.
VPD is typically expressed in the units kPa (kilopascals) with the range for most plants being 0.45 kPa to 1.25 kPa.
The numbers we shoot for (and achieve) at Golden Apple Cannabis Co. are approximately 0.9 kPa to 1.25 kPa. This works out to be generally the following temperature and RH:
o RH range of 55-60%
o Day canopy air temperature in the range of 76-78’F
o Average leaf temperature 2 degrees Fahrenheit cooler than canopy air temperature
Temperature Differential (DIF):
DIF is the difference in the highest day time (lights on) temperature and the lowest night time (lights off) temperature. Control over DIF will give you more control over a plant’s height and internode spacing (the distance between the stems or nodes on the plant) without the use of dangerous or untested chemicals or plant growth regulators.
Plant height or stem length is simply the sum of the lengths of each of the internodes. Therefore, to control plant height one must manage internode number, internode length, or both. The number of nodes and the length of each internode (the distance from one node to the next) are strongly influenced by temperature. As DIF increases, so does the internode length of most plants. It is important to understand that the effect of DIF on internode length is due to increased cell elongation, and not an increased number of individual plant cells. Plants respond rapidly to changes in DIF, with altered growth rates that are often observable in as little as 24 hours.
Keeping daytime temps lower than nighttime temps, called “negative DIF,” make plants have lower internodal length. “Positive DIF,” keeping daytime temps higher than nighttime temps, causes stems to grow longer. More recently researchers have discovered this effect comes from differential expression of enzymes that degrade gibberellins, a group of hormones that regulate plant growth.
At Golden Apple Cannabis Co. we strive for a 0 to -5 degree DIF in our gardens.
Carbon Dioxide (C02):
By using the energy of light, plants can convert carbon dioxide (CO2) and water into carbohydrates and oxygen in a process called photosynthesis. As photosynthesis requires light, this process only happens during the day. We often like to think of this as plants `breathing in carbon dioxide and `breathing out oxygen.
Plants use CO2 for growth, it is the essential building block for photosynthesis (along with light and water). When growing in sealed rooms, or controlled environment agriculture (CEA), one must supplement CO2 for the plants, just as one would have to supplement oxygen if humans were living in a sealed space.
The current levels in the atmosphere are about 300-400 parts per million (ppm). However, most crops show that for any given level of photosynthetically active radiation (PAR), or light increasing the CO2 levels will increase the photosynthesis over ambient CO2 levels. Cannabis happens to be one of these crops. For the majority of CEA crops net photosynthesis increases as CO2 levels increase from 340–1,200 ppm.
In short, by supplementing CO2 into the cultivation environment we are able to aid photosynthesis which means better growth, more yields, and the ability to run a sealed room. We run a typical CO2 concentration of 800-900 ppm during the plants “daylight” hours.
However, during the plants dark cycle, or “nighttime” photosynthesis stops and the plants stop “breathing in” C02. Therefore, it is the goal to decrease C02 to less than 400 ppm.
One last point concerning C02 is that during the dark cycle we also vent air out of the grow environment to prevent ethylene levels building up. Even low ppm concentration of ethylene gas can affect flower (inflorescence) size, plant growth rate, senescence, etc. If grow rooms or greenhouses are not vented at least once, ex., at the end of the day, and Co2 is used at 1,000 to 1,500 ppm (esp the latter), ethylene can buildup to levels that will lower yield.
HEPA, Air Filtration, and Air Flow:
Air purification is one of the most overlooked variables of an indoor garden because most airborne pathogens are invisible, but ignoring this factor could invite all sorts of problems into the grow environment.
Most of the problematic bacteria, molds and fungi that can potentially be found in CEA rooms are transmitted through the air—powdery mildew, black spot and botrytis are just three examples. Given the right conditions, these opportunistic fungi quickly wreak havoc on otherwise healthy plants and can decimate an entire crop in a short period of time. The spores are invisible to the naked eye and are often overlooked by novice growers. Once a crop is infected, it can take serious work to entirely eradicate the fungi from the room. Eliminating any potential airborne fungi with an air purification device is by far the best defense against infection.
In addition to bacteria, molds, and fungus we also have to be aware of pest insects, any potential pollen, Volatile organic compounds (VOCs) which are chemical compounds (gases) found in the air, etc.
In order to help mitigate these potential catastrophic issues, we use High Efficiency Particulate Air (HEPA) filtration systems along with Carbon Filtration systems.
Carbon filters are one of the most commonly used air purification devices by indoor horticulturists. Carbon filters use activated carbon—carbon treated with oxygen, which opens up millions of small pores in the carbon. This allows a large surface area to attract and neutralize VOCs and some chemicals and fumes that might be present in a grow environment. A chemical reaction occurs in the carbon that, in turn, causes the carbon to absorb the airborne material.
HEPA filters are paper-like filters that remove particles from the air down to .3 microns in size. Many mold spores are in the 1 to 20 micron size range, making HEPA filters a good choice for trapping mold spores. HEPA filters will also filter larger particulates out of the air.
Air flow and air circulation is also a component of the environment. We shoot for an air movement during both night and day cycles for the plants to be around 2 cfm per ft2. We make sure that all leaves are visibly affected by the fans and air movement.
The goal of proper air movement is to reduce leaf temperature, closer to that of night time air canopy temperature, to prevent formation of dew on leaves. Dew is a major vector of powdery mildew and other fungal pathogens. This is known in the CEA world as Horizontal Air Flow (HAF).