Growing cannabis is a delicate process, balancing many factors that need to be tweaked and adjusted in order to optimize plant growth and use resources efficiently. The interplay between the different parameters necessary for plant growth is where complication is introduced into the process. By increasing one factor, say temperature, you affect every other parameter, and can send your plants into a stress response.
Leaves wither, roots shrink, and plants die. This is never more evident than in the interactions between humidity, temperature, and transpiration. Hopelessly interlocked with each other, the exchange between the plant and its environment is one that needs to be closely monitored. Armed with accurate data, a grower can find the rate of transpiration and deduce how fast their plants’ metabolism is working, and adjust the environment to make it grow even faster. It’s for that reason that growers should be aware of the Vapor Pressure Deficit of their grow environments.
But first, what does this all mean?
What is Transpiration?
Transpiration is a process that occurs within a plant, where water and nutrients in the soil are absorbed through the roots and sweated out through pores in their leaves known as stomata. Plants need to expel excess water in order to keep cool and grow strong. 90% of plant water is transpired, while only 10% is used by the plant for growth.
When an environment is too humid, plants will not be able to expel water into the air. When air has too much water vapor in it, the water that plants produce will stay on them instead of evaporating, which can oversaturate the plant and lead to mold and pests. In an environment that is too dry, plants will over-transpire and release more moisture than it can replace.
Plants transpire more rapidly in the light than in the dark. Stomata react to light by opening up more. When a stomata is open, it not only allows transpiration to occur, it allows the plant to absorb more CO2.
Light also inadvertently speeds up transpiration by warming the leaf. Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises. The plant then draws more water from the ground to expel on its leaf surface for protection and cooling. This effect is a bit of a double edged sword, as it can raise the amount of water needed to sustain the plant and keep it cool.
VPD Theory
In order to monitor a plant’s ability to transpire, growers need to look at the Vapor Pressure Deficit. This is a fancy term used to describe the amount of room there is in the air for more moisture. In order to find the VPD, you only need to know two things.
Saturation Vapor Pressure – The Saturation Vapor Pressure is the maximum amount of moisture a room can hold before it starts to produce precipitation in the form of dew, mist or rain.
Actual Vapor Pressure – The actual Vapor Pressure of a space is the amount of moisture currently in the air.
With the SVP and AVP, you can calculate the relative humidity of your space. RH is the proportion of water the air is currently holding relative to its maximum capacity. You find this number by looking at the AVP over the SVP and multiplying by 100 to turn the answer into a percentage. If RH is 100% any additional moisture to the environment will precipitate out of the air as liquid water (dew, mist, etc).
With the RH, you can see how much water is in the air, but it doesn’t necessarily give an accurate prediction of how well a plant will transpire. This is because RH can be affected by changes in temperature. At 100% RH, air is considered fully saturated, but if you were to raise the temperature, you’d also raise the total amount of water the air could hold, which lowers the RH. At higher temperatures, more water can be stored in the atmosphere. The amount of water approximately doubles with every 20F-degree increase in temperature. This extra water increases the pressure of the environment on the plants, and makes the air heavier.
While RH measures the amount of water in the air over what it could possibly hold, VPD measures the difference between the total vapor pressure of the air on plants when fully saturated and the vapor pressure exerted by the air at its current saturation. Basically, it’s how heavy the air would be if it was fully saturated minus how heavy it is at its current levels. This is measured in kiloPascals, and is a more accurate way to deduce the rate of transpiration of the plant and therefore the rate at which its metabolism is working.
Leaves are considered fully saturated, so when they’re in an environment that is less saturated, they transfer their moisture into the atmosphere at a rate dictated by the VPD. A high VPD draws moisture out of the plant, because the difference between the plant and the environment is larger. Plants that have evolved to prioritize carbon gain over water conservation, such as crops, will be more likely to increase their transpiration with increasing VPD.
At 0 VPD, the air is too full of moisture to absorb any more from the plants, so the plants cannot transpire. They will soak up less water through their root systems, and their leaves and soil will remain oversaturated, producing mold, mildew, and rot.
The relationship between RH and VPD is a bit tricky. As temperature increases, so does the SVP of the environment. In order to maintain RH, you must increase AVP by introducing more water into the environment. This additional water creates more pressure on the plants, so even if you’ve maintained a 70% RH as you’ve increased temperatures from 60F-75F, you’ve increased the total amount of pressure the environment can tolerate, and in turn raised the VPD from .55 kPa to .90 kPa.
How do you Manipulate VPD?
VDP is intrinsically tied to many facets of plant growth. By regulating the VPD of a growing environment, you can manipulate the metabolism of the plant and make it grow faster and heartier. It’s important to monitor because it can also adversely affect plants if left unchecked. For instance, as VPD increases, stomata get smaller and CO2 uptake gets reduced. However, we’ve also learned that as VPD increases, the plant transpires faster due to the larger difference in vapor pressures between the air and the plants. As transpiration increases, the roots pull in more nutrients and water from the soil, and run through photosynthesis faster. If this happens too fast, it can confuse and stress out the plant, but at certain VPD sweet spots, plants move fast and efficiently without getting stressed. These targets change as the plant grows through different stages, and depend on the conditions of the room.
There are only a few key ways growers can manipulate VPD. By changing the room temperature to increase or decrease SVP; by adjusting the humidity with a dehumidifier to keep air from becoming oversaturated, or by increasing light intensity. At higher light intensities, stomata open wider and transpiration occurs faster. More water is pulled from the ground up into the air, and photosynthesis occurs at a faster rate. HPS lighting systems will increase the temperature of the plant, which may seem beneficial, but in practice, it causes the plant to over exert itself, using water at a rate faster than it can consume it. This can dry out soil and stress plants, hindering growth and lowering yield. With high-intensity LED lights, growers have more control on both temperature and light intensity, and are therefore able to manipulate both temperature and light intensity independently to achieve specific results.
Fohse’s most successful clients are masters at using VPD to push the boundaries of what plants can do under high-intensity, high-PPFD, high DLI environments. Utilizing the precision controls on our low heat, high-intensity LEDs, these accomplished growers create an environment in which every aspect is able to be finely tuned to grow large, healthy crops.
Fohse customers new to using high-intensity light can take advantage of the expertise of our long list of cultivation experts who have had success with our fixtures. To find out how, visit FOHSE.COM, and request your free light plan.
