That way, VPD is essential when it comes to offer the best conditions for the development of plants, also to understand its water requirements according to the nutrients used and thus adjust them correctly to meet the grower’s expectations. We have seen that the best way to keep a suitable VPD value is to control the temperature and/or relative humidity, which is something we have multiple options for, as we will see later.
Low levels of VPD combined with high relative humidity (light red zone) may cause nutrient deficiencies, guttation phenomenon, different diseases or weak growth; while a high SPD value combined with low relative humidity (dark red zone) it could cause wilting, curled leaves, poor growth or crisp leaves.
In the chart above you can see the right temperature and moisture values to maximize the development of cannabis plants. The green zone shows you optimal values of VPD, in which it would be ideal to maintain the environment. The orange zone shows correct values, in which plants should not have problems to develop normally. However, red areas show values in which your plants will be very far from hit their full potential, since the relation between temperature and moisture is not optimal.
Saturated steam pressure values chart
Let’s see an example: imagine that we have a grow room at 28°C with a relative humidity of 75%. If we take a look at the SVP chart, we will know that at 28 degrees Celsius corresponds to 3779 Pa. Let’s apply the formula:
If you usually read the indoor growing section of our Blog, you probably already know most of the systems to control the climate for this cultivation technique. However, let’s make a brief review for those who are not sure how to set up an indoor grow.
With relatively high values of VPD (between 5 and 12) plants tend to open their stomata and release a considerable amount of water vapor into the environment (increasing their transpiration). This increase in transpiration results in an increase in the plant’s photosynthetic activity, which will need more nutrients and will improve its overall growth, both during growth and bloom. The best range is between 7.5 and 10.5, which we have marked in green. However, we must be careful that VPD is not too high (dark red zone) since the plant would close its stomata to avoid releasing too much water vapor to the environment, which would result in fast dehydration due to excessive transpiration.
Often, marijuana growers who have their plants in indoor grow tents or rooms with artificial lighting, take into account parameters such as temperature and relative humidity to maximize plants growth and bloom, thus obtaining abundant harvest with top quality flowers. As we will see later, these factors affect different processes of the plant, such as transpiration or nutrient uptake.
In order to calculate the vapor pressure deficit in our grow space, we just need to know the temperature and relative humidity inside it. We also need to know the saturated vapor pressure (SVP) values for a given temperature, which are listed below.
Temperature and relative humidity play an important role in indoor marijuana cultivation since much of the plant’s activity depends on these factors.
Understanding VPD and Transpiration Rates
As we will discuss, plant transpiration drives plant growth and vapor pressure differential (VPD) drives plant transpiration. Temperature and humidity both impact the vapor pressure differential, a factor that must be considered when making decisions about facility systems—especially when it comes to the tricky deliberations surrounding heating, ventilation, and air conditioning (HVAC) equipment selection. In order for plants to thrive in an indoor grow space, the VPD needs to be at a particular level, which can be different for every situation and every stage of growth. Since the temperature and humidity affect the VPD, the temperature and humidity both need to be at the correct levels—at the right “setpoint”—which means heat and moisture are going to need to be added or taken away from the grow room at different times.
Temperature and humidity are defining factors for VPD, sometimes called vapor pressure deficit, which is what truly affects the health of a cannabis plant. Vapor pressure is the pressure at which liquid becomes a vapor. Here is a real-world example of vapor pressure in action: When you boil water on the stove, you heat the water, increasing the pressure to a point at which it reaches the vapor pressure of the atmosphere around it and becomes steam. In cannabis cultivation, VPD refers to the difference between vapor pressure within a plant and the vapor pressure of the air surrounding the plant. VPD is responsible for driving a process in the plant known as transpiration, which directly impacts plant health.
A common misconception is that the transpiration of cannabis plants will affect the relative humidity within a grow room. In reality, that’s backwards—using the paradigm that the room conditions affect the plants’ ability to transpire rather than the plants’ transpiration affecting the room condition is a good perspective to have when reading this article. In an ideal setting, the room setpoint conditions (temperature and humidity) would be held perfectly stable and would never deviate regardless of what happens in the room. Keeping the room conditions perfectly stable is the job of the mechanical equipment used to control the environmental conditions of the room. If we can maintain any given set of room conditions, the question is: under what conditions does cannabis thrive?
Transpiration is a process in which water and other essential nutrients move through a plant from cell to cell. It is also how plants regulate their own temperature and obtain the carbon dioxide they need out of the air. VPD drives transpiration and the nutrient uptake from the roots of a plant to the upper area of a plant. Water movement occurs as a result of plants releasing water vapor into the air through openings called stomata—almost as if they are sweating.
- Wet and dry bulb temperature readings: Dry bulb temperature is the temperature reading most of us are familiar with; the temperature that is shown on the thermostat in a home. Wet bulb temperature is the temperature that a thermometer reads when its bulb is wrapped in a moist cloth. The wet bulb temperature indicates how much moisture is present in the air. When relative humidity is at 100%, the wet and dry bulb temperatures are equal. If the difference between the dry and wet bulb temperatures is small, there is a large amount of moisture in the air. There is so much moisture in the air that it is similar to having a wet rag around the thermometer bulb. If there is a large difference between the dry and wet bulb temperature readings, the air is dry.
What Are the Metrics?
This article explores how vapor pressure differentials (VPD) affect transpiration rates in cannabis plants. Transpiration is a process in which water and essential nutrients move through the plant from cell to cell. Understanding of VPD at different temperatures and relative humidities, and its effect on this process, is important to gain maximum plant growth. We also cover the impact of set points on growth environments as well as energy costs, along with an analysis on how data surrounding watering rates and transpiration can affect equipment selection.
We all learned about the water cycle in junior high—rain, evaporation, clouds, rain, and so on. This cycle takes on increased importance in the cannabis industry because maintaining the right space conditions for an indoor grow is essential to the success of a cultivation operation. Temperature and humidity play a large role in how cannabis plants will function, directly impacting both plant yield and overall quality. We must look beyond the simplified precipitation cycle and understand that space conditions directly affect a plant’s ability to sweat, or transpire.
This article explores how vapor pressure differentials (VPD) affect transpiration rates in cannabis plants. Understanding of VPD at different temperatures and relative humidities, and its effect on this process, is important to gain maximum plant growth.