Vapor Pressure Deficit (Best Cannabis VPD)
The global cannabis industry is rapidly expanding, with the report of Grand View Research estimating the legal cannabis market to be worth USD 16.7 billion in 2022. The industry is projected to grow at a compound annual growth rate of 25.4% from 2023 to 2030.
This industry is profitable, with multiple entry points to it, becoming a grower is an attractive option. However, to succeed as a cannabis grower, understanding the ideal growing conditions, including vapor pressure deficit (VPD), is critical.
Meanwhile, most indoor growers will encounter some tricky plant growth problems. Mold growth, slow and stunted growth, nutrient deficiencies, and overwatering are common problems for cannabis growers in the growing process.
These problems can occur when the vapor pressure deficit levels in the grow room are either too high or too low. A VPD that is too high can lead to transpiration stress and reduce the plant's ability to absorb essential nutrients. While a VPD that is too low can create humid conditions that are conducive to mold and mildew growth.
Maintaining the right cannabis VPD levels is essential for healthy growth and can help to prevent these problems. It is important to monitor VPD levels and adjust the environment.
But what exactly is VPD? Why is it so important? How can you use it? Read this article, and you'll know the answers.
What does VPD Stand For?
VPD meaning: "VPD is the abbreviation of Vapor Pressure Deficit. VPD is the difference between the amount of moisture in the air and how much moisture the air can hold when it is saturated. "
This is the definition of VPD provided by Wikipedia, and VPD has an important regulatory role in indoor cultivation applications such as greenhouses and grow rooms.
vpd formula: VPD = Saturation Vapor Pressure (SVP) - Actual Vapor Pressure (AVP)
VPD kPa: one of the measurement units is kilopascals (kPa). It is a unit of pressure. Of course, it has other units of measurement, such as Paslas (Pa), hectopascal (hPa), millibars (mb), pounds per square inch (psi, lb/sqft), or kilograms per square meter (kg/sqm).
100 Pa = 1 hPa = 1 mb = 0.1 kPa
However, kPa is the most commonly used unit of measurement for VPD in the horticulture and agriculture industries because the numbers in VPD kPA are clean with whole numbers.
Influencing factors：The value of Vapor Pressure Deficit (VPD) is affected by several factors
① Temperature: As the temperature rises, the air's capacity to hold moisture also increases, leading to a decrease in VPD.
② Humidity: As the humidity increases, the air's moisture increases, which reduces the VPD.
③ Air pressure: Changes in air pressure can affect the VPD by changing the air's ability to hold moisture.
④ Plant type: Plants have different transpiration rates and water requirements, which affect the VPD .
⑤ Light intensity: Higher light intensity can lead to increased transpiration and evaporation, increasing the VPD.
⑥ Wind speed: Higher wind speeds can increase transpiration and evaporation, increasing the VPD.
⑦ Altitude: At higher altitudes, the air pressure is lower, which can affect the VPD.
What is VPD in Plants
The optimal value of Vapor Pressure Deficit (VPD) depends on the type of plant and its growth stage.
For most plants, an ideal VPD range is between 0.4 kPa and 1.6 kPa. This range provides enough moisture for the plant to transpire and absorb nutrients while allowing for sufficient evaporation to cool the plant and prevent overheating.
However, for some plants, such as succulents and cacti, a higher VPD is acceptable, as they are adapted to arid conditions. On the other hand, for plants such as ferns and orchids, a lower VPD is preferred, as they are adapted to humid environments.
In general, monitoring VPD and adjusting the growing conditions as needed is recommended to maintain an optimal range for the specific plants being grown.
The benefits of maintaining the right temperature and humidity in your grow room:
- Optimal Plant Growth
- Pest and Disease Control
- Energy Savings
- Better Quality
- Increased Yields
Next, let's look at what range of VPD values are appropriate for cannabis.
Best VPD for Cannabis
In general, the VPD for cannabis is between 0.6 to 1.5.
But just like other plants, the ideal VPD range for cannabis plants can vary depending on the plant's growth stage. During different stages of growth, the plant's transpiration rate and water requirements will change, and so will the VPD range.
Ideal VPD for Seeding
Seedlings are delicate and sensitive to environmental changes, so maintaining appropriate VPD levels is crucial to ensure healthy growth and development.
For seedlings, a low VPD range is recommended, 0.6-1.0 kPa is a lower range compared to later stages of growth. A lower VPD range with higher humidity levels helps to reduce the risk of water stress on the seedlings while still providing adequate moisture for growth.
VPD for Veg
During the vegetative stage, cannabis plants require a higher VPD range compared to the seedling stage. The recommended VPD range for the vegetative stage is 0.8-1.2 kPa.
At this stage, plants are actively growing and require higher humidity levels to support transpiration and nutrient uptake.
Best VPD for Flowering
During the flowering stage of cannabis growth, plants are producing buds and flowers, and they require slightly lower humidity levels to prevent mold growth and maintain terpene production.
In this initial growth stages, your cannabis plants require low VPD as it keeps the stomata open. Allowing plants to increase their absorption rates for better growth.
However, once your plants have reached the flowering stage, it is better to maintain a higher VPD level. This prevents the growth and development of mold and the spread of other pathogens to your established crop.
The ideal VPD range is considered to be between 1.0-1.5 kPa. Maintaining appropriate VPD levels is critical to ensure optimal plant growth and development, maximize yields, and potency.
Here is a list of recommended VPD ranges for each growth stage of cannabis, it will be clearer.
Each Growth Stage
The Ideal Cannabis VPD ranges
The plant is just starting to grow roots and leaves during this stage. A low VPD in the range of 0.6-1.0 kPa is recommended to keep the environment moist and prevent water stress.
The plant is actively growing leaves and stems. An ideal VPD veg range of 0.8-1.2 kPa is recommended to promote healthy growth and prevent excessive water loss.
The plant produces buds and flowers. A best VPD for flowering is 1.0-1.5 kPa, it allows for adequate transpiration and prevent mold or Powder Mildew from forming.
It is important to note that the ideal VPD range will vary depending on various factors such as temperature, humidity and air movement. Therefore, it is critical to monitor and adjust VPD as needed.
Importance of VPD
Vapor Pressure Deficit (VPD) is an important factor in controlling the growth and health of plants. In real-world terms, VPD plays a critical role in regulating plants' water uptake and transpiration. By controlling the VPD, you can regulate the quantity of water and nutrients available to the plant. It can affect its growth, yield, and quality.
VPD and Transpiration
Since VPD cannabis cultivation involves moisture transpiration in your plants. It is important to first go back to what the transpiration rate is.
Transpiration of plants
Transpiration is the process by which water is lost from a plant through its leaves. This occurs as water moves from the roots to the leaves through tiny tubes called xylem and then evaporates from leaves' surface into the air.
Transpiration helps regulate the plant's temperature and provides water and nutrients to the rest. It is also a way for the plant to remove excess water, which can build up in the leaves and cause damage. In short, transpiration is a crucial process for the survival and growth of plants.
Stomata are tiny pores on the leaves' surface that allow plants to take in carbon dioxide for photosynthesis and release oxygen and water vapor through transpiration. The plant controls the opening and closing of stomata to regulate the exchange of gases and water vapor.
The size of the stomatal opening is influenced by various factors, including light, temperature, humidity, and the plant's water status.
When the plant is experiencing high humidity, low light, or low temperature, it will close the stomata to reduce water loss and conserve energy. Conversely, when conditions are favorable for photosynthesis, such as high light levels and low humidity, the stomata will open to allow for maximum gas exchange.
Carbon dioxide uptake
Carbon dioxide uptake refers to the process by which plants take in carbon dioxide from the air and use it as a source of carbon for photosynthesis.
Photosynthesis is the process by which plants convert light energy, carbon dioxide, and water into chemical energy stored in the form of sugars and starches. During photosynthesis, the carbon dioxide is absorbed by the leaves through the stomata, and the light energy is captured by chlorophyll and other pigments in the chloroplasts.
Nutrient uptake by roots
The roots of a plant are responsible for obtaining water and nutrients from the soil, which are then transported through the plant's xylem tissue to the rest of the plant. The roots absorb nutrients in the form of ions and other dissolved substances, which are taken up by specialized root cells called root hairs.
VPD Requirements for Cannabis
The optimal Vapor Pressure Deficit (VPD) range for Cannabis plants can vary depending on the stage of growth and the desired outcome.
It's important to note that these ranges are general guidelines and may vary depending on the strain, temperature, and humidity levels in your grow room. Monitoring VPD regularly and adjusting your climate control system as needed can help ensure that your plants receive optimal growing conditions.
Too high of a VPD can lead to excessive transpiration and water stress, reducing growth and yield. On the other hand, a VPD that is too low can create a moist environment that is conducive to mold and Powder Mildew growth. Therefore, it's essential to monitor and adjust VPD regularly to ensure optimal growing conditions for your Cannabis plants.
VPD and Indoor Climate
We know that VPD is highly influenced by growing room temperature and humidity levels. Therefore, it is important to maintain these factors at the right levels to ensure maximum transpiration for growing buds.
Cold temperature below 65°F slows down plant growth due to a reduced rate of photosynthesis. Moreover, anaerobic pathogens seek this opportunity to attack your weaker roots, resulting in mold and disease.
Similarly, if the temperature goes above 80°F, that’s no good for your cannabis crop either, as the plant would need to use part of its energy to dissipate heat.
On the other hand, if your grow room is at 100% relative humidity. No further water can be absorbed by the air, and the plant’s transportation process will completely stop. This also indicates that SVP has been reached.
But, if the relative humidity percentage is too low, the stomata will close in an effort to preserve the plant’s moisture content. Also stunting the transpiration process.
looking for a system to control temperature and humidity?
Altaqua Grow Room HVAC system may help, it controls temperature, humidity, and airflow to create an ideal indoor growing environment for your plants at each grow stage.
VPD at Night
During the night (or lights off period for indoor growers), VPD is typically lower as plants are not actively transpiring. Therefore, the ideal VPD range during the night is generally lower compared to the daytime.
For most plants, including cannabis, the ideal nighttime VPD range is around 0.8-1.0 kPa. This range helps to maintain a balance between plant water uptake and water loss through transpiration while minimizing the risk of plant stress or disease.
Cannabis vpd chart
VPD charts are graphical representations that show the relationship between temperature, humidity, and VPD.
VPD charts can also be used to help growers make informed decisions about irrigation, ventilation, and other growing conditions to maintain an optimal VPD for their plants.
how to read VPD chart
To read a VPD chart, you need to understand the following:
X-axis: The x-axis represents temperature, usually in degrees Celsius or Fahrenheit.
Y-axis: The y-axis represents relative humidity, expressed as a percentage.
Zones: The chart is divided into different zones, each with a different color or shading, representing different VPD ranges. For example, green may indicate an optimal VPD range for most plants, while red may indicate a VPD that is too high and could cause stress.
Here is the Cannabis VPD chart.
The VPD chart for cannabis growers is similar to a standard VPD chart but with ranges that are specific to the needs of cannabis plants.
- Vegetative growth: 0.8~1.2 kPa
- Flowering: 1.0~1.5 kPa
The chart would then be divided into different zones, each with a different color or shading, representing different VPD ranges for cannabis plants. For example, a green zone may indicate an optimal VPD range for the vegetative stage, while a yellow zone may indicate a VPD that is approaching the upper limit for the flowering stage.
How to Use VPD Charts
To use a VPD chart, follow these steps:
Step 1：Measure the temperature and relative humidity
Use a thermometer and hygrometer to measure the temperature and relative humidity in your growing area.
Step 2：Locate the point on the chart
Find the point on the VPD chart that represents the current temperature and relative humidity.
Step 3：Identify the VPD range
Look at the color or shading in the area where your point is located on the chart. This will tell you the current VPD range for your plants.
Step 4：Compare to the ideal range
Compare the current VPD range to the ideal range for your plants, as indicated on the chart.
Step 5：Make adjustments as needed
If the current VPD range is not within the ideal range for your plants, make adjustments to your growing conditions as needed to bring the VPD back into the ideal range.
VPD Chart for flower of cannabis
VPD charts help cannabis growers optimize growing conditions for their plants by showing the ideal Vapor Pressure Deficit range for each plant species.
By regularly measuring temperature and relative humidity, plotting the current VPD range on the chart, and adjusting growing conditions as needed, growers can maintain an optimal VPD for healthier, more vigorous plant growth and better yields. Especially at this stage of flowering, the vpd chart is very important.
To better understand how Vapor Pressure Deficit affects your plants, it is important to study it in depth. There are two terms that directly control the VPD of your grow room.
Saturation Vapor Pressure (SVP): SVP is the maximum amount of water vapor that air can hold at a given temperature. As the temperature increases, the air can hold more water vapor, and the SVP increases. SVP is determined by the temperature and is an important factor in calculating VPD.
Actual Vapor Pressure (AVP): AVP is the amount of water vapor that is actually present in the air at a given temperature. AVP is affected by both the temperature and the relative humidity of the air. Higher temperatures and higher relative humidity levels result in higher AVP values.
AVP and SVP are interlinked, as the maximum AVP that your grow room can tolerate is its SVP. It is important to fully understand these terms, as it will help you calculate your grow room’s VPD below.
Two Methods of Calculating VPD
Vapor Pressure Deficit (VPD) can be calculated using two methods:
This method calculates the VPD using the temperature and relative humidity in the grow room.
The formula: VPD = SVP - AVP
②Wet Bulb Temperature Calculation
This method calculates the VPD using the temperature and wet bulb temperature in the grow room.
The formula: VPD = SVP at dry bulb temperature - SVP at wet bulb temperature
The dry bulb temperature is the temperature recorded by a thermometer in the grow room. The wet bulb temperature is the lowest temperature that can be achieved through evaporation.
How to Calculate the VPD of Air?
To calculate the VPD of air, you need to know two things: the air temperature (°C or °F) and relative humidity (%). Then, you can use either the dew point calculation method or the wet bulb temperature calculation method.
➪For the dew point calculation method:
1. Calculate the dew point temperature (°C or °F) using a dew point calculator or formula.
2. Subtract the dew point temperature from the air temperature to find the VPD.
➪For the wet bulb temperature calculation method:
1. Measure the air temperature and relative humidity.
2. Use a psychrometric chart or calculator to find the corresponding wet bulb temperature.
3. Subtract the wet bulb temperature from the air temperature to find the VPD.
How to Calculate the VPD of Leaves?
To calculate the VPD of leaves, you need the following values:
1. Leaf temperature (Tleaf)
2. Air temperature (Tair)
3. Relative humidity (RH)
Once you have these values, you can use the following formula to calculate the VPD of leaves:
VPD = eair-eleaf
eair = saturation vapor pressure at air temperature (Tair)
eleaf = saturation vapor pressure at leaf temperature (Tleaf).
The saturation vapor pressure can be calculated using the Antoine equation.
If you find the calculations difficult, we have also provided links to the calculators. So if you need it, please click on it and try it out!
Utilization of VPD
VPD Detection Equipment
VPD detection equipment includes:
1. Hygrometers and thermometers to measure temperature and relative humidity.
2. Data loggers and sensors to continuously monitor and record VPD.
3. Portable VPD meters to measure VPD in the field.
4. Computer programs to calculate VPD using temperature, relative humidity, and pressure readings.
1. Decagon 5TM, EC-5 Portable VPD Meter
2. Delta-T Devices VPD2 Evaporation and Transpiration Sensor
3. Rotronic HygroLog HL-NT Data Logger with VPD Calculation
4. Extech RHT3 Hygro-Thermometer with VPD Calculation.
How Growers Can Influence and Change VPD
Growers can influence and change VPD in several ways:
Way 1: Control the temperature
By controlling the temperature, growers can change the vapor pressure in the grow room. In addition, they can use
HVAC equipment to keep the temperature within a desirable range.
Way 2: Control the humidity
By controlling the humidity, growers can change the vapor pressure in the grow room. They can use dehumidifiers to reduce humidity and humidifiers to increase humidity.
Way 3: Adjust the air flow
By adjusting the air flow, growers can help distribute moisture evenly throughout the grow room and prevent it from collecting on the leaves.
Way 4: Control the light
By controlling the amount and duration of light, growers can influence the transpiration rate of the plants, which affects VPD.
Relationship between VPD and HVAC
In addition to the four methods mentioned above, there is another method: the use of HVAC.
HVAC systems provide temperature and humidity control, which are key factors in determining VPD.
For example, HVAC systems can be used to maintain a temperature range that is suitable for plant growth and to regulate the humidity levels to prevent mold and other air quality problems. By controlling the air flow and air exchange, HVAC systems can also help to maintain a healthy indoor environment with an appropriate VPD.
How to Create the Best Growing Environment with VPD
To create the best growing environment with VPD, you need to consider the following steps:
Step 1: Measure the temperature and relative humidity of your grow room regularly.
Step 2: Use a VPD calculator to determine the VPD range for your plants based on their growth stage and the temperature and humidity readings.
Step 3: Use HVAC systems and dehumidifiers to control the temperature and relative humidity in your grow room, aiming to keep the VPD within the recommended range for your plants.
Step 4: Monitor your plants for signs of stress, such as discoloration or stunted growth, and adjust your HVAC and dehumidification systems accordingly.
Step 5: Regularly inspect your plants for signs of powdery mildew or mold, which can be a result of low VPD or high humidity.
By maintaining the right VPD for your plants, you can improve their growth, increase yields, and reduce the risk of disease and pest problems.
Q1. What is VPD in growing?
VPD is important for plant growth because it affects the ability of plants to transpire. Plants require a certain level of VPD for optimal transpiration and photosynthesis.
Q2. Is VPD more important than humidity?
While both VPD and humidity are important, VPD can be a more useful indicator of plant health and growth than humidity alone. This is because VPD takes into account both temperature and humidity, whereas humidity only measures the amount of moisture in the air.
Q3. How to lower VPD value?
To lower VPD values, you can increase the humidity in the growing area or reduce the temperature. Increasing humidity levels will decrease the VPD value by increasing the amount of moisture in the air. Lowering the temperature will decrease the VPD value by reducing the amount of moisture that the air can hold.