The Vapor Pressure Deficit, or VPD, refers to the difference between the amount of moisture in the air and how much moisture the air can hold when it is saturated. The optimal VPD range in grow tents is between 0.8 and 1.2 kPa. The optimal VPD range differs in three plant growth stages, including 0.8 kPA for seedlings, 1.0 kPA for vegetative, and 1.2-1.5 kPA for flowering.
The vapor pressure deficit in the grow tent is crucial for indoor plants’ optimal and healthy growth. When the vapor pressure deficit surpasses 1.2 kPa, it causes higher transpiration, leading to heat injuries and increased leaf temperature. Conversely, when the vapor pressure deficit is lower than the minimum threshold (less than 0.8 kPa), it causes low transpiration, leading to slower plant metabolism and slower plant growth. Due to low transpiration, plants are more prone to diseases, decay, and death.
The vapor pressure deficit uses the VP calculator to determine the target levels necessary for optimal vegetation growth. The vapor pressure deficit is more important than the relative humidity. The VPD is unaffected by temperature changes, providing a more precise assessment of plant transpiration and water loss compared to relative humidity. To control the vapor pressure deficit, increase or decrease the temperature, light intensity, and humidity to ensure proper growing conditions for indoor plants.
Table of Contents
What is Vapor Pressure Deficit (VPD) in Grow Tent?
Vapor Pressure Deficit (VPD) in a grow tent refers to the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated. It’s a measure of the drying power of the air around your plants, indicating how much potential the air has to evaporate water from the leaves.
The maximum amount of water vapor that air holds at a certain temperature is called saturation vapor pressure (SVP). The current actual amount of water vapor in the air is known as the actual vapor pressure (AVP).
The optimal range for plant growth is between 0.8 kPA and 1.2 kPA. Maintaining an optimal VPD is crucial as it affects the plant’s transpiration rate – the process of losing water from plant leaves to the atmosphere. The transpiration process is vital for nutrient uptake, cooling of the plant, and overall growth and productivity.
A high VPD (greater than 1.0 kPa) indicates the air can still hold much water. A low VPD indicates the air is near saturation. However, a VPD of zero means the air is 100% saturated, and the plants cannot transpire effectively, as Michigan State University explains.
When the VPD is in optimal range, plants take up CO2 for photosynthesis more efficiently and grow better. When the VPD is lower, it leads to issues like plant stress, slowed growth, or increased susceptibility to diseases. A higher VPD reduces stomatal conductance and photosynthesis, increasing plant water losses through transpiration. On the other hand, when the VPD is low, plants cannot transpire, causing diseases and eventually death, according to a study from the New Phytologist.
The 3 stages of growing plants include seedlings, vegetative, and flowering stages, with the optimal VPD range of:
- 0.8 kPA for seedlings
- 1.0 kPA for the vegetative stage
- 1.2-1.5 kPA for the flowering stage.
The image below depicts the optimal VPD range for 3 stages of growing plants.
Why is VPD Important for Plants in Grow Tent?
VPD is important for plants in grow tents to improve their yields. The VPD regulates optimal transpiration, allowing water evaporation from plants through tiny openings in the leaves, known as stomata. The nutrients are transported from the roots to the rest of the plant, cooling it down and improving the overall growth and development of the indoor growing plant.
As the VPD controls the opening and closing of the stomate, it also regulates the transpiration rate, CO2 uptake, plant stress, and nutrient uptake. The VPD must be in the optimal range to prevent the stomate closure, causing decay and death of the plants.
For instance, a higher VPD (more than 1.2 kPa) leads to higher transpiration, causing heat injuries and increased leaf temperature. The plants will dry due to high temperatures, and the evaporation rate from the leaf’s stomata will be too fast, encouraging plants to dry faster. Conversely, low VPD (less than 0.8 kPA) causes low transpiration, leading to slower plant metabolism and inhibiting development.
How Vapor Pressure Deficit Affects Evaporation?
The optimal VPD influences 5 key factors necessary for evaporation and healthy growth of the grow tent vegetation.
- Affects Stomata opening: VPD influences the opening and closing of stomata, small pores on the leaf surface responsible for gas exchange. When VPD is optimal, the stomata functions properly, facilitating efficient CO2 uptake for photosynthesis.
- Allows CO2 uptake & photosynthesis: When the VPD is within an optimal range, the stomata open, allowing CO2 to enter the leaf tissues. CO2 is used in photosynthesis to produce glucose, which is the primary energy source for plant growth.
- Improves transpiration: VPD also drives transpiration, the process where water evaporates from the plant’s leaves. Transpiration creates a suction effect that draws more water (and dissolved nutrients) up from the roots to the leaves and evaporates into the atmosphere. As this water is lost, it pulls in more CO2, further fueling photosynthesis.
- Supports nutrient uptake: Through transpiration, plants draw up nutrients dissolved in water from the roots to other parts of the plant. However, when the VPD is too low (meaning the air is too humid), the rate of transpiration decreases, reducing the driving force for nutrient uptake and CO2 absorption.
- Prevents plant stress: Inappropriate VPD levels cause plant stress. Too high VPD leads to excessive water loss, causing yellowing leaves, reduced yield, stunted growth, and wilting. Too low VPD leads to low transpiration rates, reducing nutrient uptake and disease issues, such as bacterial, fungal, and pest diseases, mold and mildew.
Does VPD Affect Your Plants CO2 Uptake?
Vapor Pressure Deficit (VPD) plays a crucial role in the CO2 uptake of indoor growing plants by controlling the opening and closing of stomata, the microscopic pores found on plant leaves. The CO2 uptake necessary for photosynthesis is 1200 parts per million – PPM.
When the VPD is optimal, the stomata open wider, allowing for more efficient CO2 uptake, which is vital for photosynthesis. This process of photosynthesis involves plants converting light energy into chemical energy using water and CO2.
However, when VPD levels are too high, the stomata tend to close as a defense mechanism against excessive water loss through transpiration. Stomata closure restricts the plant’s ability to absorb CO2, which can negatively impact photosynthesis and, consequently, the plant’s growth and health. The major disadvantages of the aggravated photosynthesis process involve decay in plants, such as diseases by bacteria, fungi and pests, yellowish leaves, and dryness.
Studies published in Scientia Horticulturae Journal, Volume 293, 5 February 2022, titled “Effect of vapor pressure deficit on the photosynthesis, growth, and nutrient absorption of tomato seedlings” have shown that by reducing the VPD, plants can increase the assimilation of CO2. However, reduced transpiration and water transport, resulting from the reduced VPD, may impair their absorption of nutrients, leading to aggravated health and ill growth of plants. Keep the VPD at optimal range and ensure healthy development for the indoor vegetation, allowing more nutrients and regular photosynthesis.
To control the optimal CO2 amount, you can use the CO2 regulator. A CO2 regulator is a device that controls the amount of carbon dioxide (CO2) being released into an indoor growing environment, such as a grow room or a grow tent. The regulators are often used in hydroponics, where maintaining optimal CO2 levels is crucial for plant health and productivity.
Why Vapor Pressure Deficit (VPD) is More Important Than Relative Humidity (RH)?
Vapor Pressure Deficit, unaffected by temperature variations, provides a more precise assessment of plant transpiration and water loss than relative humidity.
As Heidi Wollaeger & Erik Runkle from Michigan State University, relative humidity measures the amount of water vapor present in the air compared to its maximum capacity. This capacity fluctuates with temperature, with warmer air able to hold more water than cooler air. For instance, air at 80°F (26°C) can retain double the water quantity compared to air at 60°F (15°C).
However, a more precise indicator of water loss from a leaf is Vapor Pressure Deficit (VPD), which is not influenced by temperature. VPD measures the difference between the current amount of moisture in the air and the maximum amount it could hold when fully saturated, often expressed in units like pounds per square inch (psi) or kilopascals (kPa).
A high VPD value (more than 1.0 kPa) signifies that the air can still accommodate a substantial amount of water. Therefore, there’s a significant gradient between the nearly saturated plants and the air, allowing the plants to perspire and dry out over time. A low VPD shows that the air is almost saturated. If VPD reaches zero, it means the air is completely saturated, making effective plant transpiration impossible.
How Do You Calculate VPD in the Grow Room?
To determine VPD in the grow room, use a VPD calculator. The VPD calculator is a tool that helps you determine the ideal environmental conditions for plant growth. It calculates the difference between the amount of moisture in the air and how much moisture the air can hold when it is saturated.
To calculate the optimal VPD for your indoor growing plants, you need the relative humidity (RH) and temperature. To calculate relative humidity, use this formula: AVP / SVP x 100 = RH%. The current SVP is the maximum the AVP can be, and the RH equals 100%. When the AVP reaches SVP, additional moisture, such as dew, precipitates out of the air as liquid water. To get the VPD = SVP – AVP, define how much room there is in the air for more water vapor.
The image below depicts the formula for determining VPD in the air and leaves.
How to Determine the Vapor Pressure Deficit (VPD) in the Air?
To determine the Vapor Pressure Deficit (VPD) in the air:
First, compute the Saturation Vapor Pressure (SVP). The formula is:
SVP = 610.78 * e^(T / (T + 237.3) * 17.2694)
In this equation:
- “T” stands for temperature measured in Celsius degrees.
- “e” is Euler’s Number, a constant in mathematics approximately equal to 2.71828.
- The outcome, SVP, is expressed in Pascals. To convert it to Kilopascal, divide by 1000.
With the SVP determined, you can now calculate the VPD using the following formula:
VPD = SVP * (1 – RH/100)
Here, “RH” represents relative humidity.
How to Determine the Vapor Pressure Deficit (VPD) in Leaves?
To determine the Vapor Pressure Deficit (VPD) in leaves:
First, calculate the air Saturation Vapor Pressure (ASVP). The formula is the same as the one used for air VPD.
Next, determine the leaf Saturation Vapor Pressure (LSVP). This formula is identical to the one used for ASVP, but it uses the leaf temperature (usually 1-3 °C or 2-5 °F cooler) in the calculation.
Finally, compute the leaf VPD using the following equation:
Leaf VPD = LSVP – (ASVP * RH/100)
The RH represents the relative humidity.
How to Find the Right VPD Across Different Growth Phases?
Grow tent plants have 3 different stages: seedlings/clones stage, vegetative stage and flowering stage. The ideal VPD for the 3 stages includes:
Seedling/Cloning Stage: During the first phase, plants have not yet developed robust root systems to absorb water efficiently. Therefore, a low VPD (0.8 kPa) is desirable to reduce transpiration and prevent the plant from losing too much water.
Vegetative Stage: As plants grow and their root systems develop, they can handle higher rates of transpiration. A moderate VPD (1.0 kPa) is typically recommended for this stage. A moderate VPD allows for more substantial transpiration, which in turn promotes nutrient uptake and growth.
Flowering Stage: In this stage, a slightly higher VPD (1.2-1.5 kPa) is often recommended. Higher VPD cause plants to close their stomata to conserve water, which can limit CO2 intake and potentially affect yield. However, too low VPD can increase the risk of mold and disease.
How to Calculate Vapor Pressure Deficit Using a VPD Chart?
To calculate VPD using a VPD chart, follow these 4 steps:
- Measure temperature and humidity: These measurements are expressed in degrees Fahrenheit or Celsius for temperature, and relative humidity is expressed as a percentage.
- Find Saturated Vapor Pressure (SVP): Find the SVP value on a VPD chart or table that lists SVP values for different temperatures.
- Calculate Actual Vapor Pressure (AVP): Determine the AVP by multiplying the SVP by the relative humidity (expressed as a decimal). So if your relative humidity is 60%, you’d multiply the SVP by 0.60.
- Calculate VPD: Subtract the AVP from the SVP to get the VPD. The resulting value tells you how much more water vapor the air could potentially hold.
How to Control VPD in Grow Tent?
You need to manipulate the VPD to ensure the proper stamina of the indoor growing plants.
To change the VPD conditions, you can increase/decrease the temperature (run a heater or reduce AC/increase AC) to increase/decrease VPD. You can also increase/decrease humidity and run a humidifier/dehumidifier to decrease/increase VPD.
Finally, you can increase/decrease light intensity in grow tents (move lights closer or farther, add or remove grow lights) to increase/decrease VPD for optimal range and proper growth conditions of your plants.
How to Lower Vapor Pressure Deficit in Grow Tent?
To lower vapor pressure, increase humidity. Use humidifiers or spray water into the air to increase humidity. Decrease temperature by using an AC or improve the ventilation system to lower VPD. To lower VPD, reduce light intensity. Move lights further from the plants, or use fewer light fixtures. If you have a 2×2 grow tent and use smaller LED lights (20-30 watts), remove one light and then monitor the VPD and temperature. Remove another light if necessary. For 4×4 tents, dim or remove two lights and monitor the VPD and temperature values accordingly. If it’s not sufficient, remove one or two lights more and check the readings again.
How to Upper Vapor Pressure Deficit in Grow Tent?
To increase vapor pressure deficit in the grow tent, decrease humidity. Use dehumidifiers or improve ventilation to allow more air circulation. You can also increase the temperature to upper VPD. Add a heater or use heat-producing lights to decrease temperature. Intensify lighting system to raise leaf temperature and increase VPD. Move lights closer to plants and use more powerful light fixtures. 2×2 tents require 30-40 watts of LED lights per square foot. Add one light, monitor the VPD values, and add one more if necessary. For 4×4 tents, add two lights, check the readings, and repeat the process for optimal VPD range.
How to Find the Right Grow Light for the Grow Tent?
To find the right grow light for your grow tent, strive for a range of 32-50 watts per square foot in your growing area. If your tent is specifically for vegetation, around 25 watts per square foot suffice. Therefore, for a 3′ x 3′ tent, use an LED light with power consumption ranging from 225 watts (for vegetative growth) to 450 watts (for flowering stage).
Look at the chart to find the optimal grow lights for your grow tent:
| Grow tent size (ft) | Total Wattage of the grow lights (W) |
| 2×2 | 200 |
| 4×4 | 500 |
| 4×8 | 1000 |
| 6×6 | 1200 |
What is VPD at Night?
The plants close their stomata at night, as they have no light available for photosynthesis. The closure of the stomata prevents water from escaping through the open pores and reduces the transpiration process.
During the night, plants essentially “breathe” in a process known as respiration. They take in oxygen and give off carbon dioxide, opposite to the process of photosynthesis that occurs during the day. Respiration is necessary for plant growth and maintenance, as plants produce CO2 for healthy vegetation through the respiration process.
You can use the night period to reduce RH and prevent mold and mildew. You can also keep the VPD close to the daytime ranges to ensure plants produce CO2 levels through stomata during respiration.
What is the Ideal Night Period VPD in a Grow Tent?
Keep the night VPD close to the daytime VPD (0.8-1.2 kPA) to ensure continuous growth and an undisturbed process of respiration. However, plants need different growing conditions and VPD levels for their 3 stages of growth. The VPD levels must be adjusted to improve vegetation health.
The image below briefly describes ideal night period VPD in a grow tent.
Nighttime VPD for seedlings and clones
- The ideal nighttime VPD for the first stage is between 0.6 and 1.0 kPa, (best 0.8 kPA)
- Clones are essentially young plants that are in the process of establishing their root systems, which makes them particularly susceptible to stress. Strive for a more humid environment and maintain the Vapor Pressure Deficit (VPD) nearer to the lower limit of the typical range.
Nighttime VPD for vegetation stage
- The ideal nighttime VPD for the vegetation stage is between 0.8 and 1.2 kPa (best 1.0 kPA)
- During the vegetative stage, plants are larger and stronger. As a result, you can lower the humidity in your surroundings to raise the Vapor Pressure Deficit (VPD). Lowered humidity enhances the intake of water and nutrients. However, avoid excessively increasing the VPD as higher levels cause the plant’s stomata to shut, reducing their ability to absorb CO2. CO2 absorption is crucial during the vegetative phase because it is the primary component that enables plants to grow substantially.
Nighttime VPD for the flowering stage
- The ideal nighttime VPD for the flowering stage is between 1.0 to 1.5 kPA (best 1.2 kPA)
- In the last stage, plants are robust but avoid excess moisture to prevent diseases, mold, and mildew, causing decay and death in plants.
How to Monitor VPD at Night?
To monitor VPD at night, adjust the humidity (40-55%) by the temperature (65-80°F/18-26°C) to keep the VPD in range and limit any plant stress. It’s important to avoid condensation, which means you should watch out for uncontrolled temperature drops between daytime and night.
Indoor growers monitor VPD at night every 4 hours to ensure there is no change in the temperature and humidity levels. Keeping VPD in check is important to prevent moisture from gathering on the leaves overnight, which creates a perfect environment for powdery mildew. For most plants, including cannabis, the ideal nighttime VPD range is around 0.8-1.0 kPa.
