For years growers have opted to keep grow rooms drier in order to prevent mildew and other diseases on plants, and have done this primarily by controlling relative humidity (RH). However, RH is not the best measurement to accurately predict plant transpiration or water flow through the plant. The environment is not just based on RH but also on temperature. Since warm air can hold more water than cooler air, a day at 64 degrees F with 60 percent humidity will feel much drier than a hot day at 95 degrees F with the same RH.
Vapor pressure is not affected by temperature but is the absolute measure of water vapor in the air at a given temperature. As a result, Vapor Pressure Deficit (VPD) is a much more effective and reliable way to monitor and predict transpiration and maximize plant growth and production. We’ll look at some definitions and how VPD can be used to achieve a more successful harvest.
What is Vapor Pressure Deficit (VPD)?
To understand VPD, there are a few other terms that we also need to understand.
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Transpiration: This is the method by which plants move water and nutrients from their roots through the plant to the leaves, where it evaporates (transpires) into the air. Transpiration drives plant growth, allows plants to regulate their temperature, and absorb CO2 from the air. Just as humans perspire when conditions are hot, plants transpire more when the temperature rises.
- Relative Humidity: The amount of moisture in the air relative to its saturation level at a given temperature.
- Dew Point: This is the point of complete saturation, or 100 percent RH at a given temperature.
- Vapor Pressure Deficit (VPD): The difference between the amount of moisture in the air and how much moisture the air can hold when at saturation (dew point) at a set temperature.
Understanding how this difference can be used to control transpiration in plants helps maximize plant growth and flowering at different stages. Vapor Pressure Deficit is usually measured in kilopascals (kPa) and sometimes in millibars (mb).
How Does VPD Relate to Transpiration?
Vapor pressure Deficit has a direct correlation to plant transpiration rate, or how fast water and nutrients travel through the plant. As mentioned earlier, transpiration also drives plant growth.
A low VPD means there is a small difference between the vapor pressure of the air and the leaf, indicating the air is near saturation, which slows transpiration. As the VPD increases, the air can hold more water, so more water and nutrients are pulled up through the roots, bringing moisture and nutrients to the plant before evaporating through the leaves, stimulating growth.
It is important to keep the VPD in the range best suited to the plant growth stage, since too low or too high VPD can cause problems. As the temperature in a grow house increases, this “sweet spot” narrows, while cooler temperatures give the grower a wider effective VPD range. While monitoring VPD is essential for optimal plant growth, both temperature and humidity levels should be set to help create the desired VPD.
- Low VPD inhibits plants from evaporating water (lowers transpiration rate). This blocks nutrients from traveling to the roots. It also puts plants at a greater risk of mildew and disease because they are holding too much water.
- High VPD produces too much transpiration (evaporation), which creates a risk of dehydration. Plants close their stomata to conserve water, CO2 uptake decreases, transpiration increases and plants experience more stress.
How Can VPD Be Changed?
Changes in temperature, humidity, and light intensity can influence the vapour pressure deficit (VPD). Changing any one of these factors while maintaining the other two can affect VPD in the following ways.
Temperature
Increasing or decreasing the temperature will directly increase or decrease VPD.
- ↑ Temperature = ↑ VPD
- ↓ Temperature = ↓ VPD
Relative Humidity
Increasing or decreasing relative humidity conversely decreases or increases VPD.
- ↑ Relative Humidity = ↓ VPD
- ↓ Relative Humidity = ↑ VPD
Light Intensity
Increasing or decreasing the light intensity by moving indoor grow lights closer or farther to/from the plants (increase or decrease in leaf/canopy temperature) will directly increase or decrease the VPD.
- ↑ Light Intensity (move closer to plants) = ↑ VPD
- ↓ Light Intensity (move farther from plants) = ↓ VPD
How Does Controlling VPD Maximize Growth?
By controlling the VPD, growers can encourage plant growth at each stage: seedling, vegetative, and flowering, while preventing disease or dehydration at critical stages.
- Seedlings and Cuttings: At this stage, plants need to hold on to nutrients to encourage initial growth, so a lower VPD is needed. This slows the movement of moisture, allowing the plant to absorb more nutrients before transpiring excess moisture. A range of 0.5 to 0.9 kPa is a good range for seedlings or cuttings.
- Vegetative Growth: This stage requires a moderate VPD level to maintain growth without becoming too saturated with moisture, which can cause mildew and other diseases to form and take hold. A range of 0.8 to 1.1 kPa is considered ideal for this stage.
- Flowering: The flowering stage requires a high VPD since too much moisture in the air or around roots commonly causes plant infection. A range of 1.0 to 1.5 kPa is best for this stage.
A vapor pressure deficit chart is a good way to see how RH and temperature affect VPD and where to set these environmental factors to control VPD. For additional information on controlling temperature and humidity in a grow room, see this article.
An efficient tool for controlling temperature year-round is a mini-split air conditioner heat pump which can be installed in rooms without windows or ducting and can be used for heating as well as cooling. For more specific information on these AC units in growing applications, take a look at this article.
Most growing spaces tend to be more humid and a commercial dehumidifier designed for growing applications can be very effective at helping to reduce and maintain humidity levels.
In Summary
Vapor Pressure Deficit, a more stable, predictable measurement of the grow room environment, provides the most reliable way for growers to control plant growth at various stages while preventing diseases, rot, and undue stress and dehydration in plants.
This Indoor Growing page can be a useful resource for seeing other tools for indoor growing, including grow lights, controllers, grow tents, and more.
If you have questions or need assistance finding equipment for your indoor grow space, please contact our helpful customer representatives at 1-800-934-9194.
