With indoor gardening becoming ever more technologically advanced, older ideas and theories are continually being replaced with more scientific, plant physiological based concepts. One of these important concepts relates to plant transpiration and how it is driven by the water content of the surrounding air. This process has traditionally been measured as a percentage known as relative humidity (RH).

While most of us are already familiar with the concept of RH—it is widely used in weather reporting—with plant growth it’s not always the most appropriate measurement when attempting to control a crop’s environmental conditions. Vapor pressure deficit (VPD) combines the effects of both humidity and temperature into one value; it’s basically a measure of the drying capacity of the air, which in turn drives transpiration, an essential plant process.

Relative Humidity vs. Vapor Pressure Deficit

Vapor pressure deficit can be a little difficult to comprehend at first, since most of us are already familiar with RH percentages and ideal levels for a greenhouse or indoor garden. The problem with RH is that it does not take into account the effect of temperature on the water-vapor-holding capacity of the air.

Cold air holds much less water vapor than hot air; the water-holding capacity of air doubles with every 20°F increase in temperature. Air at 80°F can hold twice the amount of water vapor compared to air at 60°F. This means the water vapor content of the air at any given RH value can vary considerably depending on temperature.

Here is an example: In a 3,000-square-foot greenhouse with 10-foot ceilings, at 80 per cent RH there would be 14 pounds of water vapor in the air at 50°F. At this same humidity at 70°F, the air holds 28 pounds of water. At this higher temperature, the water vapor in the air, which drives transpiration, is significantly different, yet the RH value is the same.

Thus, plants at 80 per cent RH will have a difference in growth response if the temperature is 50°F compared to 70°F. For a closer look, Table 1 shows the effects of RH and temperature on VPD. For the same humidity, the VPD is higher at a higher temperature, which increases the rate of transpiration.

The problem with using RH means it’s hard to set one optimal RH value, when temperatures tend to vary throughout a 24-hour period. For this reason, many growers prefer to use VPD as a more accurate measure of the water vapor content of the air and how this affects plant growth.

Vapor Pressure Deficit

Vapor pressure deficit is the difference between the amount of moisture currently in the air and how much moisture the air could hold when it is fully saturated. In other words, it is a measure of how close the air is to saturation. A VPD of zero means the air is 100 per cent saturated with water vapor.

A low VPD of around 0.3 kPa means the air is still holding a lot of water vapor, while a high VPD means very dry air with minimal water vapor, but a considerable amount of drying capacity. From a plant perspective this is important as VPD directly affects the rate of transpiration, which is the water lost from the foliage surface.

VPD can also be described as the potential of the surrounding air to pull water vapor from the foliage (i.e., transpiration). Transpiration not only cools the plant, but carries essential minerals up the plant to where they are needed for tissue development.

Once air reaches a fully saturated state, termed the dew point, water will form condensation, leading to leaf wetness, which increases the plant’s susceptibility to certain disease pathogens. Technically, VPD more closely matches what the plant experiences in relation to temperature and humidity effects on growth and transpiration.

Vapor pressure deficit is measured in units of pounds per square inch (psi), millibars (mb) or kilopascal (kPa). A further point for confusion is that RH percentage and VPD values have an inverse relationship—a high VPD means the air has a high capacity to hold water (i.e., a low humidity), while a low VPD means the air is near saturation, already holding a lot of moisture (i.e., a high humidity).

Plants don’t benefit from overly dry atmospheres, as dry atmospheres rapidly suck the moisture from the foliage and can lead to a reduction in photosynthesis, fruit size and growth.

If the water flow from roots to shoots in the xylem vessels is not fast enough to replace the water lost via transpiration, the plant starts to wilt and tissue damage will occur. If too much water is being lost via transpiration, the plant will shut its stomata in an attempt to retain turgor pressure and prevent desiccation and wilting.

When the stomata shut to prevent water loss, photosynthesis cannot occur since CO2 cannot be taken in from the surrounding air. Plant growth and yield will be slowed if this occurs too often. On the other hand, plants won’t thrive in overly wet environment (low VPD), as this slows transpiration and can result in increases in disease outbreaks.

When transpiration is reduced due to a low VPD (high humidity) in the surrounding air, a number of problems can develop, including tip burn in crops such as lettuce and blossom-end rot in tomatoes and peppers due to calcium deficiency.

If low VPD (high humidity) conditions exist at the same time as high temperatures, the plant gets stressed out as it can’t evaporate enough water from its foliage to cool its tissue and it will overheat. Cell damage, wilting and reduced growth can all occur when hot plants can’t effectively cool themselves via transpiration due to low VPD.

Vapor Pressure Deficit Values

One of the main reasons why growers may be hesitant to switch to using VPD over percentage RH recommendations is the lack of widely published optimums for different crops or stages of development. Oftentimes VPD is used in greenhouse production to identify disease risk and/or used to predict and calculate crop water requirements as part of an automated irrigation program.

However, smaller growers may simply want to use VPD to maintain an optimal environment. VPD recommendations, while they are a more accurate way of determining the effect of water vapor held in the air on plant growth, vary between greenhouse crops and indoor plants, and during processes such as cloning. The ideal VPD may also depend on other factors within both the plant and growing system.

If air temperature is continually set and maintained at just one level, then use of RH is not an issue. However, where temperatures vary and the potential water vapor-holding capacity of the air changes during the growth process, then VPD is the more accurate method of measurement.

For many fruiting crops such as greenhouse tomatoes, the optimal VPD is often set around 0.8-0.9 kPa, at an average air temperature of 75°F. This would give an optimal RH of 70 per cent. At this level plants can respond with a good rate of transpiration for growth, while not risking stomatal shut down due to a very high rate of moisture loss from the foliage.

For rooting cuttings during the cloning process, the VPD is run much lower (higher humidity), typically around 0.3 kPa to retain moisture levels around the foliage and help prevent desiccation.

One interesting aspect of VPD, which is gaining traction with researchers, is the use of this environmental aspect to manipulate plant growth and quality under certain circumstances, since it is one way of applying mild plant stress.

In greenhouse tomatoes it has been found that running higher VPD (lower humidity) increased the soluble solids (sugar) content of the fruit while lowering the water content, thus giving an improved compositional quality. However, fruit fresh weight was also reduced. Encouraging high rates of transpiration needs to be carried out with caution as plants require sufficiently large, healthy root systems.

This suggests that increasing VPD to push transpiration rates, along with using techniques such as increased EC, deficit irrigation and other practices that impart a mild stress on plants, can be used to improve compositional quality of fruits and vegetables. General recommendations for mature, actively growing plants are that average VPD does not exceed 1.6-1.8 kPa when using this process to increase fruit compositional quality.

Pushing VPD to higher levels (extremely low humidity) can have negative effects on plant growth, including reduced photosynthesis, reduced fresh weight yields, plant stunting and physiological problems such as leaf curl or burn. Many growers find that some experimentation with VPD in their indoor gardens is useful.

How to Control VPD

High VPD (low humidity) is a little easier to adjust than a low VPD (high humidity) as putting water vapor back into the air can be easily achieved with some light fogging, misting, humidifier equipment or damping down in the growing area. If only a small amount of humidification is required in a limited space, an open pan of water can provide enough evaporation to increase the humidity (lower the VPD).

Evaporative coolers tend to lower the VPD of the air fairly effectively under warm growing conditions. However, having a low VPD (high humidity) is a more common problem as the plants themselves, with their large surface area of transpiring foliage, tend to lose surprisingly high volumes of water through transpiration and this adds to the humidity of the surrounding air.

This humid air, termed the boundary layer, needs to be removed from directly around the foliage, otherwise further transpiration could be restricted. The best way of doing this is with a continual stream of fresh, drier air that not only increases the VPD directly surrounding the leaf surface but also replenishes CO2 for photosynthesis.

For very humid climates (low VPD), sometimes the only option is a dehumidifier. If the outside air being brought in to cool and dehumidify an indoor garden is naturally humid, it can’t absorb much more moisture from transpiration, so using a dehumidifier is often useful for smaller areas under these conditions.

Providing the correct VPD creates an environment that drives transpiration at an optimal rate, while helping prevent disease outbreaks. VPD can also be a useful tool for applying mild plant stress to provide beneficial effects on plant compositional quality. Developing an awareness of the difference between RH and VPD also aids in an understanding of plant physiology and how the environment influences transpiration and other plant processes.