Your environmental controls might be working perfectly. That doesn't necessarily mean your plants are experiencing the conditions you think they are.

Most commercial cannabis cultivation facilities monitor temperature, relative humidity and vapor pressure deficit (VPD). Modern environmental-control systems can track these parameters continuously, generate alarms and automatically adjust HVAC, dehumidification and ventilation equipment.

But there is an important limitation that isn't always obvious: your climate-control system only knows what its sensors are measuring.

And the environment at the sensor isn't necessarily identical to the environment surrounding the plant leaves.

In commercial cultivation, understanding this distinction can improve environmental consistency, irrigation management and troubleshooting.

What is vapor pressure deficit?

Vapor pressure deficit describes the difference between the amount of water vapor present in the air and the amount required for saturation at a specified temperature. It is typically expressed in kilopascals (kPa).

Unlike relative humidity alone, VPD incorporates the effect of temperature on the atmosphere's capacity to contain water vapor. The Food and Agriculture Organization describes vapor pressure deficit as the difference between saturation vapor pressure and actual vapor pressure, and as an indicator of the atmosphere's evaporative capacity.

See the FAO reference on vapor pressure and VPD calculations.

Air VPD vs. leaf VPD: what's the difference?

Most environmental-control systems calculate VPD using air temperature and relative humidity. This produces what is generally treated as air VPD.

However, transpiration occurs at the leaf. A more direct estimate of the vapor-pressure gradient driving transpiration accounts for the temperature of the leaf itself. This is commonly described as leaf VPD or leaf-to-air VPD.

The distinction matters because leaf temperature can differ from surrounding air temperature. An actively transpiring leaf may be cooler than the surrounding air because evaporation removes heat. A leaf absorbing substantial radiant energy, experiencing restricted airflow or undergoing water stress may be warmer.

Consequently, two cultivation areas with identical air temperature and relative humidity can have different leaf temperatures and therefore different leaf-to-air vapor-pressure gradients.

A 2025 Cannabis sativa study reported both atmospheric VPD and leaf VPD, reinforcing the usefulness of separating the two measurements in cannabis research. The study's cultivar-specific results should not be treated as universal commercial setpoints.

Read the cannabis study in Frontiers in Plant Science.

A practical example: the same room, two different VPD readings

Consider a hypothetical flowering room:

Measurement	Value
Air temperature	26°C
Relative humidity	65%
Calculated air VPD	Approximately 1.18 kPa
Measured leaf temperature	24°C
Estimated leaf-to-air VPD	Approximately 0.80 kPa

Illustrative calculation, not actual Growhaus cultivation data.

The dashboard shows approximately 1.18 kPa. But when measured leaf temperature is incorporated, the estimated leaf-to-air vapor-pressure difference is approximately 0.80 kPa.

The takeaway isn't that one number is correct and the other is wrong. Both measurements describe something useful. The problem occurs when growers assume they are interchangeable.

Why this matters commercially

Suppose a cultivation manager increases dehumidification because the dashboard indicates that VPD is below the facility's target.

Before making that adjustment, it is useful to know whether leaf temperature has been measured and whether the climate sensors represent the conditions experienced by the crop.

Changing room humidity based on incomplete information could move the crop farther from its intended operating conditions. The better approach is to understand the measurement first, then determine whether an adjustment is justified.

Sensor placement: the environmental variable nobody talks about enough

Imagine a commercial flowering room with multiple cultivation benches, overhead lighting, circulation fans and conditioned air entering through supply ducts.

A temperature and humidity sensor is mounted near the center of the room. The environmental-control system maintains the prescribed setpoints. Everything appears stable.

But what is happening elsewhere?

• The air immediately below lighting fixtures may be warmer.
• The air inside a dense canopy may be more humid.
• An area exposed to a supply-air stream may experience different conditions from the center of a bench.
• Leaves near a circulation fan may exchange heat and water vapor differently from leaves in a relatively stagnant area.

The dashboard may be reporting accurate values for its sensor location while failing to reveal important environmental differences across the crop.

This is why representative sensor placement matters as much as sensor accuracy.

Five common sensor-placement mistakes

1. Mounting sensors too far above the canopy

A sensor positioned well above the crop may measure conditions that differ from those around the leaves. This becomes more important when the room develops vertical temperature or humidity gradients.

2. Placing sensors directly in supply-air streams

Sensors exposed to concentrated conditioned airflow may disproportionately reflect the supply-air environment rather than representative crop conditions.

3. Allowing radiation to influence temperature readings

A sensor exposed to radiant energy without appropriate shielding can register a temperature that does not accurately represent surrounding air temperature. Because VPD calculations depend on temperature, that error also affects calculated VPD.

4. Using one sensor to represent a large cultivation room

One sensor can be useful for control, but one sensor cannot necessarily describe conditions across a large or highly variable cultivation area. Representative spot measurements or multiple strategically positioned sensors can help identify environmental differences that a single control point misses.

5. Failing to adjust sensor positions as the crop develops

The canopy changes throughout the cultivation cycle. Plants become taller, denser and more effective at intercepting light. A sensor position that was representative early in vegetative growth may no longer be suitable during late flowering.

Why leaf temperature matters

Leaf temperature reflects several interacting processes, including radiant energy absorbed by the leaf, heat exchange with surrounding air, transpiration and evaporative cooling, airflow around the leaf, and plant water status.

For a commercial grower, leaf-temperature measurements can provide useful additional context. An infrared thermometer or suitable thermal-imaging instrument may be used to compare representative leaves across an area.

However, thermal measurements must be collected consistently. Viewing angle, emissivity settings, measurement distance, leaf selection and instrument limitations can influence results.

Leaf temperature should be interpreted as another diagnostic measurement—not a direct measurement of stomatal conductance or overall plant health.

What happens when VPD is too low?

Low VPD indicates that the surrounding atmosphere is approaching saturation and the driving force for transpiration becomes smaller.

High humidity also increases the likelihood of condensation when plant or structural surfaces cool below the dew-point temperature. Free moisture on plant surfaces can favour certain fungal diseases.

However, low VPD does not automatically mean a disease outbreak will occur. Disease development depends on the pathogen, host, environment and other conditions.

What happens when VPD is too high?

High VPD increases atmospheric demand for water. When adequate water is available and stomata remain open, transpiration may increase. If water loss exceeds the plant's ability to replace it, water stress can develop.

The response depends on cultivar, growth stage, root-zone conditions, light intensity and other factors. This is one reason to avoid adopting a universal VPD chart without evaluating the actual production system.

Why universal VPD charts can be misleading

Many cultivation resources provide suggested VPD ranges for propagation, vegetative growth and flowering. These charts can be useful as educational starting points, but they rarely capture the full complexity of commercial cultivation.

A chart may not account for:

• Whether the values represent air VPD or leaf VPD.
• Differences in leaf temperature.
• Cultivar-specific responses.
• Lighting intensity and spectrum.
• Root-zone water availability.
• Environmental gradients across the canopy.
• Differences between daytime and nighttime conditions.

A VPD number is a measurement, not a complete cultivation strategy.

How to audit environmental measurements in a commercial grow room

Step 1: Verify the primary sensors

Confirm sensor locations, calibration status, maintenance history and the measurements used by the environmental-control system. Where appropriate, compare readings with a suitable calibrated reference instrument.

Step 2: Map the growing area

Divide the cultivation room into representative monitoring areas. Consider proximity to supply and return air, lighting, circulation fans, doors, corners and dense canopy sections.

Step 3: Measure canopy-level conditions

Record temperature and relative humidity at representative crop locations using appropriate equipment. Compare these readings with the primary climate-control sensors.

Step 4: Compare leaf temperatures

Use an appropriate measurement method to examine representative leaves. Perform repeated measurements under consistent lighting and environmental conditions.

Step 5: Review environmental trends

Compare measurements during different portions of the lighting cycle. Investigate whether environmental differences become more pronounced during peak lighting, dehumidification cycles, irrigation events or lighting transitions.

Step 6: Connect the findings to irrigation

Atmospheric conditions affect plant water demand. Environmental measurements should therefore be considered alongside substrate water content, irrigation frequency, root-zone EC and other crop-management information.

For more detail, read High-Frequency Fertigation in Coco: What Commercial Growers Should Monitor.

Step 7: Make controlled adjustments

If a significant environmental problem is identified, determine whether the cause is sensor placement, airflow distribution, equipment performance or an unsuitable operating setpoint. Change one operating variable at a time where practical, and verify the resulting conditions.

Better environmental control starts with better information

A commercial cultivation facility can invest heavily in environmental-control equipment and still struggle with inconsistency.

Sometimes the problem isn't the equipment's capacity. Sometimes the problem is the information used to control it.

A well-designed monitoring strategy should answer three questions:

Are the sensors accurate?

Are they measuring representative conditions?

Are those measurements being interpreted correctly?

Understanding the relationship between air VPD, leaf temperature and canopy conditions gives cultivation managers a more complete picture of the environment they are controlling.

The goal is not to add sensors everywhere. It is to collect the right measurements from the right locations—and use them to make better decisions.

Relevant Growhaus resources

• climaLINK Climate Sensor
• Cannabis Drying Room Design: Airflow, Load and Environmental Consistency
• Reducing Microbial Failures in Commercial Cannabis
• Growhaus Crop Advisory and Facility Support

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About the author
George Dickinson is co-founder of Growhaus Supply and has more than three decades of experience in commercial greenhouse production and controlled-environment agriculture. His background includes large-scale greenhouse cultivation and practical operating challenges faced by commercial growers.