Search for a VPD chart with CO2 and you will find plenty of confident answers: add CO2, run the room warmer and drier, shift the chart. It is a tempting shortcut. It is not a reliable operating rule.
There is no universal “CO2 VPD chart” for cannabis. CO2, light intensity, leaf temperature, airflow, irrigation, cultivar, and canopy density all interact. A chart based on air temperature and RH cannot see most of that. Under LEDs, the leaf-to-air temperature gap can change again, so copying an HPS-era air-VPD target can put the actual leaf condition somewhere else entirely.
That does not mean VPD charts are useless. They are a good common language and a fast way to see where a room is headed. The better move is to label the chart correctly as air VPD, measure what the canopy is doing, and calculate leaf VPD from real leaf temperature before changing setpoints. For the basic chart mechanics, see how to read a VPD chart. For the broader framework, our complete cannabis VPD guide covers the fundamentals.
Why CO2 does not come with its own VPD target
VPD is a water-vapor measurement. It is calculated from temperature and relative humidity. CO2 is not an input in that calculation, so adding CO2 does not mathematically create a new VPD number or a new chart.
CO2 can still change how the crop behaves at a given leaf VPD. In a controlled gas-exchange study of four cannabis varieties, elevated CO2 increased net photosynthesis and water-use efficiency while reducing stomatal conductance and transpiration. The size of the response varied by variety. That matters because stomata regulate both CO2 entry and evaporative cooling. If transpiration changes, leaf temperature can change, and leaf temperature changes the vapor-pressure gradient at the leaf surface. [1]
That is the missing step in most “CO2 chart” advice. CO2 may alter the plant response, but it does not tell you which air-temperature or RH setting will produce the same leaf temperature, leaf VPD, root-zone demand, or harvest result in every room.
The research is useful, but keep its limits in view. Chandra and colleagues measured short-term leaf gas exchange under controlled conditions, not whole-room yield across modern commercial cultivars and lighting systems. It supports measuring your crop rather than assuming enrichment automatically justifies a more aggressive dry-room target. [1]
Light intensity is part of the CO2 conversation
CO2 and light are not separate knobs. They meet in photosynthesis. If light is limiting, more available CO2 has less opportunity to be converted into carbon gain. If light is raised, the heat load, canopy uniformity, irrigation demand, and leaf-temperature pattern can all change at the same time.
A cannabis study by the University of Guelph illustrates why single-variable rules fail. In a flowering crop grown under LEDs, inflorescence dry weight increased across the tested range of canopy light intensity. But leaf-level photosynthesis did not predict whole-plant yield cleanly, and leaf responses changed with crop stage and the localized light each leaf received. The authors specifically caution that single-leaf photosynthesis is limited for predicting whole-canopy growth and yield. [2]
For an operator, this means “we added CO2” is not enough context for a VPD decision. Log at least the actual canopy PPFD or lighting program, CO2 concentration, canopy air temperature, canopy RH, leaf temperature, irrigation or substrate response, and stage. Then compare runs that are truly comparable. If the lighting program or airflow changed with the CO2 program, you changed more than one variable.
LEDs can change leaf-to-air temperature
LEDs are not automatically cooler at the leaf. They usually deliver less radiant heat toward the canopy than HPS, but leaf temperature is the result of radiation, air movement, transpiration, fixture distance, spectrum, and the local air temperature around the leaf. A leaf may run below, near, or above the air temperature depending on those conditions.
There is direct horticultural evidence that light source can shift leaf temperature. In a controlled tomato and rose experiment, leaves under HPS were 0.9 to 1.3°C warmer than leaves under the tested LED treatment. That does not supply a cannabis correction factor, but it does establish the practical point: changing fixtures can change leaf temperature even when the room sensor appears steady. [3]
CO2 makes that measurement more important. When elevated CO2 reduces stomatal conductance and transpiration, the cooling contribution from evaporation can change. Whether the leaf actually becomes warmer in your room depends on the rest of the energy balance. Do not assume the direction. Measure it. [1][4]
This is also why a generic “LED VPD chart” is only a starting point. It cannot know your fixture’s radiant output, your canopy airflow, your density, or whether the crop is cooling itself normally that day. Our article on leaf VPD vs. air VPD goes deeper on the distinction.
Air VPD and leaf VPD are not interchangeable
Air VPD uses air temperature to calculate saturation vapor pressure. Leaf VPD, more precisely called leaf vapor-pressure difference, uses leaf temperature for the saturation side and the water-vapor pressure of the surrounding air for the other side. The latter better represents the gradient water sees as it leaves the internal air spaces of a leaf. [4]
The math is straightforward:
- Air vapor pressure = saturation vapor pressure at air temperature × RH/100
- Leaf VPD = saturation vapor pressure at leaf temperature − air vapor pressure
If leaf temperature equals air temperature, the values match. When they differ, the values differ. Ohio State University extension gives a simple example: at 20°C air temperature and 60% RH, air VPD is 0.94 kPa. With an 18°C leaf, leaf VPD is 0.66 kPa. With a 22°C leaf, it is 1.24 kPa. Same room sensor reading, three materially different interpretations of the water-vapor gradient. [4]
When communicating with your team, label the number. “VPD 1.2” is incomplete. Write air VPD or leaf VPD, name the sensor location, and record whether leaf temperature was measured or assumed.
How to measure leaf temperature without fooling yourself
An infrared thermometer is a practical starting tool. Use it to find a room-specific leaf-to-air offset, not to prove that every leaf is identical.
Take paired canopy readings
- Measure air temperature and RH at canopy height, close to the leaf being read. A wall sensor or a reading near the return grille is not necessarily canopy air.
- Measure several healthy, fully expanded upper-canopy fan leaves, not one convenient leaf. Include more than one position in the room.
- Take readings at a repeatable point in the photoperiod. If you are assessing a new CO2 or lighting program, repeat the same route before and after the change.
- Record the leaf-to-air offset, along with light status, fixture setting, CO2 status, airflow conditions, and cultivar or room. An offset is an observation, not a permanent controller setting.
Leaf temperature varies with radiation, air current speed, stomatal conductance, and transpiration. That is why the question is not “what is the correct LED offset?” It is “what is the offset across this canopy under this operating condition?” [4]
Respect the limits of an IR reading
IR tools read a surface temperature within a measurement spot. Get close enough that the spot is leaf tissue rather than a mix of leaf, floor, trellis, and background. Follow the manufacturer’s instructions for the instrument and be consistent about where and when you point it. A thermal camera can show spatial variation that a point reading misses, but it still needs sensible sampling and a nearby canopy-air measurement.
A useful check is the pattern rather than a single decimal. If one end of the room repeatedly has a different leaf-to-air offset, investigate airflow, light distribution, canopy density, irrigation, and sensor placement before rewriting the entire VPD strategy.
A practical CO2 and LED workflow
| Step | What to do | Why it matters |
|---|---|---|
| 1. Establish a baseline | Log canopy PPFD, CO2, air temperature, RH, leaf temperature, and irrigation response under the existing program. | You need a before-state to judge a change. |
| 2. Change one operating variable deliberately | When possible, avoid changing CO2, lighting intensity, temperature, RH, and airflow all at once. | Otherwise, a result cannot be attributed to any one change. |
| 3. Re-measure leaf temperature | Repeat the same canopy route after the room settles into the new program. | CO2 and fixture changes can alter leaf-to-air temperature. |
| 4. Calculate and label leaf VPD | Use the paired leaf and air readings in a leaf-VPD calculation. | This replaces a generic chart offset with your measured condition. |
| 5. Watch crop and root-zone response | Review plant posture, irrigation demand, dryback, runoff, and environmental uniformity alongside VPD. | A VPD value is one part of a coupled production system. |
| 6. Keep stage-specific context | Compare like stages and cultivars. Keep flower risk management separate from vegetative growth objectives. | One chart cannot substitute for stage and crop context. |
The VPD calculator lets you enter a leaf-temperature offset to compare air and leaf values. Use the measurement as the input, not a default offset copied from the internet.
What to do with your existing chart
Keep it, but stop treating it as a universal prescription. A standard chart is normally an air-VPD lookup table unless it explicitly states a leaf-temperature assumption. Before adopting any CO2 or LED version, ask four questions:
- Is it calculating air VPD or leaf VPD?
- What leaf-to-air temperature offset, if any, does it assume?
- What lighting intensity, light source, airflow, and crop stage was it built around?
- Does that match this room today?
For a wall reference, use our printable VPD chart, then record the leaf-temperature assumption beside it. For stage context, see VPD in veg and VPD in flower. Those ranges are starting points, not a license to run a crop harder simply because the CO2 controller is on.
The bottom line
CO2 enrichment may change cannabis photosynthesis, stomatal conductance, transpiration, and water-use efficiency. LEDs may change leaf-to-air temperature. Neither fact produces a universal CO2 VPD chart or LED VPD chart.
Measure canopy air, measure representative leaf temperature, calculate leaf VPD, and log the conditions around the number. That takes a standard chart from a generic colored grid to a tool grounded in the room you actually operate.
References
- Chandra, S., Lata, H., Khan, I. A., & ElSohly, M. A. (2011). Photosynthetic response of Cannabis sativa L., an important medicinal plant, to elevated levels of CO2. Physiology and Molecular Biology of Plants, 17, 291-295. https://doi.org/10.1007/s12298-011-0066-6
- Rodriguez-Morrison, V., Llewellyn, D., & Zheng, Y. (2021). Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science, 12, 646020. https://doi.org/10.3389/fpls.2021.646020
- Bergstrand, K.-J., Mortensen, L. M., Suthaparan, A., & Gislerød, H. R. (2016). Acclimatisation of greenhouse crops to differing light quality. Scientia Horticulturae, 204, 1-7. https://doi.org/10.1016/j.scienta.2016.03.035
- Kubota, C. (2023). VPD leaf vs. VPD air: Two different ways to determine VPD. e-GRO Edible Alert, Volume 8, Number 16. e-GRO, a multi-university extension program.
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