Quick Answer: VPD (Vapour Pressure Deficit) is the measure of how hard air is pulling moisture from your plant’s leaves. Target 0.4 to 0.8 kPa for seedlings, 0.8 to 1.2 kPa in veg, 1.0 to 1.5 kPa in early flower, and 1.2 to 1.6 kPa in late flower. Use leaf temperature, not air temperature, when reading the chart. If VPD is too high, raise humidity or lower temperature. If too low, do the opposite.
I spent two full grow cycles chasing temperature and humidity as separate numbers before someone on a grow forum pointed me toward VPD. I was hitting 75°F and 55% RH religiously and thinking I had the environment dialled in. What I did not realise is that 75°F at 55% RH gives you a completely different plant experience than 80°F at 55% RH, even though the humidity number looks identical. VPD combines both variables into a single number that actually tells you what the plant is experiencing. Once I started reading VPD instead of humidity alone, consistency improved across every grow from that point forward.
This guide covers what VPD is, how to read a chart, the correct targets for every growth stage, and how to adjust your environment when you fall outside the ideal range.
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Table of Contents
What VPD Actually Is
VPD is a measure of the correlation between temperature and humidity. A higher humidity indicates a lower vapour pressure deficit. Plants will struggle to transpire if the VPD is incorrect, which can cause nutrient deficiencies and watering issues because the plant cannot breathe properly.
In plain language: VPD tells you how much moisture the air around your plants is actively pulling from their leaves. Think of it as the air’s thirst level.
VPD is the difference between the amount of moisture in the air and the maximum amount of moisture the air can hold at a given temperature. When that gap is small (low VPD), the air is nearly saturated and plants cannot release moisture efficiently. When the gap is large (high VPD), the air is dry and plants transpire rapidly.
The critical thing most growers miss when they first encounter VPD: it is measured in kilopascals (kPa), not percentages. You cannot compare a VPD reading to a humidity percentage directly. They measure different things.
Why VPD Matters More Than Humidity Alone
Humidity tells you one half of the story. VPD tells you the whole story.
A reading of 60% relative humidity means something completely different at 68°F (20°C) versus 86°F (30°C). At 68°F and 60% RH, your VPD sits around 0.6 kPa, which is in the seedling/early veg range. At 86°F and the same 60% RH, your VPD jumps to around 1.4 kPa, which is mid-flower territory. Same humidity number, completely different plant experience.
VPD directly affects nutrient uptake. When VPD is optimal, plants are more efficient. When VPD is poor, they utilise nutrients less effectively, even when there are enough of them.
This is why growers who switch from monitoring humidity alone to monitoring VPD often report better results from the same nutrient programme without changing a single product. The nutrients were always there. The plant was simply unable to uptake them efficiently because the VPD was off.
Leaf Temperature vs Air Temperature
The key step most growers skip is using leaf temperature rather than air temperature, since VPD is driven by conditions at the leaf surface. Leaf temperature usually differs from air temperature, running roughly 2 to 4°F cooler under LED lighting and warmer under HPS.
This distinction matters in practice. If your thermometer reads 78°F but your plants are under an LED, the leaf surface is likely around 74 to 76°F. Reading VPD off the chart using 78°F will give you a slightly inaccurate result. Using 75°F gives you the actual VPD your plants are experiencing.
Measuring with an inexpensive infrared thermometer pointed at a canopy fan leaf is the most accurate approach. If you only have air temperature, subtract 2 to 4°F under LED or add 2 to 5°F under HPS as a rough approximation.
For most home growers without an infrared thermometer, the LED correction (subtract 2 to 3°F from your air temperature reading) is close enough. Grab a basic infrared thermometer on Amazon for around $15 if you want precision.
The VPD Formula
You do not need to calculate VPD manually. The chart below handles it. But understanding the formula helps you make sense of why both temperature and humidity affect VPD simultaneously.
VPD [kPa] = SVP at ambient temperature minus (RH% divided by 100) multiplied by SVP at ambient temperature. SVP (vapour pressure of saturation) is a physical constant that depends on temperature.
A more complete formula is: SVP = 0.6108 × e^(17.27 × T / (T + 237.3)) where T is in Celsius. Example: 78°F (25.6°C) at 55% RH gives SVP = 3.29 kPa, so VPD = 3.29 × 0.45 = 1.48 kPa, which is ideal for peak flower.
Again, the chart does this automatically. The formula is here so you understand the mechanism, not because you need to use it by hand.
The Cannabis VPD Chart
Read this chart by finding your leaf temperature across the top row and your relative humidity down the left column. The cell where they intersect is your current VPD in kPa. Match that number to your current growth stage using the colour key below.
Temperature across the top (°F / °C): 62°F/17°C | 65°F/18°C | 68°F/20°C | 71°F/22°C | 74°F/23°C | 77°F/25°C | 80°F/27°C | 83°F/28°C | 86°F/30°C
| RH% | 62°F | 65°F | 68°F | 71°F | 74°F | 77°F | 80°F | 83°F | 86°F |
| 80% | 0.24 | 0.28 | 0.33 | 0.40 | 0.47 | 0.55 | 0.64 | 0.74 | 0.85 |
| 75% | 0.30 | 0.36 | 0.42 | 0.50 | 0.59 | 0.69 | 0.80 | 0.93 | 1.07 |
| 70% | 0.37 | 0.43 | 0.51 | 0.61 | 0.71 | 0.83 | 0.97 | 1.12 | 1.28 |
| 65% | 0.43 | 0.51 | 0.60 | 0.71 | 0.84 | 0.97 | 1.13 | 1.31 | 1.50 |
| 60% | 0.49 | 0.58 | 0.69 | 0.81 | 0.96 | 1.11 | 1.29 | 1.50 | 1.71 |
| 55% | 0.56 | 0.66 | 0.78 | 0.92 | 1.08 | 1.25 | 1.46 | 1.69 | 1.93 |
| 50% | 0.62 | 0.73 | 0.87 | 1.02 | 1.20 | 1.39 | 1.62 | 1.87 | 2.14 |
| 45% | 0.69 | 0.81 | 0.96 | 1.13 | 1.32 | 1.53 | 1.78 | 2.06 | 2.35 |
| 40% | 0.75 | 0.88 | 1.04 | 1.23 | 1.44 | 1.67 | 1.94 | 2.25 | 2.57 |
| 35% | 0.81 | 0.96 | 1.13 | 1.33 | 1.57 | 1.81 | 2.10 | 2.43 | 2.78 |
Colour key by growth stage:
- Green (ideal): 0.4 to 0.8 kPa → Seedling and clone
- Blue (ideal): 0.8 to 1.2 kPa → Vegetative
- Yellow (ideal): 1.0 to 1.5 kPa → Early to mid flower
- Orange (ideal): 1.2 to 1.6 kPa → Late flower
- Red (stress zone): Above 1.6 kPa → Too dry, plants close stomata
Screenshot this chart and tape it to the outside of your tent. A 2-second glance during daily checks is faster than pulling up an app, and it does not stress plants when you open the door repeatedly.
VPD Targets by Growth Stage
Seedling and Clone: 0.4 to 0.8 kPa
Young seedlings and fresh clones have minimal root systems and cannot handle high transpiration rates. Keep VPD at 0.4 to 0.8 kPa by maintaining 75 to 80°F and 65 to 80% relative humidity. A propagation dome helps maintain these conditions.
At this stage, seedlings absorb most of their moisture through leaves rather than roots. Low VPD reduces the transpiration demand on an underdeveloped root system, preventing drought stress before the plant can support itself through the root zone. This directly pairs with the seedling heat mat setup — the warmth from the mat helps you hit the right temperature while a humidity dome covers the top.
As roots establish over 5 to 7 days, gradually lower humidity and raise VPD toward vegetative targets. Never jump VPD by more than 0.2 to 0.3 kPa in a single day. Abrupt environmental shifts stress plants just as surely as being outside the target range does.
Vegetative Stage: 0.8 to 1.2 kPa
The ideal range of VPD to target is around 0.8 to 1.1 kPa for cannabis during the vegetative phase. At this stage plants are actively building root mass, stem structure, and canopy. Optimal VPD drives the nutrient uptake that fuels all of that growth.
In the vegetative stage, a VPD of around 1.0 kPa is considered ideal. At this stage, plants are actively growing and need to uptake maximum nutrients. Maintaining the right VPD ensures they can do this efficiently.
A practical target for most home growers in veg: 77°F (25°C) air temperature with 60 to 65% relative humidity. At those values your VPD lands around 1.0 to 1.1 kPa, which sits squarely in the vegetative sweet spot. Cross-reference with the chart above to confirm against your specific setup.
Early to Mid Flower: 1.0 to 1.5 kPa
Cannabis in peak flower performs best at VPD 1.2 to 1.6 kPa. This is typically achieved at 77 to 80°F with 45 to 55% relative humidity.
The transition from veg to flower is the most important VPD adjustment point in the grow cycle. As the plant flips to 12/12 and begins producing flowers, it can handle and benefit from higher transpiration rates. Trichome and resin production is supported by the slightly drier conditions of the 1.0 to 1.5 kPa range.
Run your humidity controller with the dehumidifier outlet set to trigger above 55% at this stage. Pair that with the temperature management from your VPD controller and the combined system maintains the early flower zone automatically.
Late Flower: 1.2 to 1.6 kPa
In the final 2 weeks before harvest, many growers push VPD to 1.5 to 2.0 kPa by dropping humidity to 35 to 40% to stress the plant slightly. This late-flower push has two practical benefits: it significantly reduces bud rot risk as dense buds trap less ambient moisture, and mild water stress in the final weeks is associated with increased resin and trichome production as the plant redirects resources toward reproduction.
Low VPD below 0.6 kPa means the air is nearly saturated — plants close their stomata, transpiration stops, CO2 uptake plummets, and growth slows dramatically. It also creates the perfect conditions for botrytis (bud rot) and powdery mildew in flower. Always keep VPD above 0.8 kPa in veg and above 1.0 kPa in flower.
This is the single most important VPD rule in the guide. Bud rot does not announce itself. By the time you see brown inside a dense cola, the infection is already days old. Maintaining VPD above 1.0 kPa through the entire flower period removes the environmental condition that botrytis requires to establish.
How to Fix VPD When You Are Out of Range
If VPD is too high, raise humidity or lower temperature. If too low, do the opposite. Never change both at once — you will lose track of which fix worked. Give the tent time to stabilise before the next adjustment. Rapid back-to-back changes create yo-yo conditions that stress plants.
This single piece of advice prevents most VPD management mistakes. Here is a practical decision tree:
VPD too high (above target for your stage): The air is too dry and pulling moisture from leaves faster than roots can replace it. Fix in this order:
- Raise relative humidity first. Add a humidifier connected to your humidity controller and let it run until humidity rises 5 to 8%.
- If raising humidity is not enough or not practical in late flower, lower temperature slightly. Dropping canopy temperature by 2 to 3°F reduces VPD meaningfully.
- Confirm by re-reading the chart at the new temperature and humidity values before making further changes.
VPD too low (below target for your stage): The air is too humid and plants cannot transpire efficiently. Fix in this order:
- Run your dehumidifier. If you have a dual-stage controller, your dehumidifier outlet should already be handling this automatically.
- Increase airflow from your oscillating fans to help move humid air away from the leaf surface.
- Check your inline fan is sized correctly for your tent. An undersized inline fan means humid air is not being exchanged frequently enough. Use the CFM calculator to verify.
VPD in range but inconsistent: If your readings jump around significantly across the day, the issue is usually lights-off humidity spikes. Plants stop transpiring in the dark, releasing less moisture while the room cools. This drops VPD toward the lower boundary or below. Running your dehumidifier on a light-independent schedule rather than only during lights-on stabilises the environment across the full 24-hour cycle.
The Fastest Way to Get Into Range Right Now
If you are new to VPD management and want to hit the right zone immediately without calculating anything, these temperature and humidity targets will land you close to optimal for each stage:
| Stage | Air Temp Target | RH Target | Expected VPD |
| Seedling | 77°F (25°C) | 70 to 80% | 0.5 to 0.8 kPa |
| Vegetative | 77°F (25°C) | 60 to 65% | 1.0 to 1.1 kPa |
| Early flower | 77°F (25°C) | 50 to 55% | 1.2 to 1.5 kPa |
| Late flower | 75°F (24°C) | 40 to 45% | 1.4 to 1.6 kPa |
These are starting points assuming LED lighting and standard indoor growing conditions. Adjust based on your actual chart readings using leaf temperature once you have established a baseline.
What Equipment You Need to Monitor and Control VPD
To monitor: A thermometer and hygrometer combination sensor is the minimum. The Inkbird IBS-TH2 or Govee Bluetooth sensor both provide continuous temperature and humidity logging to an app for around $15 to $20. Place the sensor at canopy level, shaded from direct light.
To measure leaf temperature accurately: An infrared thermometer pointed at a fan leaf gives you the most precise leaf temperature for chart reading. Available for around $15 on Amazon.
To control automatically: A VPD controller like the AC Infinity Controller 69 Pro calculates VPD continuously from its integrated sensor and adjusts connected fans and devices automatically. A humidity controller like the Inkbird IHC-200-WiFi manages the humidifier and dehumidifier side of VPD simultaneously.
Running both together means you can set your target VPD range per growth stage and walk away. The system handles temperature through fan speed and humidity through connected devices without manual intervention. This is the setup most serious home growers land on after a few cycles of manual management.
VPD and Your Nutrient Programme
VPD affects how efficiently your plants absorb the nutrients you feed them. VPD directly affects nutrient uptake. When VPD is optimal, plants are more efficient. When VPD is poor, they utilise nutrients less effectively, even when there are enough of them.
This is why experienced growers who dial in VPD often report better results from the same nutrient programme they have always used. The nutrients were not the limiting factor. The plant’s ability to absorb them was. Correct VPD removes that bottleneck.
For autoflower growers especially, where the compressed lifecycle means every week of suboptimal conditions has a proportionally larger impact on final yield, VPD management is one of the highest-leverage variables you can control. See the autoflower nutrient guide for the feeding schedule that pairs with these VPD targets stage by stage.
FAQ
What is the best VPD for cannabis? A good VPD range depends on growth stage: 0.4 to 0.8 kPa for seedlings and clones, 0.8 to 1.2 kPa for vegetative growth, 1.0 to 1.3 kPa for early flower, and 1.2 to 1.5 kPa for late flower. The target rises as the plant matures because larger plants can handle stronger transpiration, and drier late-flower air reduces bud rot risk.
Should I use air temperature or leaf temperature for the VPD chart? Leaf temperature. If you only have air temperature, subtract 2 to 4°F under LED or add 2 to 5°F under HPS as a rough approximation.
What happens if VPD is too low? Low VPD below 0.6 kPa means the air is nearly saturated — plants close their stomata, transpiration stops, CO2 uptake plummets, and growth slows dramatically. It also creates the perfect conditions for botrytis and powdery mildew in flower.
What happens if VPD is too high? The air is too dry and pulls moisture from leaves faster than roots can replace it. Plants close their stomata to conserve water, which also stops CO2 absorption and nutrient uptake. Above 1.6 kPa you are in the stress zone for most growth stages. Above 2.0 kPa, irreversible wilting and leaf curl become likely.
Do autoflowers need different VPD targets than photoperiods? No. VPD targets are driven by growth stage and plant size, not by whether the strain is autoflowering or photoperiod. A seedling autoflower needs the same 0.4 to 0.8 kPa range as a photoperiod seedling. A flowering autoflower needs the same 1.0 to 1.5 kPa as a photoperiod in flower.
Can I manage VPD without a dedicated controller? Yes. Manual monitoring with a thermometer and hygrometer combined with the chart above is entirely workable. Check readings twice daily, once during lights-on and once just before lights-off. Adjust your humidifier or dehumidifier manually based on readings. A VPD controller automates this process but is not required to achieve good results, particularly for a beginner who wants to understand the environment before automating it.