Most growers track temperature and relative humidity (RH) as two separate numbers. Neither number on its own shows how fast a plant loses water through its leaves. This water loss is called transpiration. Vapour pressure deficit (VPD) combines temperature and humidity into one number that shows how strongly the air dries the canopy. This guide shows how to calculate VPD and which targets suit each growth stage. It also shows how day and night temperature affect the crop, and how to stop water forming on buds at night.
Here is an example. One room is at 25 °C and 60% RH, and another is at 30 °C and 60% RH. The RH is the same, but warm air holds more water. So the plants in the 30 °C room lose water much faster. VPD shows this difference, and RH alone does not.
Part 1 covers how VPD works, targets by stage, day and night temperature, and condensation at night. Part 2 covers dehumidifiers, air conditioning, airflow and carbon filters, bud rot and powdery mildew prevention, climate controllers and a daily checklist.
How Vapour Pressure Deficit Works
What relative humidity does not tell you
RH shows how full the air is compared with the most water it can hold at that temperature. That maximum changes with temperature. So the same RH means different things in a cool room and in a hot room. VPD measures the actual gap between the water the air could hold when saturated and the water it holds now. VPD is given in kilopascals (kPa), a unit of pressure.
A higher VPD means the air is drier for the plant, so the plant transpires faster. A lower VPD means the air is close to saturation. Transpiration then slows down, and the risk of disease goes up.
Step 1: how much water the air can hold
Saturation vapour pressure (SVP) is the most water vapour the air can hold at a given temperature. It is calculated with the Magnus-Tetens formula:
SVP = 0.6108 × exp[(17.27 × T) / (T + 237.3)]
T is the temperature in °C, and SVP comes out in kPa. “exp” is the exponential key on a scientific calculator, or the EXP function in a spreadsheet.
Values from the formula at normal indoor temperatures:
| Temperature (°C) | SVP (kPa) |
|---|---|
| 18 | 2.06 |
| 20 | 2.34 |
| 22 | 2.64 |
| 24 | 2.98 |
| 26 | 3.36 |
| 28 | 3.78 |
| 30 | 4.24 |
| 32 | 4.75 |
SVP climbs steeply as the air warms. From 20 °C to 30 °C it rises by about 80%, from 2.34 to 4.24 kPa. That is why two rooms at the same RH can have very different VPD.
Step 2: how much water the air holds now, and the gap
Actual vapour pressure (AVP) is the water vapour in the air right now. VPD is the gap between SVP and AVP:
AVP = SVP × (RH / 100)
VPD = SVP − AVP = SVP × [1 − (RH / 100)]
Leaf VPD versus air VPD
Water evaporating from a leaf cools it. So the leaf surface usually runs 1–3 °C cooler than the air. Leaf VPD uses the leaf temperature to calculate SVP. It uses the air temperature and RH to calculate AVP. Leaf VPD is the number that matters to the plant. It controls how the leaf’s pores (stomata) actually behave. If you can, measure canopy temperature with an infrared thermometer and calculate leaf VPD directly.
Leaf VPD = SVP(T_leaf) − SVP(T_air) × (RH / 100)
The formula treats the inside of the leaf as saturated at leaf temperature. The air around the leaf holds the amount of water set by air temperature and RH.
Worked example: the air is 26 °C and 55% RH, and the leaf measures 24 °C.
- SVP at leaf temperature: SVP(24 °C) = 2.98 kPa
- AVP of the air: SVP(26 °C) × 0.55 = 3.36 × 0.55 = 1.85 kPa
- Leaf VPD = 2.98 − 1.85 = 1.13 kPa
- Air VPD in the same room = 3.36 × 0.45 = 1.51 kPa
Between 24 and 28 °C, each 1 °C that the leaf runs below the air lowers leaf VPD by about 0.2 kPa. This gap is not fixed. It depends on the light fixture, light intensity, airflow and how fast the plant transpires. Some fixtures give off more radiant heat, and under them the canopy can be as warm as the air or warmer. The fixture types are compared in Indoor Grow Lights: LED vs HPS vs CMH. Measure the gap with an infrared thermometer instead of assuming a number.
The quick lookup table in the next section shows air VPD. A controller that reports leaf VPD will read lower than that table at the same room conditions.
Translator’s note: a VPD inside the target range does not prove the room is right. From the SVP table, a room at 32 °C and 70% RH has an air VPD of 4.75 × 0.30 = 1.43 kPa. That is inside the vegetative range. But 32 °C is above every day-temperature target in this guide. Thai rooms without air conditioning often run at 30–35 °C. Check the temperature first, then VPD.
What to do now: for one week, record air temperature, RH and leaf temperature at canopy height twice a day. Take one reading in the middle of the light period and one 30 minutes after lights-off. Calculate both air VPD and leaf VPD. This shows how much cooler than the air your leaves run, so you know which table figures fit your room.
Targets for Each Growth Stage
Vapour pressure deficit, temperature and humidity by stage
| Stage | VPD (kPa) | Day temp (°C) | Night temp (°C) | RH (%) |
|---|---|---|---|---|
| Clone / propagation | 0.6–1.0 | 24–26 | 22–24 | 70–80 |
| Seedling | 0.8–1.1 | 23–26 | 21–24 | 65–75 |
| Vegetative | 1.0–1.6 | 25–28 | 20–24 | 55–70 |
| Stretch (flower weeks 1–2) | 1.2–1.7 | 25–28 | 21–24 | 50–65 |
| Early flower (weeks 3–5) | 1.3–1.6 | 24–27 | 20–23 | 50–60 |
| Late flower (weeks 6–7) | 1.3–1.8 | 22–26 | 18–22 | 40–55 |
| Ripening / flush (week 8 on) | 1.5–1.7 | 20–24 | 16–20 | 35–45 |
A clone is a cutting rooted from a mother plant (Mother Plants & Cloning). Stretch is the fast height gain in the first weeks after the switch to flowering (Stretch: Controlling Height During the Flowering Transition). Flushing means giving plain water or a weak solution before harvest. Whether it helps is covered in Pre-Harvest Flushing: Science or Myth?.
Every VPD figure in this guide’s stage tables is air VPD. This covers the table above and, in Part 2, the flower-week table and the summary table. Each figure was calculated with the Magnus formula. It uses the middle of the day-temperature range and covers the RH range shown. Leaf VPD is usually lower, because leaves are usually cooler than the air.
Quick lookup: air VPD (kPa) at a given temperature and RH
| Temp (°C) | 40% | 45% | 50% | 55% | 60% | 65% | 70% | 75% | 80% |
|---|---|---|---|---|---|---|---|---|---|
| 18 | 1.24 | 1.13 | 1.03 | 0.93 | 0.82 | 0.72 | 0.62 | 0.52 | 0.41 |
| 20 | 1.40 | 1.29 | 1.17 | 1.05 | 0.94 | 0.82 | 0.70 | 0.58 | 0.47 |
| 22 | 1.59 | 1.45 | 1.32 | 1.19 | 1.06 | 0.93 | 0.79 | 0.66 | 0.53 |
| 24 | 1.79 | 1.64 | 1.49 | 1.34 | 1.19 | 1.04 | 0.89 | 0.75 | 0.60 |
| 26 | 2.02 | 1.85 | 1.68 | 1.51 | 1.35 | 1.18 | 1.01 | 0.84 | 0.67 |
| 28 | 2.27 | 2.08 | 1.89 | 1.70 | 1.51 | 1.32 | 1.13 | 0.95 | 0.76 |
| 30 | 2.55 | 2.33 | 2.12 | 1.91 | 1.70 | 1.48 | 1.27 | 1.06 | 0.85 |
Why very humid air can leave new leaves short of calcium
Calcium travels from the roots to new leaves mainly with the water that transpiration pulls up through the plant. The plant moves very little calcium from older leaves to new growth. When VPD stays very low, the plant transpires slowly. Less water moves through it, so less calcium reaches the youngest leaves. This can happen even when the nutrient solution has enough calcium. New growth can then show calcium-deficiency signs although the feeding is correct. Before you add more calcium, bring VPD into the stage range and check the airflow at the canopy. Leaf symptoms are compared in Nutrient Deficiency & Toxicity: Reading Your Leaves.
Translator’s note: in Thailand, the hard part is usually reaching the low end of the RH ranges. Clones and seedlings at 65–80% RH are easy to keep in a humid climate. Flowering at 40–60% RH is hard, especially in the rainy season. Any air brought in from outside is then very humid. Size dehumidifiers for the late-flower targets, not the veg targets.
What to do now: find your current stage in the table and compare it with your logged readings. If VPD is below range in flower, fix humidity first, before you change the feeding. Dehumidifier sizing is in Part 2. This tells you whether the problem comes from climate or from nutrition.
How VPD Changes Over 24 Hours
VPD changes through the day and night. The biggest swings come when the lights switch on or off. A data logger is a sensor that saves a reading every 1–5 minutes. Its record shows whether your equipment keeps up with these swings.
Lights-on
After lights-on, the canopy and room warm quickly, often by several degrees in the first hour. The dehumidifier or air conditioner may respond slowly. Then RH falls only slowly and VPD climbs.
Example: at lights-off, the room is 22 °C and 60% RH, and the leaf is 20 °C. Leaf VPD is about 0.75 kPa. Thirty minutes after lights-on, the air is 27 °C, RH is still 58%, and the leaf is 25 °C. Leaf VPD = SVP(25 °C) − SVP(27 °C) × 0.58 = 3.17 − 2.07 = 1.10 kPa. That is a jump of about 0.35 kPa in half an hour.
Lights-off
After lights-off, the room cools. At first, the amount of water in the air stays about the same. SVP falls as the air cools, so RH rises and VPD drops. A room at 26 °C and 45% RH that cools to 22 °C reaches about 57% RH, with no change in moisture. If it cools to 20 °C, RH reaches about 65%.
In late flower, this is a high-risk time for Botrytis, also called bud rot or grey mould. Water can condense on and inside dense buds before the room settles.
Three habits that reduce the swings
- Keep the dehumidifier running through the dark period. Plants still transpire at night, only more slowly, so water keeps entering the air.
- Lower the temperature step by step over the last hour of the light period, not all at once. Slower cooling gives the dehumidifier time to remove water as RH rises.
- Do not irrigate shortly before lights-off. Each irrigation adds moisture just as the room starts to cool.
Dew point: why water forms on buds after lights-off
Dew point is the temperature at which the air becomes saturated with the water it already holds. Any surface colder than the dew point collects liquid water. You see the same effect on a glass of iced water on a hot day. Leaves usually run cooler than the air, and dense buds hold moisture inside. If a leaf or bud surface drops below the dew point, a film of water forms. Botrytis needs this free water to start.
| Air temp (°C) | Dew point at 40% RH (°C) | at 50% RH | at 60% RH | at 70% RH |
|---|---|---|---|---|
| 20 | 6.0 | 9.3 | 12.0 | 14.4 |
| 22 | 7.8 | 11.1 | 13.9 | 16.3 |
| 24 | 9.7 | 13.0 | 15.7 | 18.2 |
| 26 | 11.5 | 14.8 | 17.6 | 20.1 |
| 28 | 13.4 | 16.7 | 19.4 | 22.0 |
If leaf temperature drops below the dew point, water condenses and the disease risk becomes critical. Keep leaf temperature at least 2 °C above the dew point at all times. For example, a room at 24 °C and 60% RH has a dew point of 15.7 °C. Leaves in that room should stay at 17.7 °C or warmer.
Translator’s note: night humidity in Thai conditions
- Rainy season. A fan that pulls outside air in at night brings in very humid air. The dehumidifier then has to remove that water as well.
- Air conditioner alone. An air conditioner removes water only while it is cooling. Once the dark room reaches the set temperature, the air conditioner cools less or stops. RH then climbs even though the room feels cool. A separate dehumidifier keeps RH down at night.
- Cold air on the canopy. Do not point the air conditioner outlet straight at the plants. Cold air can pull leaf temperature down toward the dew point.
- Power cuts. If the power fails during the dark period in late flower, the dehumidifier and fans stop and RH rises. Some devices do not restart on their own when power returns. A small UPS (a battery unit that keeps a device running through a short cut) can power the controller and data logger. The logger then records how long the room went unprotected.
What to do now: look up the dew point for your usual night temperature and RH. Measure leaf temperature 30 minutes after lights-off. If the leaf is less than 2 °C above the dew point, run the dehumidifier all night and cool the room more slowly. Then, while the room is empty or in veg, switch the power off and on once at the breaker. Note which devices restart with their settings. After a real outage, you then know which devices to restart by hand.
Temperature by Stage
| Stage | Day (°C) | Night (°C) | DIF (°C) | Why |
|---|---|---|---|---|
| Clone | 24–26 | 22–24 | +2 to +4 | Low stress, high humidity |
| Vegetative | 25–28 | 20–24 | +4 to +6 | Encourages stem growth and vigour |
| Early flower | 24–27 | 20–23 | +4 to +6 | Supports stretch and the start of flowering |
| Late flower | 22–26 | 18–22 | +4 to +6 | Slower metabolism favours resin production |
| Ripening / finish | 20–24 | 16–20 | +4 to +8 | Cool nights help keep terpenes |
Day–night temperature difference (DIF)
DIF is the day temperature minus the night temperature. With a positive DIF (day warmer than night), the stem sections between leaf pairs (internodes) grow longer, so plants grow taller. A zero DIF gives compact plants. A negative DIF (night warmer than day) gives very compact plants. Cool nights at the end of flowering make a large positive DIF. This brings out more aroma and more purple colour (anthocyanin) in cultivars that carry the genes for it.
| DIF strategy | Range | Effect |
|---|---|---|
| Positive DIF | +4 to +8 °C | Longer internodes, taller plants |
| Zero DIF | 0 °C | Compact growth, medium internodes |
| Negative DIF | −2 to −4 °C | Very compact growth, shorter internodes |
| Cool-night finish | +6 to +10 °C | Strongest terpene and anthocyanin expression |
Cannabinoids and temperature
Day temperatures around 25 °C are the usual target in flowering. Long periods of heat at the canopy raise plant stress and water demand. This guide gives no single temperature at which cannabinoids are lost. Published studies differ in cultivar, light level and how long the heat lasted. Keep the canopy inside the stage ranges above, and avoid long heat peaks.
Terpenes
Many terpenes (aroma compounds) evaporate easily. This is especially true of monoterpenes such as myrcene, limonene and linalool. So heat late in flower and during drying increases their loss. Growers often run high light intensity while keeping the canopy inside the late-flower temperature range. This is a common way to protect aroma. How much it helps depends on the cultivar and the environment.
Translator’s note: in Thailand, it takes a lot of electricity to hold a room at 20–26 °C when it is 30–35 °C outside. The lights are usually the largest heat source in the room. Many growers therefore run the light period during the cooler night hours. The air conditioner then carries the light heat while the outside air is coolest. With this schedule, the dark period falls in the hottest part of the day. Check that the room does not become warmer in the dark than in the light. Otherwise you get the compact growth of a negative DIF without planning it.
What to do now: set the night temperature on your controller to the day setting minus the DIF you chose for this stage. Measure plant height and internode length once a week. This shows whether your DIF gives the height you want before stretch makes it hard to correct.
Cool Nights at the End of Flowering: Colour, Resin and Aroma
Purple colour (anthocyanin)
Purple and dark colours in cannabis come from pigments called anthocyanins. They build up when night temperatures drop to 15–18 °C. This response depends on genetics. Cultivars without the anthocyanin genes will not change colour at any temperature. In cultivars with the genes, steady cool nights in weeks 6–9 of flowering give the strongest colour.
Cool-night finish schedule
| Period | Night (°C) | Day (°C) | Purpose |
|---|---|---|---|
| Flower weeks 7–8 | 16–18 | 22–25 | Keep terpenes, start colour |
| Flower weeks 8–9 | 14–16 | 20–24 | Strongest colour, resin ripening |
| Final 48–72 hours | 14–16 | 18–22 | Starch depletion, final ripening |
Caution: nights below 14 °C can slow the plant’s metabolism too much. Trichome ripening may then stall. Check trichomes with a loupe (a small hand magnifier) before you commit to nights below 15 °C. How to read trichomes is explained in Harvest Timing: Trichome Maturity.
This schedule is an optional finish. The standard ripening range in the stage tables is 16–20 °C at night. Use 14–16 °C only when you choose to run a cool-night finish.
Translator’s note: in tropical climates such as Thailand, nights below 18 °C need dedicated cooling and raise running costs. Colour comes from genetics. A cultivar without the genes gives nothing back for that extra electricity. In most Thai rooms, keeping leaves above the dew point at night matters more than cool nights.
What to do now: try the cool-night finish only on a cultivar known to turn purple, and only on a small batch first. Finish a second batch of the same cultivar at normal ripening temperatures. Compare colour, aroma and the extra electricity used. This tells you whether cool nights pay for themselves in your room.
References
- Backer, R. et al. (2019). “Closing the Yield Gap for Cannabis: A Meta-Analysis of Factors Determining Cannabis Yield.” Frontiers in Plant Science, 10, 495.
- Caplan, D., Stemeroff, J., Dixon, M. and Zheng, Y. (2018). “Vegetative propagation of cannabis by stem cuttings: effects of leaf number, cutting position, rooting hormone, and leaf tip removal.” Canadian Journal of Plant Science, 98(5), 1126–1132.
- Chandra, S. et al. (2011). “Temperature response of photosynthesis in different drug and fiber varieties of Cannabis sativa L.” Physiology and Molecular Biology of Plants.
- Rodriguez-Morrison, V., Llewellyn, D. and 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.
- Eichhorn Bilodeau, S. et al. (2019). “An Update on Plant Photobiology and Implications for Cannabis Production.” Frontiers in Plant Science, 10, 296.
- Jin, D. et al. (2020). “Secondary Metabolites Profiled in Cannabis Inflorescences, Leaves, Stem Barks, and Roots for Medicinal Purposes.” Scientific Reports.
- Punja, Z.K. (2021). “Emerging diseases of Cannabis sativa and sustainable management.” Pest Management Science.
- Saloner, A. and Bernstein, N. (2021). “Nitrogen supply affects cannabinoid and terpenoid profile in medical cannabis (Cannabis sativa L.).” Industrial Crops and Products, 167, 113516.
- ASHRAE (2023). ASHRAE Handbook — HVAC Applications, chapter on environmental control for animals and plants.
Legal Note
This article is for education only and is not legal advice. In Thailand, cannabis flower (ช่อดอกกัญชา) is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568. The Notification was issued under the Thai Traditional Medicine Protection Act B.E. 2542 and has been in force since 26 June 2025 (explained here). Flower may be sold only with a PT 33 (ภ.ท.33) prescription, for at most 30 days. Advertising and online sale of flower are prohibited. The Ministerial Regulation on Controlled Herbs (No. 2) B.E. 2569 came into force on 30 April 2026 and tightens licence criteria for cannabis flower (see the 2026 regulations). Check every licence requirement with the Thai authorities before you start growing. The draft Cannabis Control Act is not yet law. This forum does not help anyone buy or sell controlled substances.
If your readings do not match these targets, post your log below this topic. Include the stage, air temperature, RH and leaf temperature at lights-on and 30 minutes after lights-off. Other growers can then compare it with their own rooms.
Continued in Part 2: Grow Room Climate Control: Dehumidifiers, Air Conditioning, Airflow, Mould Prevention and Controllers (Part 2 of 2)