VPD Explained: The One Chart That Links Temperature and Humidity

Vapour pressure deficit (VPD) is the gap between how much moisture the air is holding and how much it could hold when fully saturated at your plants' leaf temperature, measured in kilopascals (kPa). It rolls temperature and humidity into a single number that predicts how hard your plants transpire. Targets run from 0.4 kPa for seedlings up to 1.6 kPa in late flower, with one worked example per stage:

Get VPD right and transpiration, nutrient uptake and stomatal behaviour fall into line. Get it wrong and you can hit "perfect" temperature and humidity on paper while your plants still stall. Many other temperature and RH pairs hit the same VPD, so treat each example as a starting point.

Who this guide is for: UK indoor growers who already understand grow-room temperature and humidity separately and now want to tie them into one target they can dial in.

What is VPD and why does it matter for growing?

The formula is simple: VPD = saturation vapour pressure (SVP, how much water vapour the air could hold at a given temperature) minus actual vapour pressure (AVP, how much it is holding). Bone-dry air gives a large deficit; near saturation it is close to zero. The numbers that matter to a grower sit between about 0.4 and 1.6 kPa.

Plants pull water up from the roots and release it as vapour through pores (stomata) in their leaves. That flow, transpiration, carries nutrients up the plant and cools the leaf. VPD is the pull that draws the vapour out:

• VPD too low (air too humid/cool): little pull. Transpiration slows, calcium and other water-borne nutrients lag behind, and the humid, still air invites mould and botrytis, part of the wider high-humidity pest and mildew risk.
• VPD too high (air too dry/warm): aggressive pull. Plants transpire faster than the roots can resupply, stomata close in self-defence, and growth stalls. That can look like a deficiency or lockout, so check VPD before changing the feed.

The saturation figures are pure physics: fully saturated air holds about 2.34 kPa of water vapour at 20°C (68°F) and 3.17 kPa at 25°C (77°F).

Why does leaf temperature matter more than air temperature?

Here's the part most beginners miss: VPD should be calculated at the leaf surface, not the air. A transpiring leaf is cooler than the air around it, and saturation vapour pressure is set at the evaporating surface. Across horticulture references, a healthy leaf under grow lights typically runs 1–3°C (2–5°F) cooler than the surrounding air. Calculate from air temperature alone and you'll read a higher VPD than your plants experience, so you over-humidify and run the room too wet.

The error can move you a full growth stage. Take a room at 25°C (77°F) and 60% RH:

Calculated from the standard saturation-vapour-pressure equation for the same 25°C / 60% RH room.

Same room, three verdicts. Measure leaf temperature where you can; a VPD thermometer is a laser-style infrared gun that reads leaf and air VPD directly. Where you can't, assume the leaf sits about 2°C (3.6°F) below your air reading, the "leaf offset" calculators and controllers let you set.

VPD chart: ideal kPa by temperature and humidity

Read air temperature down the side and relative humidity across the top, with the leaf taken as 2°C (3.6°F) below the air. For flower, look for cells between about 1.2 and 1.6 kPa.

Calculated with the saturation-vapour-pressure equation in FAO Irrigation and Drainage Paper 56, e° = 0.6108 × exp[17.27T ÷ (T + 237.3)] with T in °C: VPD = e° at the leaf minus RH × e° at the air.

The reliable rule is low VPD early, higher VPD later: gentle transpiration while roots establish, then a harder pull to drive feeding and keep the late-flower canopy dry. If in doubt, sit mid-band.

The RH figures sit inside the target humidity by stage. The seedling RH is for an open room; under a closed propagator dome use the schedule in our propagation masterclass.

How do you hit your VPD targets?

You don't buy a "VPD machine". You hit VPD with the temperature and humidity kit you already run, guided by a canopy-level sensor:

1. Measure at the canopy. Put a thermo-hygrometer such as the Hygrometer Temperature & Humidity Sensor With Probe at plant-top height. A gauge on the tent floor can read several degrees and several percent off. For an app-connected option, the Air Comfort Temp & Humidity Sensor logs up to 100 days of readings to your phone over Bluetooth, and our guide to choosing a smart thermo-hygrometer covers the rest.
2. Read (or estimate) leaf temperature. The VPD Thermometer gives the real figure; failing that, set a ~2°C leaf offset in your calculator or controller.
3. Move VPD with whichever lever is furthest off target. To lower it, see the next section. To raise it, add extraction, a dehumidifier or a touch more warmth, and size your extraction fan before you buy.
4. Automate it. Controllers such as the AC Infinity Controller 69 Pro have a dedicated VPD mode: set high and low VPD triggers and the controller drives your fans, humidifier or dehumidifier to hold the band, combining air temperature, humidity and a leaf-temperature offset. A temperature and humidity controller such as the GAS 13 Amp Enviro Controller V2 holds the temperature and RH pair that gives your target, with separate day and night settings.

The kit overlaps almost entirely with your temperature and humidity setup, so treat VPD as the target and those two guides as the how: don't buy new gear for VPD alone. We'd sooner see a grower hold a steady middle-of-the-band reading than chase the grid every hour, so change one lever at a time and let the room settle before you judge it.

How do you lower VPD in a grow tent?

Add moisture to the air or reduce its drying effect:

• Run a humidifier. The quickest lever in a dry UK winter tent. The SonicAir Humidifier Pro is one option; the Humidity Control collection has the rest.
• Ease back extraction. Slowing the fan stops the room dumping the moisture the plants add. A speed controller such as the SMS Smart Controller 6.5 Amp MK2 makes it a dial rather than a guess.
• Cool the canopy. Lower air temperature cuts VPD; in a hot summer tent it is the main lever.
• Soften the airflow. Strong air straight onto the canopy sweeps away the humid layer at each leaf and lifts VPD, so use a clip fan such as the CLOUDRAY S6 Gen 2 for gentle movement, not a gale.

Does VPD change between day, night and winter?

Yes. When the lights go off, air temperature falls. Cooler air holds far less moisture, so relative humidity climbs even though no water was added, and VPD drops, sometimes sharply. A little drift is fine; a collapse is the danger, because near-zero night VPD makes a botrytis and powdery-mildew factory. Rules of thumb:

• Keep the lights-off temperature drop moderate, roughly 2–8°C (4–14°F) below your lights-on figure. The G.A.S. Day & Night Heater Controller lets a heater hold a separate night set-point.
• Run a dehumidifier or keep gentle air movement overnight so RH doesn't spike.
• Don't let night VPD fall below about 0.4 kPa. At that level transpiration stalls, and as VPD slides toward zero the air reaches its dew point and condensation forms on the leaves.

You're not chasing an identical day and night number, you're stopping VPD from crashing while the lights are off.

Winter adds a daytime twist. Cold UK air at 5°C (41°F) and 80% RH, once heated to 20°C (68°F) with no moisture added, drops to about 30% RH, so a heated room runs a high VPD while cold nights collapse it again. Our guide to winter grow room conditions covers the seasonal picture. A controller with a VPD or humidity trigger earns its keep here, holding the floor automatically at 3am.

FAQ

What is a good VPD for flowering? Aim for roughly 1.2–1.5 kPa through early-to-mid flower and up to about 1.6 kPa in late flower and ripening, calculated at the leaf. Higher VPD late in flower keeps the canopy drier, which reduces mould risk around dense, mature buds. If your genetics look stressed at the top of the range, ease back toward 1.2 kPa.

Do I need to measure leaf temperature for VPD? Ideally yes. A VPD thermometer gives the true leaf figure, and leaves usually run 1–3°C (2–5°F) cooler than the air. If you can't measure it, set a leaf offset of about 2°C (3.6°F) in your calculator or controller. Using air temperature alone overstates VPD and tends to leave you running the room too wet.

What VPD do seedlings and clones need? Aim for 0.4–0.8 kPa, calculated at the leaf. Young plants have few roots, so a gentle pull keeps them from drying out. In an open room that means about 24°C (75°F) and 70% RH; under a closed propagator dome, follow our propagation masterclass.

Recommended kit and further reading

Here's where we'd point a UK grower:

• Air Comfort Temp & Humidity Sensor: a wireless canopy sensor that logs temperature and humidity to your phone, so you catch drift before the plants show it.
• AC Infinity ventilation & climate control: fans and climate controllers from one maker, some with a VPD mode.
• Fans, Filters and Environment and Fan Speed Controllers: the levers you move VPD with.
• Grow Room Tools & Accessories: thermometers, hygrometers and bits for dialling a room in.

Related guides: grow room temperature by stage · grow room humidity control · grow room ventilation · how CO2 raises your optimum temperature · grow room setup checklist.

Why trust The Grow Superstore

We've been helping UK indoor growers since 2009, both online and from our physical store at 6 Wulfric Square, Bretton, Peterborough. VPD is one of the questions we talk through in store almost every week. The advice above is the same walkthrough we give across the counter: measure at the canopy, allow for leaf temperature, and pick the band for your stage. Pop in for a free in-store consultation, or get in touch and we'll help you choose a sensor or controller that reads VPD directly.