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. As a working guide, aim for roughly 0.4–0.8 kPa for seedlings and clones, 0.8–1.2 kPa in veg, and 1.2–1.6 kPa in flower.

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.

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?

Vapour pressure deficit is the difference between the saturation vapour pressure (how much water vapour the air could hold at a given temperature) and the actual vapour pressure (how much it is holding). When the air is bone dry the deficit is large; when it's close to saturated the deficit is near zero. The numbers that matter to a grower sit between about 0.4 and 1.6 kPa.

Why does it drive growth? Plants transpire — they pull water up from the roots and release it as vapour through pores (stomata) in their leaves, and that flow carries dissolved nutrients up the plant while cooling the leaf. VPD is the pressure gradient that pulls that 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.
• 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.

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). Warmer air holds much more — which is why the same relative humidity feels completely different at 20°C (68°F) and 30°C (86°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. An actively transpiring leaf is cooler than the air around it, and because saturation vapour pressure is set at the evaporating surface, the leaf's temperature is what counts.

Across horticulture references, a healthy leaf under grow lights typically runs 1–3°C (2–5°F) cooler than the surrounding air. Ignore that offset and calculate from air temperature alone, and you'll read a higher VPD than your plants actually experience — so you over-humidify and run the room too wet.

The size of that error is large enough to move you a full growth stage. Take a room sitting at 25°C (77°F) and 60% RH:

VPD calculated from...	Leaf assumption	VPD result	Reads like
Air temperature only	Leaf = air (25°C)	~1.27 kPa	Flowering
Leaf 2°C cooler	Leaf 23°C (73°F)	~0.91 kPa	Vegetative
Leaf 3°C cooler	Leaf 22°C (72°F)	~0.74 kPa	Seedling / clone

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

Same room, three different verdicts. The takeaway: measure leaf temperature with an infrared (IR) thermometer where you can, and where you can't, assume the leaf sits about 2°C (3–4°F) below your air reading — the "leaf offset" most calculators and controllers let you set.

What is the ideal VPD chart by growth stage?

Younger plants want a low VPD (gentle transpiration while roots establish); as the plant matures you raise VPD to push harder feeding and, late in flower, to keep the canopy dry.

The target VPD column is what you're aiming for. Each row then gives one worked example — an air temperature and RH that, with the leaf sitting about 2°C below air, lands inside that band; the final column shows where it lands. Many other temperature/RH pairs hit the same VPD, so treat each example as a starting point, not the only combination that works.

Stage	Target VPD (kPa)	Example air temp	Example RH	VPD at leaf
Seedlings & clones (propagation)	0.4–0.8	24°C (75°F)	70%	~0.6 kPa
Vegetative	0.8–1.2	26°C (79°F)	60%	~1.0 kPa
Early–mid flower	1.2–1.5	26°C (79°F)	50%	~1.3 kPa
Late flower / ripening	1.2–1.6	25°C (77°F)	45%	~1.4 kPa

Published VPD ranges vary a little between manufacturers and growers — some run flower as low as 1.0 kPa, others as high as 1.6 kPa — so treat these as bands to aim inside, not exact set-points. The direction of travel is the reliable part: low VPD early, higher VPD later. If in doubt, sit in the middle of the band and adjust to how the plants respond.

The seedling row asks for warm-but-humid; the late-flower row asks for a drier canopy. Those are exactly the by-stage temperature and humidity targets from our companion guides — VPD just expresses both at once (see our grow room temperature guide and our humidity control guide).

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. The workflow:

1. Measure at the canopy. Put a thermo-hygrometer at plant-top height, where the leaves live — a gauge on the tent floor can read several degrees and several percent off.
2. Read (or estimate) leaf temperature. An IR 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 raise VPD: more extraction, a dehumidifier, or a touch more warmth. To lower it: a humidifier, less extraction, or a lower temperature. Airflow across the canopy also lifts VPD at the leaf by refreshing the humid boundary layer against each leaf.
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.

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.

Does VPD change between day and night?

Yes — and this is where a lot of otherwise well-run tents come unstuck. When the lights go off, air temperature falls. That cooler air holds far less moisture, so relative humidity climbs even though no water was added. Both effects push VPD down, sometimes sharply.

A little drift is fine; plants transpire less in the dark anyway. The danger is a collapse — if night VPD falls near zero, the tent becomes a botrytis and powdery-mildew factory overnight. Practical rules:

• Keep the lights-off temperature drop moderate — roughly 2–8°C (4–14°F) below your lights-on figure, matching stage.
• Run a dehumidifier or keep gentle air movement overnight so RH doesn't spike into the 80s and 90s.
• Don't let night VPD fall below about 0.4 kPa — that low, transpiration stalls and the still, humid air invites mould and botrytis; as VPD keeps sliding 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. 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 pushes feeding and reduces the 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 — an infrared 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 in your calculator or controller. Calculating from air temperature alone overstates VPD, so chasing that inflated figure tends to leave you over-humidifying and running the room too wet.

What VPD is too high or too low? As a rough rule, below about 0.4 kPa the air is too humid and still — transpiration slows and mould risk rises; above about 1.6 kPa the pull is aggressive enough that stomata start closing and growth stalls. The safe working window for most stages sits between those two figures, moving up as the plant matures.

Is VPD more important than temperature and humidity? It doesn't replace them — it combines them. You still set temperature and humidity with the same kit; VPD just tells you whether that pair is right for the stage. Two rooms with identical humidity can have very different VPD if their temperatures differ, which is why VPD is the better single target to chase.

How do I lower VPD in my grow tent? Add moisture or reduce the drying effect: run a humidifier, ease back your extraction fan, or drop the temperature slightly. The quickest lever in a dry UK winter tent is usually a humidifier; in a hot summer tent it's cooling the canopy. To raise VPD instead, add extraction, a dehumidifier, or a little warmth.

Recommended kit and further reading

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

• Canopy thermo-hygrometer with VPD readout — a smart sensor such as the AC Infinity CLOUDCOM B1 tracks canopy temperature and humidity (and VPD, in the app) in real time, so you catch drift before the plants show it.
• AC Infinity ventilation & climate control — smart controllers with a dedicated VPD mode that drives your fans, humidifier and dehumidifier to hold your target band automatically.
• Fans, Filters & Environment — extraction, circulation and humidity kit, the levers you actually move VPD with.
• Humidity Control — humidifiers and dehumidifiers for the moisture side of the equation.
• Systems, Parts & Testers and Grow Room Tools — meters, testers and the odds and ends for dialling a room in.

Most relevant collection: if you only click one link, start with AC Infinity ventilation & climate control — a VPD-aware controller is the single biggest step from guessing to holding a target.

Related guides: Grow Tent Too Hot? Summer Temperature Control · Grow Room Humidity Control: Targets by Stage and the Kit That Holds Them

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Why trust The Grow Superstore

We've been helping UK indoor growers since 2009, both online and from our physical store in 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.