Does VPD need leaf temperature, or is air close enough?
Air is not close enough, and the reason isn’t what most VPD charts imply.
By Curtis Olson — retired software developer, home grower, and the builder of the device this site is about. More about me and why I built it. · Last reviewed
The short answer
VPD is a statement about how hard the air is pulling water out of a leaf. That calculation needs the temperature of the surface the water is leaving from. Air temperature is a stand-in for it, and the substitution is worse than the VPD charts suggest. Not because it’s off by some steady amount you could just subtract — because how far off it is keeps changing, and so does which way it’s off.
What the difference looked like on my own tent
One morning last August my tent was steered on air-derived VPD and read 1.035 kPa — comfortably inside the veg target band. Eight minutes later the same tent, measured off the leaf surface with an infrared sensor, read 0.612 kPa: below band.
That is a gap of 0.42 kPa, which is roughly the width of the entire veg band. One number said “you’re fine, hold here.” The other said “you’re under, do something.” Same tent, same morning, same plants.
That is one tent on one morning and I’m not going to dress it up as a general constant — that would be exactly the mistake this page is about. What it is: an existence proof that the two numbers can disagree by enough to change the decision.
Why you can’t just subtract a couple of degrees
That’s the tempting patch — knock two degrees off for LED, carry on. I believed a version of it myself and had it written into my own notes, right up until I went and read the paper it was supposedly derived from.
The best measurements I know of are Nelson & Bugbee (2015, PLOS ONE), who compared leaf and air temperature under sunlight, greenhouse light, HPS and LED. Their findings, in short:
- For plants that are not water stressed, leaves sat within about 2 °C of air temperature under all four light sources.
- Swapping HPS for LED moved leaf temperature by only around 1.3 °C.
- The effect of the plant’s water status and its own evaporative cooling was much larger than the effect of the light source.
- Under water stress with high light and low airflow, leaves ran hotter than air — by around 8 °C under LED, and more under HPS.
Read those together and the lamp-offset idea falls apart. The gap has very little to do with your light and almost everything to do with the plant. A leaf keeps itself cool the same way you do — by sweating. Water goes out through the pores on the underside, evaporates, and carries the heat off with it. A plant that’s drinking freely sweats freely and sits right about at air temperature. A plant that’s short of water closes those pores to hang on to what it has, stops sweating, and then just sits there under the lamp getting warmer.
So the gap is small while everything is fine, and it gets big — and turns around — right when it isn’t. The leaf goes from a touch cooler than the air to several degrees hotter than it. That’s the part that kills the correction factor: knock two degrees off for your LED, and on the day the plant is actually in trouble you’re now ten °C out, in the wrong direction, on the one reading you needed to be right. Air VPD is least trustworthy at exactly the moment it matters most.
People who build the instruments say the same. A 2026 paper describing a low-cost handheld leaf-VPD meter (a humidity sensor and an infrared thermometer, about $83 in parts) says plainly that instruments which estimate leaf VPD from a fixed leaf–air offset “may introduce condition-dependent errors.” Its leaf readings agreed with a thermal camera to within about 0.3 °C. Two limits are worth knowing: it was tested on lettuce only, and it is a spot-check tool, so it did not measure how the gap moves with water stress. That part still rests on Nelson & Bugbee.
The same patch often gets run the other way round: keep the leaf at 82 °F, and under LED that means running the room at 80–85. For a healthy, well-watered plant that is a fair starting point. The leaf really does sit within a degree or two of the air, a little below it under LED, and 82 °F (about 28 °C) is inside the range where cannabis photosynthesis peaks (the Chandra papers below). What it can’t be is a standing setting. It works the leaf out from the room, and on the day the plant stops drinking, the leaf climbs above the air instead of sitting below it. A room already at 85 is the worst place for that to happen. Use the rule to pick a starting room temperature, then check the leaf.
Heat or VPD? It isn’t one or the other
A common version of the argument goes like this: a tent held at 72–74 °F with VPD “perfect around 1”, plants growing but slowly, a cheap heater added, a clear improvement, and the conclusion that heat trumps VPD. The improvement is believable. The conclusion mixes up two different things.
- VPD is made of temperature. It is worked out from temperature and humidity together, so “heat versus VPD” compares a number with one of its own ingredients. Warm a tent without adding water and VPD rises: at 23 °C and 64 % RH the air VPD is about 1.0 kPa; at 27 °C and the same 64 % it is about 1.3. Holding 1.0 at 27 °C takes about 72 % RH. That is why a warmer tent suddenly needs a bigger humidifier.
- Temperature also sets the pace on its own. Photosynthesis in cannabis rises with temperature up to an optimum. In controlled gas-exchange measurements the peak sat at 25–30 °C in the first study, and across seven varieties in a later one it ranged from about 25 °C in some to 30–35 °C in a high-potency drug type. 72–74 °F is 22–23 °C, below that range for most plants. So a warmer tent can grow faster at the same VPD, and that is a temperature effect, not temperature beating VPD.
- And the leaf may not be at air temperature either. A healthy, well-watered leaf sits within a couple of degrees of the air, and under an LED it can sit a degree or so below it (the Nelson & Bugbee numbers above). If the leaf is 1 °C cooler than 23 °C air at 64 % RH, the VPD at the leaf is nearer 0.8 than 1.0; at 2 °C cooler, nearer 0.7. “Perfect around 1” off an air probe can be under band at the leaf, and adding heat lifts the leaf with it.
So the useful version of “heat trumps VPD” is this: choose the temperature first, because it sets how fast the plant can work, then use humidity to put the leaf VPD where you want it at that temperature. They are two dials, and temperature is the one hobby tents most often leave too low.
How to check it in your own tent
- Measure the leaf, not just the air. An IR thermometer on a healthy fan leaf in the canopy, and the VPD recomputed from that number.
- Raise the air a degree or two at a time, and add humidity so the leaf VPD stays where it was. Changing both at once is how the two effects get confused.
- Watch the growth rate for a week at each step: new nodes, new leaves, how far the top moves.
- Stop when the top leaves cup, taco or curl up under the light. That is heat, and curling leaves: heat or hunger? covers telling it apart.
What would show this is wrong: if a tent already running leaf temperature in the mid-to-high 20s °C is warmed with leaf VPD held steady and growth does not change, or slows, that plant was already at its temperature optimum and heat was not the lever. The optimum differs by variety in the measurements above, so that is a real possibility, not a technicality.
What this means if you don’t own an IR sensor
Buy a handheld infrared thermometer — the IR gun, as the forums call it. They’re about twenty dollars. Point it at a healthy fan leaf in the canopy, compare to your air temperature, and recompute VPD off the leaf number.
This isn’t a fringe opinion. AC Infinity — who make the controllers a great many hobby tents run on — sell a handheld IR thermometer for exactly this, so growers can check leaf VPD and hand-calibrate their controller against it. The largest player in the category agrees leaf temperature is the right number.
The honest caveat on the handheld approach is the same physics as above: a spot check tells you the gap at that moment, and the gap moves with the plant’s water status through the day. It’s a real improvement over ignoring the leaf. It is not the same as watching it continuously.
Where my device fits, stated narrowly
Croplock reads leaf-surface temperature from an infrared sensor in the canopy and works VPD out from that reading. That on its own is no longer unusual. Plenty of gear can adjust a tent by itself now, and some of it reports a leaf temperature too. So the claim I’ll make is a narrower one: the device checks the leaf reading before it uses it.
The reason it has to is that an infrared sensor reports the temperature of whatever it is pointed at, and it has no way to know whether that is a leaf. Over a small plant it is mostly looking at the pot, and a pot warmed by the lamp reads like a very hot leaf. So two checks run on every reading:
- Is there enough plant under the sensor? The camera has to see enough canopy for the reading to be mostly leaf.
- Could a leaf be this warm? A reading too far above the air to be a leaf is thrown out.
If either check fails, or there is no reading at all, the device steers on plain air VPD and records that it did, and why. It does not fill the gap with an assumed offset. I removed the fixed-offset fallback it used to have, for the reason above: one number can’t correct for a gap that runs cool some of the time and hot the rest.
The checks are not perfect, and the weak spot is worth knowing. Wet soil cools by evaporation the same way a leaf does, so the temperature check can’t tell the two apart. Only the camera check catches that case, and a setup with no camera doesn’t have it.
Sources
- Nelson, J.A. & Bugbee, B. (2015). Analysis of environmental effects on leaf temperature under sunlight, high pressure sodium and light emitting diodes. PLOS ONE 10(10): e0138930.
- Bench measurement, 2026-08-02, from my own tent’s logs.
- Seyhan, T.G. & Seyhan, S. (2026). Real-time leaf-level vapor pressure deficit monitoring: development, uncertainty analysis, and validation of a low-cost portable sensor platform for controlled environment agriculture. Applied Sciences 16(17): 8625. DOI 10.3390/app16178625. The fixed-offset quote is from its abstract; the 0.3 °C agreement is its leaf-temperature RMSE against a FLIR E4 (0.289 °C, n = 250).
- Chandra, S., Lata, H., Khan, I.A. & ElSohly, M.A. (2008). Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiology and Molecular Biology of Plants 14(4): 299–306. DOI 10.1007/s12298-008-0027-x — temperature optimum for photosynthesis 25–30 °C.
- Chandra, S., Lata, H., Khan, I.A. & ElSohly, M.A. (2011). Temperature response of photosynthesis in different drug and fiber varieties of Cannabis sativa L. Physiology and Molecular Biology of Plants 17(3): 297–303. DOI 10.1007/s12298-011-0068-4 — optimum about 25 °C in some varieties, 30–35 °C in a high-potency drug type.
- The humidity and VPD figures in “Heat or VPD?” are arithmetic with the Tetens saturation formula, not measurements. The step-the-temperature test is first-party practice, not a citation.
More of these at Answers. The device is at croplock.com.
From the build: In band, and sick anyway — the morning the air read 1.035 kPa and the leaf read 0.612, eight minutes apart.
From the record: One grow, on the record — the leaf ran 2.24 °C below the air at one point and 1.04 above it at another — 240 paired hourly readings, a 3.29 °C spread, and the sign changes.
Also read: Where should the probe go? · Curling leaves: heat or hunger? · Bud rot: how humid, for how long? · Work out your own VPD