Somebody in a forum tells you your humidity is wrong. Somebody else tells you humidity is the wrong number entirely and you should be running VPD. You look up a VPD chart, find a grid of numbers in five colors, and discover that your tent is sitting in the orange.
Here is what that grid is and is not. It is a genuinely better way to read a room than temperature and humidity separately, because it converts two numbers into the one thing the plant responds to. It is not a precision instrument, it does not know what a bud looks like, and most of it is unreachable for a cannabis plant anyway.
This guide gives you the chart in both Fahrenheit and Celsius, the target for every stage from seedling to the week before harvest, the arithmetic behind it so you can check us, and an honest account of how much any of it has actually been measured on cannabis. It also covers the part that almost no chart mentions: the leaf temperature sitting underneath every number on it, and what happens to your reading when that assumption is wrong.
What VPD Actually Is
Vapor pressure deficit is how hard the air is pulling water out of your plant. That is the whole idea. Everything else is bookkeeping.
Air can hold a certain amount of water vapor, and that amount goes up steeply as the air gets warmer. Warm air holds a lot; cold air holds very little. The maximum an air parcel could hold at its current temperature is called the saturation vapor pressure. What it is actually holding right now is the actual vapor pressure. The difference between those two - the room the air still has for more water - is the vapor pressure deficit.
Both are measured in kilopascals, written kPa, which is just a unit of pressure. You do not need to have an intuition for a kilopascal any more than you need one for a lumen. What you need is the range: on a cannabis plant the useful numbers run from about 0.4 kPa (very still, very humid, the air is barely pulling at all) to about 1.5 kPa (dry and thirsty air, pulling hard). Below 0.4 the plant has trouble moving water at all. Above about 1.6, most charts assume it starts closing its pores to protect itself - a sensible rule of thumb that has never been measured on cannabis.
Relative humidity, the number on your cheap hygrometer, is the actual vapor pressure expressed as a percentage of the saturation vapor pressure. That percentage is the problem. Because the denominator moves with temperature, the same relative humidity means completely different things in a cool tent and a warm one.
Run the arithmetic on 60% relative humidity at two temperatures a home grower would call normal. At 68°F (20°C), 60% humidity is a pull of 0.78 kPa - gentle. At 82°F (28°C), the very same 60% is 1.26 kPa - more than half again as strong. Nothing about your humidity readout changed. The plant went from a mild day to a brisk one.
That is the argument for VPD in one paragraph, and it is a good argument. One number, and it already has the temperature folded into it.
What the plant is actually doing with all this
A cannabis plant is, among other things, a pump. Water comes in through the roots, travels up the stem, and leaves through thousands of microscopic adjustable pores on the underside of each leaf. Those pores are stomata, and the water leaving through them is transpiration.
The plant does not transpire because it enjoys it. It transpires because the same open pores that let water out let carbon dioxide in, and carbon dioxide is what it builds sugar from. Every molecule of CO2 it takes in costs it water. The stream of water moving up the plant is also how calcium and every other nutrient that does not move on its own gets from the root zone to the new growth at the top.
So transpiration is not waste. It is the plant's delivery system, its cooling system and the price of admission for photosynthesis, all running on the same pump. VPD is the setting on that pump. Too little pull and the delivery slows to nothing. Too much and the plant shuts the pores to avoid running dry, which stops the carbon dioxide coming in at the same moment.
Charlotte Grossiord and her co-authors put the general version of this plainly in their 2020 review in New Phytologist: stomatal conductance declines under high VPD, while transpiration rises with VPD only up to a threshold, after which it levels off or falls, and the result is reduced photosynthesis and growth.2 They also make a distinction that matters more than it sounds: the quantity that governs the plant is leaf-to-air vapor pressure deficit, not the deficit of the air on its own. Hold that thought - it is the subject of a whole section below, and it is where most VPD advice quietly falls over.
The VPD Chart, in Fahrenheit and Celsius
Here is the chart. Air temperature down the side in both units, relative humidity across the top, and the leaf-to-air VPD in kilopascals where they meet. Find your row, find your column, read the number.
Two things about it that most charts leave unsaid, and that you are entitled to know before you trust a cell.
First, every number here is computed, not surveyed. Saturation vapor pressure comes from the standard equation in the United Nations Food and Agriculture Organization's Irrigation and Drainage Paper 56, the reference agronomists have used for this since 1998: es = 0.6108 exp[17.27T / (T + 237.3)], with temperature in Celsius and the answer in kPa.3 We checked our arithmetic against that document's own worked example before drawing anything, and it reproduces it exactly. If you want to verify a cell yourself, that is the equation to use, and any online VPD calculator will use the same one or a close cousin of it.
Second, this chart assumes your leaf sits 2°F (about 1°C) cooler than the air. Every leaf-to-air VPD chart you will ever see makes an assumption like that one, and almost none of them tell you. It is a reasonable default and it is frequently wrong, in both directions. The section on leaf temperature shows exactly how much it moves the answer, which is more than you would guess.
How to use it without overthinking it
You do not need to land on a specific cell. You need to be in the right neighborhood of the chart and stay there. We would rather be steady and slightly off than perfect and swinging, because a plant adjusts its stomata to the conditions it has been living in, and a swinging room keeps asking it to adjust again.
Read the number once when you set up a stage, make one change if you are outside the band, and then go look at your plants instead.
VPD Targets by Growth Stage
These are the bands we run and the ones the rest of our guides are built on. The temperature and humidity columns are the same ones in our indoor growing guide and our flowering stage guide. The VPD column is what those bands work out to. If you are not sure which stage your plant is in, our growth stages guide shows how to tell.
| Stage | Air temperature | Relative humidity | VPD | Why |
|---|---|---|---|---|
| Seedlings and fresh clones | 72-78°F (22-26°C) | 70-80% | 0.4 to 0.8 kPa | A young plant has almost no root system. Keep the pull gentle. |
| Vegetative | 71-82°F (22-28°C) | 55-65% | 0.8 to 1.2 kPa | The widest safe band of the whole grow. Hard to get wrong. |
| Flower, weeks 1 to 3 | 70-80°F (21-27°C) | 55-65% | 0.9 to 1.1 kPa | The stretch. Humidity is still high because the buds are still small. |
| Flower, weeks 4 to 6 | 70-80°F (21-27°C) | 50-60% | 1.1 to 1.3 kPa | Buds are filling. Humidity comes down as they do. |
| Late flower, week 7 to harvest | 65-75°F (18-24°C) | 45-55% | 1.1 to 1.3 kPa | Mold window. If the humidity band and the VPD figure disagree here, humidity wins. |
The shape is the point: the pull starts gentle and gets stronger as the plant gets better at handling it. A seedling with three roots and two real leaves cannot supply a hard pull, so you give it humid, still air and let it build a root system first. A mature plant in flower has the plumbing to move real volumes of water, and a firmer pull moves calcium and everything else up to the bud sites where it is needed.
If you only remember one band, remember 1.0 to 1.2 kPa. It overlaps the band for veg and for every week of flower, and no trial we know of has shown that a grower who simply held it from the first week of veg to the day before harvest would lose anything by it.
Seedlings and clones: 0.4 to 0.8 kPa
A fresh clone has no roots at all and a seedling has barely any. Neither can replace water as fast as dry air takes it, so the air has to do almost none of the work. That is why a humidity dome goes over a propagation tray: it is a VPD control device that costs four dollars. Our planting guide covers the first two weeks in full, including the leaf surface temperature target of 75-80°F (24-27°C) that goes with this band.
Do not point a fan at seedlings. Gentle air movement in the tent is fine and helps stems thicken, but a fan aimed into a propagation tray is the single fastest way to dry out a cutting that has no way to drink.
Vegetative: 0.8 to 1.2 kPa
This is the easiest stage to get right, and the one where the chart matters least. The band is wide, the plant is tough, the roots are working, and unless your room is genuinely extreme you are probably already in it. Veg is where you should be spending your attention on training the plant, not on a decimal place.
There is one real connection between VPD and training, though, and it runs the other way to what people expect. A flat, open canopy built with clips like the BudClips does not change the VPD of the room at all. What it changes is how much of the room's air actually reaches each leaf - which, as the leaf temperature section explains, is most of the battle.
Flower weeks 1 to 3: 0.9 to 1.1 kPa
The stretch. The plant is putting on height and setting bud sites, humidity is still relatively high at 55-65%, and the pull stays moderate. That figure and the two after it are our flowering guide's, not new numbers.
Flower weeks 4 to 6: 1.1 to 1.3 kPa
Buds are filling. Humidity steps down to 50-60% and the pull firms up to match. Nothing dramatic happens here if you drift a little either way.
Late flower, week 7 to harvest: 1.1 to 1.3 kPa
This is the one stage where we will tell you to ignore the VPD number if it fights with the humidity number, and the reasoning is in the section on the chart's blind spot. Run 45-55% humidity, take the cooler end of 65-75°F (18-24°C), and if the VPD reads high, let it read high.
The same targets, read backwards
In practice you do not get to pick a VPD - you get to pick a humidity, because temperature is usually whatever your lights and your house decide it is. So here is the chart turned around: your air temperature on the left, and the humidity that lands you in the 1.0 to 1.2 kPa band on the right.
| Air temperature | Humidity to hold | Notes |
|---|---|---|
| 65°F (18°C) | 36 to 46% | Cool. At our stage humidity bands you land under the VPD target in veg and early flower. Normal, and fine. |
| 68°F (20°C) | 42 to 51% | Still on the soft side at our veg humidity. |
| 70°F (21°C) | 45 to 53% | Works. Our bands land just under the VPD target here. |
| 72°F (22°C) | 49 to 56% | Our humidity bands and our VPD bands agree here. |
| 75°F (24°C) | 53 to 60% | Our humidity bands and our VPD bands agree here. |
| 77°F (25°C) | 56 to 62% | Works at every stage. |
| 80°F (27°C) | 59 to 65% | Warm. At our late-flower humidity the VPD now reads 1.35 to 1.70. Keep the humidity, let the number run high. |
| 82°F (28°C) | 62 to 67% | The top of the safe air band, and late flower reads well over target. |
| 85°F (29°C) | 65 to 69% | Still inside the photosynthesis optimum, but the humidity this row asks for is mold weather in flower. |
Read the notes column, because it is the honest part. Our humidity bands and our VPD bands only line up cleanly in the middle of the temperature range, around 72 to 77°F (22 to 25°C). Run a cool room and the humidity we recommend lands you under the VPD target. Run a warm one and it lands you over it - at 80°F (27°C), our late-flower humidity band of 45 to 55% reads 1.35 to 1.70 kPa, well above the 1.1 to 1.3 the same guide asks for. That is not a mistake in either set of numbers. It is what happens when you convert between two things that were never derived from each other, and it is the practical reason we tell you to hold the humidity and let the VPD land where it lands.
At 80°F (27°C) and above, the humidity that would put you in the VPD band is humidity you should not run in late flower at all. The chart is not wrong; it is answering a different question than the one you have. It is telling you what produces a given pull. It has no idea that a dense cola sitting in 65% humidity is a bud rot problem regardless of how elegant the vapor pressure deficit is.
Which is the other thing nobody says about VPD charts: most of the grid is unreachable. Suman Chandra and colleagues measured photosynthesis in Cannabis sativa across five temperatures and found the rate peaked at 30°C (86°F) and concluded the practical optimum was 25-30°C (77-86°F), with stomatal conductance rising with temperature only up to 30°C and falling above it.4 Add the mold ceiling in late flower and the bottom end where a cool room stalls, and the part of that big colorful grid a cannabis grower can actually use is a small box in the middle of it.
What the Research Actually Says About VPD and Cannabis
Almost every VPD chart on the internet presents its bands as settled. We went looking for what they are settled on, and the honest answer is thinner than the confidence around it.
The one controlled cannabis experiment
In 2025, Ingrid Carolina Corredor-Perilla, Tae-Hyung Kwon and Sang-Hyuck Park, working at the Institute of Cannabis Research at Colorado State University-Pueblo, published the only controlled experiment we could find that varies humidity on cannabis and reports what happened to the harvest.1 They grew ten plants of a CBD-dominant cultivar per treatment through eight weeks of veg and six of flower, at two canopy humidity levels: 37-58% against 78-98%.
The differences are not subtle. Taking their measured numbers rather than their summary percentages:
- Flower biomass: 33.8 g per plant in the drier room against 9.9 g in the humid one - a 3.4-fold difference. Flowers plus leaves, 48 g against 11.5 g.
- CBDA ran 4.9 times higher in the drier room, 27.8 against 5.7 mg/g. After decarboxylation, CBD came out at 2.80% against 0.61%. CBCA differed 13-fold.
- Flowering was delayed three weeks in the humid treatment.
- Stems ran 9.7% longer, noticeably thinner, and carried about a quarter fewer nodes - the classic stretched, weak-stemmed plant.
- Guttation, tip burn and leaf rot showed up in the humid treatment and spread.
Take that seriously, because it is the best evidence the field has. The bottom of the VPD chart is a genuine cliff and you do not want to be near it. If your tent sits at 85% humidity because you never bought a dehumidifier, this is roughly what it is costing you, and it is not a rounding error.
The one symptom in that study you can actually use
Tucked into their observations is something worth knowing, because it is one of the few signs of a humidity problem you can check with your own eyes. The humid plants showed guttation: beads of water pushed out at the tips of the leaf serrations and left sitting there.
Guttation is what happens when the roots keep pushing water up and the leaves cannot evaporate it away, so the plant forces the excess out through pores at the leaf margins. It is not dew and it is not a spray residue - the drops sit precisely on the leaf's points, and the rest of the leaf is dry. A few drops at lights-on after the dark period are normal: with transpiration shut down overnight, the pressure from the roots has nowhere else to go. The signal is heavy guttation on many leaves that is still there well into the light period. That means the air is too humid for the leaves to evaporate what the roots are sending up. It is harmless in itself. In the trial it kept company with tip burn and leaf rot, which are not.
The detail in their Table 1 that nobody quotes
Now the part that made us rewrite this whole page. The authors report leaf VPD, not just air VPD, for both treatments at each stage. In the humid room, leaf VPD ran 0.13 kPa in veg and -0.04 kPa in flower. A negative leaf VPD means the leaf was at or below the dew point of the air touching it: water was condensing on the plant rather than leaving it. That is not a low VPD, it is fog.
And in the room that won? Leaf VPD of 1.04 and 1.02 kPa in veg, and 0.60 kPa through flowering, in air held at 17°C (63°F) and 58% humidity.
Read that number again. The best-performing treatment in the only controlled cannabis experiment on this subject spent its entire flowering period at a leaf VPD of 0.60 kPa - below the bottom of the flowering band on almost every chart you will find, including the one further up this page. It still produced 33.8 g of flower per plant at 2.80% CBD.
We are not going to pretend that one data point overturns the bands. It is a single cultivar, a cold room and a CBD-dominant chemotype, and a trial designed to test a swamp against a normal room is not designed to locate an optimum. But it is a strong argument against treating 0.9 as a floor you must not fall below, and we would rather tell you that than quietly leave it out.
What that experiment does not tell you
It had two treatments, and one of them was a swamp. Nothing in that study distinguishes 0.9 kPa from 1.2 kPa, which is exactly the distinction almost every chart invites you to agonize over. A trial with two arms can tell you that one end of a range is bad. It cannot locate an optimum inside the other end.
We looked for the study that does make that comparison on cannabis. As far as we can establish, it does not exist. The stage-by-stage bands you see everywhere - ours included - are inherited from greenhouse horticulture, refined by commercial growers watching their own rooms, and passed along. That is not nothing. Greenhouse growers have been managing VPD on tomatoes and ornamentals for decades and the physiology is not species-specific in any way we have reason to doubt. But it is not the same thing as a trial, and a page that presents it as one is selling you a confidence it did not earn.
The general physiology is much better established. Grossiord and colleagues, reviewing the field across all plants, describe the shape: stomata close as VPD climbs, transpiration rises with VPD only to a threshold and then plateaus or falls, and photosynthesis and growth drop off behind it.2 Their review also makes the point that most of the field shares our problem - "few studies have isolated the physiological response of plant functioning to high VPD," because in nature high VPD arrives alongside heat and drought and is hard to separate from either.
Chandra and colleagues give us the cannabis-specific temperature envelope: photosynthesis peaked at 30°C (86°F), stomatal conductance rose with temperature only up to 30°C and was hurt above it, and transpiration kept climbing all the way to the 40°C (104°F) top of their range.4 That last detail is worth sitting with. In a hot room the plant keeps losing water long after it has stopped getting anything for it.
What we conclude from that, and what we do
We run the bands in the table above. We think they are sensible, we think they are conservative in the right direction, and we are telling you plainly that the precision implied by a chart with two decimal places is not backed by cannabis data. Our position, in one sentence: the chart is a floor check, not a set point.
That is the same stance we take on the drying rules, where the famous 60/60 rule turns out to appear in no paper at all, and on light intensity, where the commonly repeated yield plateau does not exist in the one good trial. Where the folklore and the literature disagree, we go with the literature and say so. Where the literature is silent, we say that too rather than borrowing someone else's certainty.
The Leaf Temperature Problem
Here is the part that changed how we think about VPD charts, and the reason we stopped treating the third digit as meaningful.
The chart is not built on the temperature of your air. It is built on the temperature of your leaf. Vapor pressure deficit as the plant experiences it - leaf-to-air VPD - is the saturation vapor pressure at the leaf's surface temperature minus the actual vapor pressure of the surrounding air. Since nobody measures leaf temperature, every chart substitutes an assumption. Ours assumes the leaf is 2°F (1°C) below air. Others use 3°F (about 2°C), or offer a slider.
Jacob Nelson and Bruce Bugbee worked out what that offset really is. They took radiation measurements under four sources - clear-sky sunlight in the field, sunlight in a glass greenhouse, indoor high pressure sodium, and indoor LED - and ran them through an energy-balance model.5 Three findings matter here, and all three are inconvenient for the chart.
One: the lamp matters less than the forum says. LED runs about 1.3°C cooler at the leaf than HPS under typical indoor conditions. That is real, but as they put it, "the effect of LED technology on leaf temperature is smaller than is often assumed."
Two: when the plant is healthy, the leaf sits close to the air. If plants were not water stressed, leaves under all four radiation sources were typically within 2°C (3.6°F) of air temperature. Close, but note that this is a band, not an offset - it covers leaves both above and below the air.
Three: whether the plant can drink matters far more than what you hung above it. In their near-worst case of water stress and low wind, leaves ran 6°C above air in the field, 8°C under LED, 10°C in a greenhouse and 12°C under HPS. A transpiring leaf cools itself; a leaf whose stomata have shut cannot. Their own summary is that "the effect of plant water status and leaf evaporative cooling is much larger than the effect of radiation source."
What that does to your reading
Put numbers on it. Take an ordinary tent at 77°F (25°C) and 55% relative humidity and ask what the VPD is, varying nothing but the leaf temperature across a band that sits comfortably inside Nelson and Bugbee's normal range.
| If the leaf is | Your VPD is really | Humidity you would need for 1.20 kPa |
|---|---|---|
| 4°F (2°C) cooler than the air | 1.03 kPa | 50% |
| 2°F (1°C) cooler - what the chart assumes | 1.22 kPa | 56% |
| the same as the air | 1.43 kPa | 62% |
| 2°F (1°C) warmer | 1.64 kPa | 69% |
| 4°F (2°C) warmer | 1.87 kPa | 76% |
Read the right-hand column, because that is the one you can act on. To hold 1.20 kPa at 77°F (25°C), the humidity you should be running is 50% if the leaf is 4°F (2°C) cooler than the air and 76% if it is 4°F (2°C) warmer. That is a twenty-six point spread in the only setting you actually control, produced entirely by a number nobody measures.
And notice the circularity, because it is the heart of this. The chart's offset is only valid for a leaf that is transpiring freely. A leaf transpires freely when the VPD is in a good range and the plant has water. So the chart assumes the outcome it is supposed to help you produce. When your grow is going well, the chart is roughly right and you did not need it. When something is wrong - the plant is underwatered, the root zone is hot, the medium has gone salty - the leaf runs hot, the real VPD is far above what you read, and the chart is confidently wrong in exactly the situation you needed it for.
The fix, which costs about twenty dollars
Point an infrared thermometer at a healthy fan leaf in the middle of the canopy, lights on. It reads surface temperature directly. Our planting guide already calls it the most useful cheap tool in a seedling tent, and it earns its place at every stage after that too.
Once you can see leaf temperature, a second thing falls out for free: you can tell heat stress from too much light. Leaves curling or tacoing at the edges while the leaf reads hot is thermal stress, and the answer is to cool the room or raise the lamp. Bleached, pale tips while the leaf reads cool is photon overload, and the answer is to dim. They look alike and the fixes are opposite.
How to Hit a VPD Target
Six steps, in order. Most tents need two of them.
Step 1 - Put the sensor where the plant is
A thermometer and hygrometer taped to the tent wall is measuring the tent wall. Air inside a tent is not one uniform thing: it is warmer near the light, cooler near the floor, and wetter inside a dense canopy than in the open space beside it. Clip the sensor at canopy height, in among the plants, out of the direct beam of the fixture. That reading is the only one that means anything.
If you have one sensor and two plant heights, put it at the top of the canopy, where the buds are.
Step 2 - Read the chart, or do the arithmetic
Find your air temperature row and your humidity column on the chart above. Any online VPD calculator will give you the same answer, and they are free. Write your number down; you will need it in step 4.
Step 3 - Point a fan at the canopy before you buy anything
This is the step people skip, and it is the one that fixes the most tents.
Every leaf carries a thin film of still air on its surface called the boundary layer. Water leaving the stomata has to cross it. In still air that film gets thick and humid, so the leaf is effectively breathing into a small pocket of saturated air regardless of what the room's hygrometer says. Air movement thins that film, which is why a fan makes a plant transpire faster without changing a single number on your readout. Our canopy science guide goes further on it: a thick boundary layer slows carbon dioxide exchange too, so a stagnant canopy interior can be locally starved of CO2 while the room reads perfectly fine.
That also cuts the other way and explains a real failure: a dense, untrained bush in still air has an interior nobody's meter is reading. The stagnant middle of that plant can be a mold risk while the tent average looks perfect. Opening a canopy up with training is an airflow intervention as much as a light one - it is why we build a flat canopy with clips and a net rather than letting one apex run away, and it is covered in the low-stress training guide.
A clip fan or two, moving the leaves gently, not whipping them. You want the canopy to shimmer, not to thrash.
Step 4 - Move humidity first, temperature second
If your number is off, humidity is almost always the right lever. It is cheap to change, it does not disturb anything else, and it is usually the one that drifted.
Temperature moves VPD at roughly the same rate: at 77°F (25°C), one degree Fahrenheit shifts it about as much as one to two points of humidity. But you have less freedom with it. Below about 65°F (18°C) growth slows, and above 86°F (30°C) you are past the photosynthetic optimum Chandra and colleagues measured. Move it only when humidity alone will not get you there.
Step 5 - Check the leaf
Point the infrared thermometer at a healthy fan leaf in the middle of the canopy with the lights on. If the leaf reads close to air temperature or below it, the chart's assumption is holding and your reading is fine. If the leaf reads several degrees above air, stop adjusting humidity and go find out why the plant is not drinking - it is nearly always the root zone, and our watering guide is the place to start.
Step 6 - Hold it, do not chase it
Plants acclimate to the air they live in. Our reasoning - and it is reasoning, not a trial result - is that a plant held steady spends less of its day re-adjusting its stomata than one whose conditions keep swinging. We would rather hold 1.25 kPa steady than oscillate around 1.10. Set it, check it once a day, and resist the urge to tune.
What Actually Moves the Number
| What you change | Which way VPD moves | What to watch |
|---|---|---|
| Run a humidifier | Down | The first thing to reach for when VPD is too high. Cheap, fast, safe. |
| Run a dehumidifier | Up | The first thing to reach for when VPD is too low. It also dumps heat into a small tent. |
| Raise air temperature | Up | About as strong as humidity - one degree F does roughly what one or two points of humidity do. Bounded by the 86°F (30°C) photosynthesis ceiling. |
| Lower air temperature | Down | Same lever backwards. In late flower this is how you get humidity control and mold safety at once. |
| Point a fan at the canopy | Effectively up at the leaf | It does not change the room's numbers. It thins the still layer of humid air sitting on each leaf, which is the layer the plant actually breathes through. |
| Raise light intensity | Up | More radiation on the leaf warms it, and a warmer leaf raises leaf-to-air VPD at the same room reading. |
| Increase exhaust fan speed | Toward the room outside your tent | Whatever the air in the house is, you are moving toward it. Useful in a dry basement, useless in a humid one. |
| Defoliate | Up | Fewer leaves transpiring means less water going into the tent air. A real effect and the worst reason to defoliate. |
Two entries there deserve more than a table cell.
The fan does not change your room's VPD and it changes the plant's experience more than anything else on the list. That is not a contradiction, it is the boundary layer from step 3. Your meter reads the room. The leaf lives inside its own little climate, and moving air is how you connect the two.
Raising light intensity raises VPD at the leaf, because more radiation landing on a leaf warms it, and a warmer leaf has a higher saturation vapor pressure inside it. This is where people conflate two different problems. A leaf can be too hot because the lamp is too close and dumping radiant heat on it, or a leaf can be receiving more photons than it can process. Those are not the same fault and they have opposite fixes. Our canopy science guide covers the light side; the leaf thermometer tells them apart.
When to Ignore the Chart
Four situations where the right VPD number is the wrong answer.
Late flower, because mold does not read kPa
This is the big one, and it is the chart's real blind spot.
Botrytis and powdery mildew respond to relative humidity and free water. They have no opinion about vapor pressure deficit. A tent at 80°F (27°C) and 62% humidity reads 1.11 kPa. A tent at 70°F (21°C) and 50% humidity reads 1.09 kPa. On a chart printed to two decimals those are the same room. They are not remotely the same room to a spore landing inside a dense cola, and the difference gets worse at night when the temperature falls and the humidity in that bud climbs.
Zamir Punja, reviewing cannabis disease management, puts the control measure as reducing relative humidity and moisture deposited on the inflorescences, because that is what cuts spore germination and infection.6 He also makes the observation that sits underneath this whole page: susceptibility differs with canopy architecture and bud size, because those change the microclimate and the relative humidity right at the infection site. Not the room. The half inch of air inside the cola.
Which gives you one physical thing to do about it that the chart cannot. Heavy colas that lean into each other in week eight close that half inch up. Tying them apart with a soft wire like the BudHuggers is support rather than training, which is the only kind of intervention that still belongs in flower, and it opens the gaps where the air has to move.
So the house rule, which the whole cluster runs on: in late flower, if the humidity band and the VPD figure disagree, humidity wins. Hold 45-55%, take the cool end of the temperature range, and let the VPD read high. Our bud rot guide and our powdery mildew guide own that reasoning, and it is worth knowing that no humidity threshold for either disease has ever been measured on cannabis itself - which is one more reason to be conservative rather than clever.
Drying, because that is a different job entirely
Drying flower is not growing it. There are no stomata doing anything and no photosynthesis to protect - you are managing how fast water leaves dead tissue, and the goal is slow and even rather than efficient. Our drying guide runs a humidity schedule rather than a VPD target, and says plainly that you should not calculate it: hold 60-68°F (15-20°C), follow the schedule, and the VPD takes care of itself.
Seedlings and clones under a dome
You are deliberately running a VPD far below anything the chart would call good, because a plant with no roots cannot supply a pull. The dome is doing exactly its job. Do not fix it.
Autoflowers, a little
The bands are the same - an autoflower is the same plant with a different clock. What changes is your margin. A photoperiod that spends two bad weeks in the wrong part of the chart can be given two more weeks of veg to make it up. An autoflower cannot, because it is flowering on its own schedule whatever you do. Nothing about the target moves; the cost of getting it wrong does.
Meters, Controllers and Calculators
What is worth buying, in the order we would buy it.
A decent hygrometer and thermometer, clipped at canopy height. About $15. Any of them will do. This is the whole requirement. Everything below is a convenience.
An infrared thermometer. About $20. The highest-value tool on this page, because it is the only one that measures the thing the chart is guessing at. Point-and-read, no calibration, useful at every stage.
An environmental controller, if you already own an inline fan that will talk to one. The AC Infinity Controller 69 and its siblings will hold a humidity or VPD setpoint by modulating your exhaust fan, and if you are already in that ecosystem it is a genuine quality-of-life upgrade. It is not making your plants better than a $15 hygrometer and a person who looks at it. It is saving you the looking.
A dedicated VPD meter or sensor, such as the Pulse. These log continuously, show you trends, and are genuinely lovely. They are also several hundred dollars to tell you a number you can read off a $15 hygrometer and the chart above. Worth it if you like data and can afford it. Not a requirement, and buying one before you own a fan and a dehumidifier is getting the order wrong.
An online VPD calculator. Free, and fine. They almost all use the same saturation vapor pressure equation this page does. Just check what leaf offset the one you are using assumes, because they differ, and now you know that matters.
What we would not buy: anything that promises to automate VPD without also controlling humidity, and any hygrometer so cheap it does not specify an accuracy. A sensor that is 8% off turns the whole exercise into theater.
Common VPD Mistakes
Reading the room instead of the canopy. The sensor on the tent wall, at knee height, away from the plants. Covered in step 1 and it is the most common one by a distance.
Chasing the second decimal. Adjusting a humidifier daily to move from 1.08 to 1.12 kPa. The evidence does not support that the difference exists, and the swinging costs more than the target gains.
Buying a dehumidifier before owning a fan. The fan is $20 and does more for the leaf than the dehumidifier does. It also dumps less heat into a small tent.
Running the chart's humidity in late flower. At 80°F (27°C) the chart asks for around 60-65% to sit in band. In week eight that is how you find out what bud rot smells like.
Assuming a leaf temperature and then trusting three digits. The whole leaf temperature section. If you have not measured it, your VPD is a range, not a number - so treat it as one.
Treating a high reading as a humidity problem when it is a watering problem. A plant whose stomata have closed because the root zone is too dry, too hot or too salty will run a hot leaf and a genuinely high leaf-to-air VPD, and no amount of humidifier will fix the plant. Check the pot before you check the chart.
Frequently Asked Questions
What does VPD stand for?
Vapor pressure deficit. It is the difference between the amount of water vapor the air is holding and the maximum it could hold at its current temperature, measured in kilopascals (kPa). In plain terms it is how hard the air is pulling water out of your plant. Because it already has temperature folded into it, one VPD number tells you more than a temperature and a humidity reading side by side.
What is a good VPD for cannabis?
Between 0.8 and 1.3 kPa covers veg and every week of flower. If you want one number to hold from the start of veg to the day before harvest, use 1.0 to 1.2 kPa, which overlaps every stage's band. Seedlings and fresh clones are the exception and want a much gentler 0.4 to 0.8 kPa, which is what a propagation dome gives you.
What should VPD be in veg?
0.8 to 1.2 kPa, which is what 71-82°F (22-28°C) at 55-65% relative humidity works out to. This is the widest band of the whole grow and the hardest stage to get wrong. If your tent is anywhere near normal room conditions you are probably already inside it, and your attention is better spent on training the plant.
What should VPD be in flower?
0.9 to 1.1 kPa through weeks 1 to 3, then 1.1 to 1.3 kPa from week 4 to harvest. Those come from our flowering guide's humidity bands: 55-65% early, 50-60% mid, 45-55% late, at 70-80°F (21-27°C) falling to 65-75°F (18-24°C). In late flower, if the humidity band and the VPD figure disagree, keep humidity at or below 55% and let the VPD read high.
What VPD do seedlings need?
0.4 to 0.8 kPa, at 72-78°F (22-26°C) and 70-80% relative humidity. A seedling or a fresh clone has almost no root system and cannot replace water as fast as dry air removes it, so the air has to do nearly none of the work. A humidity dome is the cheapest VPD control device there is. Do not aim a fan into a propagation tray.
How do you calculate VPD?
Find the saturation vapor pressure at your leaf temperature, then subtract the actual vapor pressure of the air. The standard equation is es = 0.6108 exp[17.27T / (T + 237.3)], with T in Celsius and the result in kPa. Actual vapor pressure is that same figure at air temperature multiplied by your relative humidity. Any free online calculator does exactly this.
How do I lower VPD in a grow tent?
Raise humidity or lower temperature. Humidity is the lever to reach for first because it is cheap, fast and disturbs nothing else - a humidifier on a cheap controller will do it. Lowering temperature works too, at about the same strength - one degree Fahrenheit does roughly what one to two points of humidity do - but you have less room to move it before growth slows below about 65°F (18°C).
How do I raise VPD in a grow tent?
Lower humidity or raise temperature. A dehumidifier is the usual answer, though in a small tent it also adds heat, which raises VPD again from the other direction. Increasing exhaust helps only if the air outside your tent is drier than the air inside it. Adding airflow across the canopy does not change the room reading but raises the effective pull at the leaf, which is often what you actually wanted.
Is 1.8 kPa too high?
For cannabis in veg or flower, it is higher than you want. Most charts treat about 1.6 kPa as the point where a plant starts closing its stomata to save water, though nobody has measured that threshold on cannabis, and closed stomata stop carbon dioxide coming in too. Before you fix the room, check the leaf: a reading that high with a hot leaf usually means the plant is not drinking, and that is a root zone problem.
Is VPD more important than humidity?
It is a better single number, and it is not more important. VPD is the better reading for growth because it accounts for temperature. Humidity is the better reading for mold, because Botrytis and powdery mildew respond to relative humidity and free water rather than to a vapor pressure deficit. Use VPD to run the plant and humidity to protect the harvest, and in late flower let humidity win.
Do I need a VPD meter, or is a calculator enough?
A calculator is enough. A $15 hygrometer clipped at canopy height plus any free online VPD calculator gives you the same number a several-hundred-dollar sensor gives you. Dedicated loggers like the Pulse, and controllers like the AC Infinity Controller 69, are real conveniences that hold a setpoint and show you trends. They are not accuracy upgrades. Buy a fan first.
How do I measure leaf temperature for VPD?
Point a handheld infrared thermometer at a healthy fan leaf in the middle of the canopy with the lights on. They cost about $20 and read surface temperature directly. It matters more than it sounds: at 77°F (25°C) and 55% humidity, a leaf 4°F (2°C) cooler than the air and one 4°F (2°C) warmer give VPD readings of 1.03 and 1.87 kPa - from the same room.
Is the VPD chart different for autoflowers?
No. An autoflower is the same plant on a different clock and wants the same bands at the same stages. What changes is your margin for error. A photoperiod can be given extra veg time to recover from a bad fortnight; an autoflower is flowering on its own schedule regardless, so the same mistake costs more. Aim for the same numbers and be less casual about drifting.
References
- Corredor-Perilla, I. C., Kwon, T.-H., & Park, S.-H. (2025). Elevated relative humidity significantly decreases cannabinoid concentrations while delaying flowering development in Cannabis sativa L. Frontiers in Plant Science, 16, 1678142. https://doi.org/10.3389/fpls.2025.1678142
- Grossiord, C., Buckley, T. N., Cernusak, L. A., Novick, K. A., Poulter, B., Siegwolf, R. T. W., Sperry, J. S., & McDowell, N. G. (2020). Plant responses to rising vapor pressure deficit. New Phytologist, 226(6), 1550-1566. https://doi.org/10.1111/nph.16485
- Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration: guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56, Chapter 3. Food and Agriculture Organization of the United Nations. https://www.fao.org/4/x0490e/x0490e07.htm
- 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. https://link.springer.com/article/10.1007/s12298-008-0027-x
- 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. https://doi.org/10.1371/journal.pone.0138930
- Punja, Z. K. (2021). Emerging diseases of Cannabis sativa and sustainable management. Pest Management Science, 77(9), 3857-3870. https://doi.org/10.1002/ps.6307
About the Author
Henrique Dias is the CEO and co-founder of BudTrainer. He is a mechanical engineer who also holds a graduate certificate in Commercial Cannabis Production from Niagara College in Ontario, Canada - the first college-level cannabis cultivation program in North America. Before founding BudTrainer, Henrique worked with Health Canada licensed cannabis producers, where he gained hands-on experience in cultivation, processing, and post-harvest management of cannabis at commercial scale. He started BudTrainer to bring that same level of craft to home growers through better-designed gardening tools and clear, science-backed education.
DISCLAIMER: Everything taught and sold by BudTrainer® is to be used strictly for legal purposes. We do not endorse the production of illegal substances and it is your duty to ensure that you are complying with the law. The words "hemp," "cannabis," "weed," and "marijuana" are used interchangeably to refer to the same plant (legal hemp with less than 0.3% THC) for the purposes of this lesson.
